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Parallel Session 1

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Applied & Industrial Physics

Location: Hall 5
Time: March 5th, 14:30 - 16:00

Speech: March 5th, 14:30, Hall 5

SELF-ASSEMBLED MAGNETIC DROPLET POPULATIONS FOR DIGITAL MICROFLUIDICS

M. Latikka1, A. Baidya1,2, M. Backholm1, A. Ballesio1, G. Beaune1, J. V. I. Timonen1, T. Pradeep2, R. H. A. Ras1,3

mika.latikka@aalto.fi
1 Department of Applied Physics, Aalto University School of Science, Espoo, Finland
2 Department of Chemistry, Indian Institute of Technology Madras, Chennai 60036, India
3 Department of Bioproducts and Biosystems, Aalto University School of Chemical Engineering, Espoo, Finland


In digital microfluidics small droplets are manipulated on an open surface for diagnostic and on-demand synthesis purposes. Droplet actuation is usually based on electrowetting on dielectric, but it can also be realized using magnetic forces, without the need for high electric fields or electricity in general. However, droplet dispensing and splitting in conventional magnetic digital microfluidics is difficult without irreversible pinning of the droplet.$^1$
Here we demonstrate ferrofluid-based digital microfluidics platform capable of droplet splitting, transport and combination while retaining droplet mobility. We form populations of ferrofluid droplets by step-wise droplet division caused by magnetic field induced instability.$^2$ In sufficient magnet field the ferrofluid droplet becomes unstable and splits into two or more daughter droplets. The resulting droplets self-assemble into symmetric patterns due to interdroplet magnetic repulsion. These droplet populations can then be easily actuated with permanent magnets. The droplets can also be recombined using dynamic magnetic fields. In addition to magnetic forces, the droplet division is also controlled by interfacial tension between ferrofluid and the surrounding fluid. As a result, division can be used to measure interfacial tension.

1. Y. Zhang and N.-T. Nguyen, Lab Chip $\textbf{17}$ (6), 994 (2017)
2. J. V. I. Timonen, M. Latikka, L. Leibler, R. H. A. Ras and O. Ikkala, Science $\textbf{341}$, 253 (2013)



Figure 1
Figure 1: a: Self-assembled droplet population created from a single 1 µl aqueous ferrofluid droplet with a permanent magnet. The droplets are immersed in silicone oil. b: 0.2 µl ferrofluid droplet in increasing magnetic field H and vertical field gradient dH created with a magnet moving closer to the droplets. First, the droplet elongates along the field (t = 70 s), then divides into two (t = 80 s). The divisions continue as the field is increased and a self-assembled droplet population is formed.

Speech: March 5th, 14:45, Hall 5

NANOCLUSTER AEROSOL EMISSIONS OF A 3D PRINTER: THE EFFECT OF NOZZLE TEMPERATURE AND FILAMENT MATERIAL

M. Poikkimäki1, V. Koljonen1, M. Närhi2, O. Kangasniemi1, O. Kausiala1, N. Leskinen1, M. Dal Maso1

mikko.poikkimaki@tuni.fi
1 Aerosol Physics, Faculty of Engineering and Natural Sciences, Tampere University, Tampere, Finland.
2 Photonics, Faculty of Engineering and Natural Sciences, Tampere University, Tampere, Finland.


The three-dimensional (3D) printers enable easy and inexpensive fabrication of items with complex shapes. For these reasons, the utilization of 3D printers is increasing in many indoor environments, such as homes, schools and offices. The widespread use of 3D printing has raised health concerns related to the operation and fumes emitted during the printing process. For instance, many printers have been found to emit large amounts of nano-sized particles during the printing operation (Stephens et al 2013; Kim et al 2015; Azimi et al 2016). The nanoparticles pose a risk to human health, which calls for detailed characterization of the emissions.

Despite numerous studies on 3D printer emissions, data on particles less than 3 nm in diameter, referred as nanocluster aerosol (NCA) by Rönkkö et al (2017), is still limited. Mendes et al (2017) were the first ones to study particle emissions from 3D printers in the low NCA size range from 1 to 3 nm. However, they reported the total particle number concentrations of the NCA, while this study presents the first results on NCA emission rates from a 3D printer operation.

The chosen 3D printer utilizes the fused filament fabrication (FFF) printing method. In this method, a thin filament material is continuously fed to a narrow nozzle in which the material is heated. The nozzle moves around the printer casing and, simultaneously, extrudes the material through the nozzle forming a desired object layer by layer onto a heated bed.

The FFF 3D printer was operated using several filament materials, such as acrylonitrile butadiene styrene (ABS), polylactic acid (PLA) and a polyester filament (nGEN), in nozzle temperatures of 210, 220, 240 and 250°C in order to detect the combined effects of the material and the temperature on the NCA emissions.

The printer situated in a contained chamber, which was diluted by filtrated air. This chamber was connected to aerosol sampling equipment, including Particle Size Magnifier (PSM A11, Airmodus) and Scanning Mobility Particle Sizer (SMPS, TSI) measuring particle number size distribution in particle size ranges of 1 – 5 nm and 5 – 50 nm, respectively. Additionally, Nanoparticle Surface Area Monitor (NSAM, TSI) measured the lung deposited surface area. The size distribution data was used to calculate the time resolved NCA emission rate taking into account the particle losses in the chamber as well as in the sampling lines.

The results show that during a typical printing process the NCA emission rate rapidly increases at the beginning of the printing session reaching a peak value after which it decreases and stabilizes into a steady-state situation. The average steady-state NCA emission rates (Fig. 1) ranged from 10$^6$ to almost 10$^{10}$ particles/second depending from the nozzle temperature and the filament material. The emission rates showed dependence on the nozzle temperature yielding higher emission rates at higher temperatures. Furthermore, the choice of filament material seems to affect the emission rate even if operated at the equal nozzle temperature.

These results can be used in proper selection of the materials and temperatures during 3D printer operation in order to reduce the particle emissions and hence, the exposure on the 3D printer users.

We thank Airmodus Ltd for the PSM A11 instrumentation employed in the measurements. M. P. acknowledges TUT Grad school for financial support.

Azimi, P. et al (2016) Environmental science & technology 50 (3), 1260-1268.
Kim, Y. et al (2015) Environmental science & technology 49 (20), 12044-12053.
Mendes, L. et al (2017) Journal of Industrial Ecology 13(2), 121-132.
Rönkkö, T. et al (2017) Proceedings of the National Academy of Sciences 114 (29), 7549-7554.
Stephens, B. et al (2013) Atmospheric Environment 79, 334-339.



Figure 1
Figure 1: The average steady-state NCA emission rates of filament materials as a function of the nozzle temperature. The error bars contribute to the maximum range of variation.

Speech: March 5th, 15:00, Hall 5

REFLECTIVE THERMOPLASMONIC SEMI-CONDUCTOR COMPOSITE FILMS IN THE NEAR-IR

K. Conley1, V. Thakore2, M. Karttunen2, T. Ala-Nissila1,3

kevin.conley@aalto.fi
1 Aalto University
2 University of Western Ontario
3 Loughborough University


Plasmon controlled propagation of near-infrared (NIR) electromagnetic waves has been demonstrated in diverse thermal and solar applications. The impinging radiation directly excites localized surface plasmon resonances, or surface-to-surface electron oscillations confined to a finite particle, which generate large extinction at the resonance wavelengths. The effectiveness of plasmons from conventional metallic particles, however, is limited due to large ohmic dissipation at high temperatures. Unlike metals, the plasmon performance in semiconductor microparticles is maintained even at elevated temperatures and they do not suffer excessive heating. The large scattering and good thermal stability has been exploited to reflect NIR and limit unwanted radiative and conductive heat exchange in coatings embedded with semiconductor microparticles at low volume fraction and allow for new applications for plasmon enhanced coatings.

We developed Multiscale Modelling methods to simulate the spectral response and directional scattering of spherical core-shell microparticles embedded in an insulating medium at low volume fraction. The spectrally-sensitive coatings exhibit controlled localized surface plasmons with broadband NIR reflectance. By adjusting the material, size, shape, and dielectric environment of the particles, the highly scattering and tunable resonance wavelength can be adjusted to match the incident radiation, such as flame or solar spectrum, greatly improving the reflectance efficiency. The plasmon enhanced coatings offer an alternative to internal structuring to guide or trap light inside of ultra thin film solar cells. This enable the physical thickness to be decreased while maintaining the optical thickness. In addition to controlling the directional photon scattering in ultra thin solar cells, these spectrally-sensitive coatings are useful within high temperature insulators and heat flux sensors for fire safety applications.


Speech: March 5th, 15:15, Hall 5

THERMOACOUSTIC REFRIGERATION TO COOL BEAM PROFILE MONITOR AT CERN

A. Holmström1, M. Aicheler2,3, E. Haeggström1

axi.holmstrom@helsinki.fi
1 Electronics Research Laboratory, Department of Physics, University of Helsinki
2 CERN
3 Helsinki Institute of Physics


Thermoacoustic heat engines either use heat to amplify (or even produce) sound or use sound energy to refrigerate. Travelling wave thermoacoustic engines [1] refrigerate through a Stirling cycle [2].

The Ionization Profile Monitor (IPM) [3] is part of the luminosity upgrade of the LHC at CERN. It is placed directly inside the vacuum of the accelerator ring and requires cooling. Previously used cooling solutions present problems: Peltier elements containing Bismuth transmute into Polonium in the harsh radioactive environment and liquid cooling is not wanted so close to the vacuum of the beam pipe. Therefore, thermoacoustic refrigeration could provide a new cooling solution which functions with a gas instead of a liquid.

Prior to constructing a prototype travelling wave thermoacoustic refrigerator, simulations are performed to determine its feasibility (IPM’s requirements are 12 W cooling power, cold temperature < 0°C). Estimates of the cooling power for a travelling wave thermoacoustic refrigerator structure containing helium with mean pressure 1 atm, sound pressure amplitude of 0.5 kPa and a 100 Hz sound frequency, has been calculated using an approximation method ([4], eq. 5.40). Viscosity and thermal conductivity in the helium were taken into account, but not the thermal conductivity of the regenerator or heat exchangers or acoustic losses caused by other structures than the regenerator. The temperature of the cold side was chosen to be 0°C and the hot side 20°C. Acoustic power of PAcoustic = 0.75 kW produced an estimated cooling power PCooling = 0.69 kW. This corresponds to a coefficient of performance COP = (PCooling / PAcoustic ) = 0.93, which is 7 % of the ideal Carnot’s COP for the fully reversible Stirling cycle. Despite the approximations, a cooling power of 0.69 kW still far exceeds the minimum required cooling power of 12 W for the IPM. Simulations of the effect of different parameters affecting the cooling power (length of regenerator, working gas, mean pressure, and temperature difference between the hot and cold side) will be presented.

References
[1] P. H. Ceperley, "A pistonless Stirling engine—The traveling wave heat engine," The Journal of the Acoustical Society of America, vol. 66, no. 5, pp. 1508-1513, 1979.
[2] G. W. Swift, "Thermoacoustic engines," The Journal of the Acoustical Society of America, vol. 84, no. 4, pp. 1145-1180, 1988.
[3] D. Bodart et al., "Development of an ionization profile monitor based on a pixel detector for the CERN Proton Synchrotron," in Proceedings of IBIC 2015, Melbourne, Australia, 2015, pp. 13-17.
[4] G. W. Swift, Thermoacoustics: A Unifying Perspective for Some Engines and Refrigerators, 2nd ed. Springer International Publishing, 2017.


Speech: March 5th, 15:30, Hall 5

NOVEL GAMMA RADIATION DETECTOR FOR FINNISH EARLY WARNING NETWORK

Sakari Ihantola1,2, Philip Holm1, Peter Dendooven2, Kari Peräjärvi1, Olof Tengblad3, Mika Kiiskinen4, Maarit Muikku1

sakari.ihantola@stuk.fi
1 STUK - Radiation and Nuclear Safety Authority
2 Helsinki Institute of Physics
3 Estructura de la Materia IEM-CSIC, Spain
4 Finnish Defence Forces


Early warning networks are a crucial part of preparedness for nuclear accidents. A comprehensive early warning network enables timely detection of abnormal levels of radioactivity in the atmosphere. This is essential for determining the right protective measures needed to minimize the possibly severe health effects caused by the radiation.

Currently early warning networks are largely based on ambient gamma dose rate measurement stations. The networks are sometimes supplemented with spectrometric stations that enable the identification of radionuclides based on the measured gamma ray energy spectrum.

Unfortunately, even the spectrometric early warning stations suffer from two major limitations. First, when the release plume reaches the vicinity of the detector, it is impossible to distinguish airborne radioactivity and fallout components from each other. Secondly, the detector container box may become contaminated with radioactive nuclides. For emergency management it is of utmost importance to know when the air is clear of radioactive materials.

The presentation summarises the progress in development of a new gamma radiation detector instrument that solves these problems. The current work includes testing of scintillator materials in Phoswich configuration and simulation of the performance of a complete detector system. The development of the new detector is done in parallel with the update of the Finnish early warning network. The final detector will be extensively tested and ready to be deployed by late 2021. The work is conducted under DEFACTO project jointly by The Finnish Radiation and Nuclear Safety Authority, The Helsinki Institute of Physics, Estructura de la Materia IEM-CSIC Spain and The Finnish Defence Forces.


Speech: March 5th, 15:45, Hall 5

HEAVY-IONS INDUCED DAMAGE ON SILICON CARBIDE (SIC) POWER MOSFETS

C. Martinella1,2, A. Javanainen1,3, U. Grossner4, R. Stark4, T. Ziemann4, R. G. Alia2, Y. Kadi2

corinna.martinella@cern.ch
1 Department of Physics, University of Jyväskylä, FI-40014 Jyväskylä, Finland
2 Engineering Department, CERN, 1211 Geneva 23, Switzerland
3 Electrical Engineering and Computer Science Department, Vanderbilt University, Nashville, TN 37235 USA
4 Advanced Power Semiconductor Laboratory, ETH Zurich, 8092 Zurich, Switzerland


Silicon-carbide (SiC) is very attractive for power devices due to its physical properties. The wide energy bandgap of 3.23 eV (4H SiC at room temperature) allows SiC devices to operate at high voltage, high temperatures and switching frequencies while achieving lower conduction losses in comparison to silicon. SiC devices are considered as promising technology for space and accelerator applications. For this reason, space agencies such as NASA, ESA and JAXA, and accelerator laboratory as CERN, have recently performed studies to investigate the radiation tolerance of SiC devices.
It has been observed that SiC MOSFETs exhibit three characteristic regions of response to heavy-ions as a function of the drain-source bias conditions (VDS) during the exposure. At low VDS, the ion-induced charge is collected with a similar multiplication mechanism as in the Si MOSFETs and no permanent damage is observed. At intermediate VDS, a unique Single Event Effect (SEE) is observed for SiC devices, that results in a gradual degradation of the device and permanent increasing in drain (ID ) and gate (IG) leakage currents with increasing heavy ion fluence. The damage is not catastrophic, but the device operation may become limited. In the third region, at sufficiently high VDS, a Single Event Burnout (SEB) occurs leading to a catastrophic failure of the device. 
This work is focused on the permanent degradation region, where a unique phenomena is observed for SiC. A mechanism describing the radiation induced leakage paths within the device structure is proposed and combined with simulations using an equivalent circuit to model it.


Heavy ion irradiations were performed at the RADiation Effects Facility (RADEF) in the Accelerator Laboratory of the University of Jyväskylä. Three types of SiC MOSFETs from the manufacturer Cree/Wolfspeed, available as bare dice were exposed individually to a fluence of 1x106 ions/cm2, while the gate voltage VGS= 0 V and the VDS was constant during the irradiation, and increased after each run. The heavy ion species used are 131Xe+35, 82Kr+22 and 56Fe+15, with at an energy of 9.3 MeV/amu. 
At sufficiently high VDS bias, the increase of similar magnitude in absolute values of the ID and IG were observed during the exposure. When the devices were exposed at VDS = 350 V and above, the ID increased at higher rate than the IG


From the electrical post-irradiation analysis of devices exposed to VDS = 300 V and VDS >350 V (Fig. 1 (a)), a model was developed to describe the heavy ion induced current path through the degraded device.
At low VDS, the current flows from drain to gate, exhibiting a linear dependence on the applied bias. Hence, the current voltage characteristics can be modeled by a simple resistor with two components Rox and Repi, that represent the oxide resistance and the epilayer resistance, respectively. At higher VDS, instead, the leakage path is mostly from drain to source, with a lower contribution of leakage from drain to gate.
It is hypothesized that the leakage through the gate oxide generates a voltage gradient within the dielectric that induces a field-effect sufficient to partially open the channel. This sets locally the MOSFET structure in a condition of “partial on-state”, which allows the current flow to the source.


The electrical equivalent for the current transport mechanism within the damage location is illustrated in Fig 1 (b). The very small part of the channel that opens as a consequence of the radiation induced leakage in the gate is modelled with a MOSFET named RADMOS. The gate terminal of the RADMOS is controlled by the potential generated in the gate oxide of the DUT. The total resistance of the gate is divided in Rox1 and Rox2 in order to represent the potential gradient within the oxide. At sufficiently high current flowing in the gate oxide, the VGS_RADMOS > Vth_RADMOS, the channel is partially opened and the currents start to flow to the source.
Fits were done for the ID, IG and Imeasurements of the irradiated devices. For VDS < 100 V, it was confirmed that the IG follows a linear behavior (i.e. ohmic), while the ID and IS follow a quadratic behavior characteristic of a MOSFET in "on state". An electrical equivalent was used to perform simulations with the parameters extracted from the fit.
The comparison between the measurements, the fit and the simulation results show a very good agreement for V <



Figure 1
Figure 1: (a) Comparison of IDVDS, IGVDS, ISVDS measurements for two devices irradiated at VDS = 300 V and VDS > 350 V. (b) Electrical equivalent for the heavy-ion induced current transport model in a degraded SiC power VD-MOSFET, valid at VDS < 100 V.

Parallel Session 2

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High Energy and Nuclear Physics I

Location: Hall 13
Time: March 6th, 11:00 - 12:30

Speech: March 6th, 11:00, Hall 12

SPECTATOR DARK MATTER

Tommi Tenkanen1, Tommi Markkanen2, Arttu Rajantie2

ttenkan1@jhu.edu
1 Johns Hopkins University
2 Imperial College London


We show that the observed dark matter abundance in the Universe can be fully accounted for by a minimally coupled spectator scalar field that was light during cosmic inflation and has sufficiently strong self-coupling. In this scenario, dark matter was produced during inflation by amplification of quantum fluctuations of the spectator field. The self-interaction of the field suppresses its fluctuations on large scales, and therefore avoids cosmological isocurvature constraints. The scenario does not require any fine-tuning of parameters and allows for a broad dark matter mass range from $1$ GeV to $10^8$ GeV. We will discuss prospects for detection.


Speech: March 6th, 11:15, Hall 12

EXPERIMENTAL PROBES OF PSEUDO-GOLDSTONE DARK MATTER

Niko Koivunen1

niko.koivunen@helsinki.fi
1 University of Helsinki


The dark matter is one of the outstanding problems of the modern physics. One of the most prominent explanations of the dark matter has been the weakly interacting massive particle (WIMP) paradigm. However the usual WIMP dark matter has been heavily challenged in the recent years due to direct-detection experiments that systematically provide more stringent bounds on the strength of the interaction between the dark matter and the ordinary matter. The direct-detection experiments happen practically at zero momentum transfer limit. The couplings of Goldstone bosons are proportional to their momentum in general. This statement is also true for the massive pseudo-Goldstone bosons. This can be exploited by making the dark matter a pseudo-Goldstone boson. For the pseudo-Goldstone dark matter the direct-detection amplitude is proportional to momentum transfer and therefore vanishes at zero momentum limit, allowing the WIMP-like pseudo-Goldstone dark matter to pass stringent direct-detection bounds naturally.

The pseudo-Goldstone dark matter still has important experimental possibilities even though the direct-detection signal is suppressed. The indirect-detection amplitude is not momentum suppressed for the pseudo-Goldstone dark matter and the future projections of the indirect-detection experiments allow for stringent exclusion limits for the models of this type [1]. The pseudo-Goldstone dark matter can also be probed in the LHC. The relatively light ($m_\chi<100$ GeV) pseudo-Goldstone dark matter could be observed at the high-luminosity LHC in the low backround vector boson fusion channel [2].


[1] Direct and indirect probes of Goldstone dark matter, Tommi Alanne, Matti Heikinheimo, Venus Keus, Niko Koivunen and Kimmo Tuominen, arXiv:1812.05996[hep-ph].

[2] Probing pseudo-Goldstone dark matter at the LHC, Katri Huitu, Niko Koivunen, Oleg Lebedev, Subhadeep Mondal and Takashi Toma, arXiv:1812.05952 [hep-ph].


Speech: March 6th, 11:30, Hall 12

TRIGGERING INTERESTING PHYSICS WITH THE CMS EXPERIMENT AT CERN

S. Laurila1

santeri.laurila@helsinki.fi
1 Helsinki Institute of Physics


To select interesting physics events among the abundant proton-proton collisions at the LHC, the CMS experiment uses a sophisticated two-level triggering system: the hardware ("Level-1") trigger is composed of custom-design hardware boards, while the High Level Trigger is a software-based system, running a streamlined version of the offline reconstruction software running on a computer farm.

From 2015 to 2018, the LHC delivered proton-proton collisions at a centre-of-mass energy of 13 TeV, with luminosities often exceeding the design value of the machine. Recent upgrades of the trigger architecture make it possible to use modern pattern recognition and multivariate analysis techniques at hardware level, enabling efficient triggering also at high luminosities. The performance of the upgraded Level-1 trigger system, and recent Finnish contributions in the operation and data quality monitoring of the trigger system, are presented.

The future HL-LHC collider will operate at a center-of mass energy of 14 TeV, and deliver even higher luminosities ($5-7$ times higher than the design value of the LHC), setting unforeseen challenges to the Level-1 trigger. To meet these challenges, the Level-1 trigger will be completely upgraded for the HL-LHC, with new features such as addition of tracking information and high-granularity calorimeter information at Level-1. The main features of the future trigger system, as well as ongoing efforts to develop new sophisticated trigger algorithms, are discussed.


Speech: March 6th, 11:45, Hall 12

COMPACT MUON SOLENOID (CMS) DETECTOR UPGRADE IN FINLAND

P. Luukka1, S. Bharthuar1, E. Brücken1, M. Golovleva1,2, A. Gädda1, S. Kirschenmann1, V. Litichevskyi1, L. Martikainen1, T. Naaranoja1, J. Ott1, E. Tuominen1

panja.luukka@helsinki.fi
1 Helsinki Institute of Physics
2 Lappeenranta University of Technology


The Compact Muon Solenoid (CMS) is a general-purpose detector at the CERN LHC. It has a broad physics programme ranging from studying the Standard Model to searching for extra dimensions and particles that could make up dark matter. The luminosity of the LHC accelerator will significantly increase in the coming years and consequently, radiation induced defects will severely affect the silicon sensor performance especially in the pixel region. In addition, detector occupancy will play a substantial role, and thus, the requirements for the detector performance will significantly increase. To meet the set goals for high-quality physics data taking, the CMS detector has thus to go through several upgrades in the coming years to preserve the efficiency, resolution and background rejection at these high luminosities.

At the nominal luminosity of the HL-LHC, the average number of interactions in a single bunch crossing is approximately 140, and the bunch crossings occur at the rate of 40 MHz. A relatively small fraction of the collisions, however, are such that they contain high transverse momentum particles originating from potentially interesting new high mass objects. Thus, it is important to distinguish the interesting events from the non-interesting ones, and thus the pileup rejection capability (i.e. the capability to distinguish the relevant hits in the tracking detectors) becomes extremely important in the high luminosity conditions. One way of improving the quality of the tracking is to have detectors with higher segmentation to reduce the risk of having several particles on the same readout channel simultaneously. Another way is to add information about the precise time of the arrival of the particles to the detector. This kind of global event timing can be achieved by upgrading the CMS experiment with a timing detector sensitive to minimum ionizing particles (MIPs) between the tracking detector and the electromagnetic calorimeters. Such a timing detector will be specialized to provide timing for the individual tracks crossing it. The added value of the timing detector is expressly quantified in terms of improved track and vertex reconstruction abilities, lepton efficiencies, di-photon vertex location, and missing transverse momentum resolution. A substantial reduction of the pileup jet rate and improved performance in b-jet identification is also expected.

The most comprehensive upgrade including the upgrades of several CMS sub-detectors, the Phase 2 upgrade, will happen during the Long Shutdown 3 (LS3) in 2024-26. In this upgrade, Finland will participate to the upgrade of the Tracker pixel detector and to the building of the new Minimum Ionizing Particle Timing Detector Endcap Timing Layer (MTD-ETL).


Speech: March 6th, 12:00, Hall 12

LATEST TOTEM RESULTS – IMPLICATIONS ON THE EXISTENCE OF A COLOURLESS THREE GLUON BOUND STATE

K. Österberg1, on behalf of the TOTEM collaboration

kenneth.osterberg@helsinki.fi
1 Department of Physics and Helsinki Institute of Physics, University of Helsinki


Hadronic high energy elastic proton proton ($pp$) and proton antiproton ($p\bar p$) scattering are traditionally described only by t-channel crossing-even exchange of a pair (or even number) of gluons, the so-called “Pomeron”, which has the parity (P) and charge parity (C) quantum numbers equal to plus. However t-channel crossing-odd exchange with PC = --, the so-called “Odderon”, corresponding to three (or odd number) of gluons, is allowed [1] and even predicted by QCD [2], but suppressed with respect to crossing-even exchange. Contrary to crossing-even exchange, that is expected to be purely imaginary, crossing-odd exchange, that may contribute to the real part of the amplitude, is not invariant for $pp$ and $p\bar p$. Hence, it is regarded that any significant difference in the elastic differential cross-section between $pp$ and $p\bar p$ at the same energy in the TeV scale region, where gluonic exchanges are expected to be dominant, is evidence for crossing-odd exchange, i.e. evidence for the existence of t-channel exchange of a colourless three (or odd number) gluon bound state. Furthermore, effects of crossing-odd exchange are expected to be visible where the imaginary part of the hadronic amplitude is suppressed e.g. at the diffractive minimum, the “dip”, or, in the measurement of the $\rho$ parameter that is the ratio of real to imaginary part of the hadronic amplitude at $t = 0$.

The TOTEM [3] experiment at CERN’s Large Hadron Collider (LHC) has recently measured the rho parameter and the total cross-section at $\sqrt s$ = 13 TeV [4], and shown that conventional t-channel crossing-even exchange based models [5] used up to now at LHC are not able to simultaneously describe both measurements. However, adding t-channel crossing-odd exchange leads to a better simultaneous description of the TOTEM measurements in the models. Now TOTEM has measured the dip and the second maximum in the elastic $pp$ differential cross-section at both $\sqrt s$ = 2.76 [6] and 13 [7] TeV. Combined with the previous measurement at $\sqrt s$ = 7 TeV [8], the TOTEM measurements show that in $pp$ the dip position moves to smaller |t|-values as the energy increases, while the cross-section ratio between the second maximum and the dip, denoted R, stays approximately constant at 1.8. This is significantly different compared the elastic $p\bar p$ differential cross-section measured by D0 at $\sqrt s$ = 1.96 TeV [9], where no dip, only a “kink”, and an R value of 1 is observed. Under the condition that effects due to the energy difference between the TOTEM and the D0 measurements can be neglected, the latest TOTEM results provide evidence for a colourless three gluon bound state exchange in the t-channel of the proton-proton elastic scattering.

[1] L. Lukaszuk and B. Nicolescu, Lett. Nuovo Cim. 8 (1973) 405.
[2] J. Bartels, L. N. Lipatov and G. P. Vacca, Phys. Lett. B 477 (2000) 17.
[3] G. Anelli et al. (TOTEM Collaboration), JINST 3 (2008) S08007.
[4] G. Antchev et al. (TOTEM Collaboration), First determination of the $\rho$ parameter at $\sqrt s$ = 13 TeV - probing the existence of a colourless three-gluon bound state, CERN-EP-2017-335 (submitted to Eur. Phys. J. C).
[5] J.R. Cudell et al. (COMPETE Collaboration), Phys. Rev. Lett. 89 (2002) 201801.
[6] G. Antchev et al. (TOTEM Collaboration), Elastic differential cross-section $d\sigma/dt$ at $\sqrt s$ =2.76 TeV and implications on the existence of a colourless 3-gluon bound state, CERN-EP-2018-341 (to be submitted to Eur. Phys. J. C).
[7] G. Antchev et al. (TOTEM Collaboration), Elastic differential cross-section measurement at $\sqrt s$ =13 TeV by TOTEM, CERN-EP-2018-338 (to be submitted to Eur. Phys. J. C).
[8] G. Antchev et al. (TOTEM Collaboration), EPL 95 (2011) 21002.
[9] V.M. Abrazov et al. (D0 Collaboration), Phys. Rev. D 86 (2012) 012009.



Figure 1
Figure 1: Elastic differential cross section in $pp$ at $\sqrt s$ = 2.76 TeV by TOTEM and in $p\bar p$ at $\sqrt s$ = 1.96 TeV by D0 as a function of the momentum transfer t. The $pp$ data have a clear dip at 0.61 $\pm$ 0.03 GeV$^2$, followed by a second maximum, whereas in the $p\bar p$ data there is no dip, only a kink. The green dashed line indicates the normalization uncertainty of the D0 measurement.

Speech: March 6th, 12:15, Hall 12

STATUS OF JUNO

Wladyslaw Henryk Trzaska1, Timo Enqvist1, Jari Joutsenvaara1,2, Pasi Kuusiniemi1, Kai Loo1,3, Maciej Slupecki1

wladyslaw.h.trzaska@jyu.fi
1 University of Jyväskylä
2 University of Oulu
3 University of Mainz


The Jiangmen Underground Neutrino Observatory (JUNO) is a medium-baseline reactor neutrino experiment, currently under construction in South China. The chosen site is equidistant from two nuclear power plants at the 53 km solar oscillation maximum. The combined projected thermal power of the reactors will be 35.8 GWth. The central detector will consist of a large acrylic sphere, 35.4 m in diameter, supported by a stainless-steel truss. The primary goal of JUNO is to resolve the neutrino mass hierarchy with at least 3σ significance by reconstructing energy spectrum of reactor neutrinos registered using 20 kT of liquid scintillator. To reach this goal an unprecedented energy resolution of 3% @ 1 MeV must be achieved, and a multitude of technical challenges solved. JUNO is also expected to improve the precision of solar oscillation parameters and the atmospheric mass-squared splitting to better than 1%. As a multi-purpose detector, JUNO can also detect geoneutrinos, neutrinos from core-collapse supernovae, search for dark matter, sterile neutrinos, and other non-standard interactions. The excavation of the experimental hall started in March 2018. JUNO collaboration has now 635 members from 77 institutes in 17 countries and continues to grow. At the moment JYFL is the only participant from Finland. This talk will present the physics case, the design, and the latest status of JUNO and briefly mention the Finnish contribution to the project.



Figure 1
Figure 1: Conceptual view of JUNO Central Detector.

Physics of Materials and Condensed Matter I

Location: Hall 5
Time: March 6th, 11:00 - 12:30

Speech: March 6th, 11:00, Hall 13

FRICTION MEASUREMENTS OF WATER DROPLETS ON SUPERHYDROPHOBIC SURFACES

M. Backholm1, M. Hokkanen1, M. Vuckovac1, V. Jokinen2, R. H. A. Ras1,3

matilda.backholm@aalto.fi
1 Department of Applied Physics, Aalto University
2 Department of Chemistry and Materials Science, Aalto University
3 Department of Bioproducts and Biosystems, Aalto University


Superhydrophobicity, that is extreme water-repellency, is a fascinating surface property found in nature on many plants and insects. As is beautifully exemplified by a lotus leaf, a water droplet on a superhydrophobic substrate will be almost spherical in shape. Due to the incredibly good resistance of the surface towards wetting, the droplet can easily roll off with only a tiny resistance. By mimicking the design found in nature, scientists have manufactured artificial superhydrophobic surfaces with self-cleaning, non-wetting, anti-icing, and anti-fogging properties, just to name a few examples. Measuring the friction force of droplets moving on these extremely slippery substrates is important to further improve their quality, as well as to understand the physics governing the drop motion. Conventional tribology tools, however, have not been able to probe the minuscule friction forces experienced by water droplets moving on the most superhydrophobic surfaces. Here, we present our work on developing a technique capable of measuring friction forces in the nanonewton range – one order of magnitude lower than the previously reported standard. Our experimental work explores existing analytical wetting and fluid dynamics models and gives guidance to designing applications such as self-cleaning windows and moisture resistant electronics.


Speech: March 6th, 11:15, Hall 13

DATABASE-DRIVEN HIGH-THROUGHPUT STUDY FOR PEROVSKITE SOLAR CELL COATING MATERIALS

Azimatu Seidu1, Lauri Himanen1, Jingrui Li1, Patrick Rinke1

azimatu.seidu@aalto.fi
1 Aalto University


In recent times, perovskite solar cells (PSCs) with an efficiency of 22 %, have revived the search for clean, affordable and efficient energy within the photovoltaic community. However, the practical realization of this hope has not been achieved due factors such as instability in moisture, oxygen and heat rich environments [1, 2]. In this study, we developed a high-throughput screening scheme and applied it to acquire candidate materials from the computational materials science database of AFLOW [3]. From more than 1.8 million entries of inorganic compounds, we collected a total of 120 binary and ternary materials that can be considered as protective coatings for the photoabsorbers in perovskite solar cells. To calculate the lattice mismatch between the perovskites and coating materials, we considered both inorganic and hybrid perovskites with Cs and MA as inorganic and organic cations respectively. We also used Pb and Sn metal cations with Br, Cl and I as halides. The selected coating materials fulfilled a series of criteria, as shown in the figure below. Aside from the commonly known metal oxides that have been used for coating, our research discovered compounds such as Si3N4 , BiF3, GaS, MoF3, B4C among others. The selected coating materials are non-toxic, nonreactive with moisture and abundant. Additionally, the coating materials have the adequate band gap energies to serve as efficient window materials to the perovskite substrate. More interestingly, we ensured minimum strain at the perovskite-coating interface by limiting the lattice mismatch to <|5|%. The focal point of this research is that, we have shown the possibilities of having a wider range of possible coating materials for PSCs. Our search does not only have the potential to stabilize PSCs against ambient conditions, but also increase their efficiency.


Reference
[1] M. A. Green, A. Ho-Baille, and H. J. Snaith, Nature Photon. 8, 506 (2014).
[2] G. Niu, W. Li, F. Meng, L. Wang, H. Dong, and Y. Qiu, J. Mater. Chem. A 2, 705 (2014).
[3] R. H. Taylor, F. Rose, C. Toher, O. Levy, K. Yang, M. B. Nardelli, and S. Curtarolo, Computational Materials Science 93, 178 (2014).



Figure 1
Figure 1: Database Curation Criterior

Speech: March 6th, 11:30, Hall 13

THEORETICAL STUDY OF SURFACE FUNCTIONALIZATION IN TWO-DIMENSIONAL TITANIUM BASED MXENES

Rina Ibragimova1, Martti Puska1, Hannu-Pekka Komsa1

rina.ibragimova@aalto.fi
1 Aalto University, Espoo, Finland


MXene phases are a new rapidly developing class of two-dimensional materials with suitable electronic, optical and mechanical properties for different applications [1]–[3]⁠. These phases consist of transition metals such as Ti, Sc, Zr, Hf, V, Nb, Ta, Cr, Mo and carbon or nitrogen atoms, and can be produced through the etching of layered MAX phases. During the etching process, surface is terminated by O, OH, and F functional groups. Theoretical studies showed that type of surface termination dramatically modify materials properties [4]–[6]⁠⁠. Thus far, several experiments have been conducted in order to understand a distribution of surface terminations but any general conclusions have not yet been made. In this work, we computationally describe the surface distribution of functional groups and its interaction with parent Ti$_{2}$C and Ti$_{3}$C$_{2}$ in the HF solution. The free Gibbs energies of formation for the distinctly terminated surfaces have been obtained by a detailed DFT model incorporating vibrational contribution and implicit solvation. The model based on the formation energy of individual ions in solution enables to link the free energies to the values of pH, temperature and work function [7]. Our results indicate the formation of O, OH and F mixture in a certain interval of work functions, which coincides with experimental data [1]⁠. Furthermore, DFT together with cluster expansion (CE), and Monte Carlo methods are employed to investigate the distribution of the functional groups on the surface. The proposed computational approach allows us to deeper understand a functionalization mechanism and introduce the range of experimental conditions for further tuning the MXenes properties.



References

[1] B. Anasori, M. R. Lukatskaya, and Y. Gogotsi, Nat. Rev. Mater. 2 (2017), 16098
[2] V. Ming, H. Huang, K. Zhou, P.S. Lee,W. Que, J.Z. Xu, and L. B. Kong, J. Mater. Chem. A 5 (2017), 3039-3068
[3] M. Yu, S. Zhou, Z. Wang, J. Zhao, and J. Qiu, Nano Energy 44 (2018), 181-190
[4] M. Khazaei, M. Arai, T. Sasaki, C. Chung, N.S. Venkataramanan, M. Estili, Y. Sakka, and Y. Kawazoe, Adv. Funct. Mater. 23 (2013), 2185-2192
[5] Y. Xie and P. R. C. Kent, Phys. Rev. B 87 (2013), 235441
[6] H.Weng, A. Ranjbar, Y. Liang, Z. Song, M. Khazaei, S. Yunoki, M. Arai, Y. Kawazoe, Z. Fang, and X. Dai, Phys. Rev. B 92 (2015), 075436
[7] M. Todorova, J. Neugebauer, Physical Review Applied 1 (2014), 014001


Speech: March 6th, 11:45, Hall 13

CASCADE OVERLAP SIMULATIONS IN METALS

F. Granberg1, E. Levo1, J. Byggmästar1, F. Djurabekova2,1, K. Nordlund1

fredric.granberg@helsinki.fi
1 Department of Physics, University of Helsinki
2 Helsinki Institute of Physics, University of Helsinki


Metals and metal alloys are the materials of choice as structural parts in most applications. Conventionally, iron and iron-based alloys have been used for a long time in many applications. In recent times, new metal alloys with exotic properties have been studied, for instance High Entropy Alloys (HEA), which could be used in some demanding applications. Both the conventional materials as well as the novel materials have been subject to thorough investigations, to obtain their properties in different environments. These studies have shown the strengths and drawbacks of different alloys, which can be utilized when choosing the perfect alloy for a certain application. Computer simulations have also been a widely used tool to study the properties of different materials. To use these materials in applications where radiation is present, like nuclear power plants or space applications, we need to know their response to irradiation. Previous studies have been conducted on the damage production in single energetic cascades, which will give a good insight on how many and which defects are produced. On the other hand, to be able to compare the radiation response with experiments and materials used in real applications, higher doses must be addressed.

To obtain higher irradiation doses, comparable with experiments, we simulate massively overlapping cascades in elemental materials and alloys, by means of Molecular Dynamics (MD). We investigate pure iron and compare it to several iron-chromium alloys, as well as pure nickel and several nickel-based EquiAtomic MultiComponent (EAMC) alloys, which are a subcategory of HEAs. [1-3] We irradiate the simulation cell homogeneously with several thousands of recoils to study the defect production and evolution of the different materials. For all materials we studied a bulk configuration, to simulate a volume deep inside the material, far from grain boundaries. For the iron sample and iron-chromium alloys we also simulated a surface configuration. A surface was introduced to include a permanent defect sink in the system, to investigate the effect of that. Both the defect amount and the dislocation structure and their evolution as a function of dose are studied in all investigated samples. The defect amount evolution was seen to be related to the nanostructural evolution of the material, such as dislocation reactions. We found that the different materials will respond differently to irradiation, both regarding the defect amount evolution as well as the defect structure evolution.


[1] F. Granberg et al. “Mechanism of radiation damage reduction in equiatomic multicomponent single phase alloys”, Phys. Rev. Lett. 116, 13 (2016), 135504
[2] F. Granberg et al. “Damage buildup and edge dislocation mobility in equiatomic multicomponent alloys”, Nucl. Inst. Meth. Phys. Res. Sec. B. 393 (2017) 114-117
[3] E. Levo et al. “Radiation damage buildup and dislocation evolution in Ni and equiatomic multicomponent Ni-based alloys”, J. Nucl. Mater. 490 (2017) 323-332


Speech: March 6th, 12:00, Hall 13

COMPUTATIONALLY-EFFICIENT MODELING OF INTERATOMIC INTERACTIONS COMBINING MACHINE LEARNING AND DENSITY FUNCTIONAL THEORY

Miguel Caro1,2

miguel.caro@aalto.fi
1 Department of Electrical Engineering and Automation, Aalto University
2 Department of Applied Physics, Aalto University


Molecular dynamics (MD) simulations are a useful tool to understand the interactions between atoms and to get insight into the processes that take place at the nanoscale and give rise to the observed properties of materials. "Classical" interatomic potentials, based on i) harmonic description of bonds, ii) partial electrostatic charges and iii) Lennard-Jones approximations for dispersion interactions, are computationally efficient but do not grant accurate representation of the real underlying physics/chemistry. They tend to fail at flexibly describing molecules in changing environments, especially when there is bond rearrangement, i.e., when chemical reactions take place. Density functional theory (DFT), on the other hand, offers a satisfactory description of interatomic interactions and can be used to characterize bond formation and annihilation. Unfortunately, DFT becomes prohibitively expensive when running MD of systems beyond a few hundreds of atoms or for time scales longer than a nanosecond. To bridge this gap between computational efficiency and accuracy, algorithmic developments that make use of machine learning techniques are being adopted by the community. In particular, the Gaussian approximation potential (GAP) framework [1] is becoming increasingly popular to describe interatomic interactions in the form of cohesive, or "total", energies. However, GAP-type interpolation can be used, in principle, also to learn local atomic properties other than total energies. Adsorption characteristics and spectroscopic signatures of atoms are possibly the most relevant examples. In this presentation, I will introduce a new method to predict adsorption energies [2] and X-ray core-excitation spectra [3], with an example application to amorphous carbon (although the method is general). I will also present a new type of atomic descriptor that allows us to improve the predictive ability of GAP models and therefore bring them closer to full DFT accuracy [2].

[1] A.P. Bartók, M.C. Payne, R. Kondor, G. Csányi. Phys. Rev. Lett. 104, 136403 (2010).

[2] M.A. Caro, A. Aarva, V.L. Deringer, G. Csányi, and T. Laurila. Chem. Mater. 30, 7446 (2018).

[3] A. Aarva, V.L. Deringer, S. Sainio, T. Laurila, and M.A. Caro. In preparation.



Figure 1
Figure 1: The inclusion of the new electronic kernel based on local density of states allows us to improve the performance of machine-learning models for adsorption energy prediction [2].

Speech: March 6th, 12:15, Hall 13

EFFICIENT METHOD FOR CALCULATING RAMAN SPECTRA OF SOLIDS WITH IMPURITIES AND ALLOYS

Arsalan Hashemi1, Arkady V. Krasheninnikov1,2, Martti Puska1, Hannu-Pekka Komsa1

arsalan.hashemi@aalto.fi
1 Department of Applied Physics, Aalto University, P.O. Box 11100, 00076 Aalto, Finland
2 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, 01328 Dresden, Germany


Raman spectroscopy is a widely used, powerful, and nondestructive tool for studying the vibrational properties of bulk and low-dimensional materials. Raman spectra can be simulated using first-principles methods, but due to the high computational cost calculations are usually limited only to fairly small unit cells, which excludes carrying out simulations for alloys. Here, we develop an efficient method for simulating Raman spectra of alloys, benchmark it against full density-functional theory calculations, and apply it to several alloys of two-dimensional transition metal dichalcogenides. The method is based on the projection of the vibrational modes of the supercell to those of the primitive cell, for which full first-principles Raman calculations are performed. This approach is not limited to 2D materials and should be applicable to any crystalline solids with defects and impurities. Furthermore, a mass approximation is adopted to efficiently evaluate the supercell vibrational modes but is limited to chemically and structurally similar atomic substitutions. To benchmark our method, we first apply it to Mo$_x$W$_{1−x}$S$_2$ monolayer in the H-phase, where several experimental reports are available for comparison. Second, we consider Mo$_x$W$_{1−x}$Te$_{2}$ in the T’-phase, which has been proposed to be 2D topological insulator, but where experimental results for the monolayer alloy are still missing. We show that the projection scheme also provides a powerful tool for analyzing the origin of the alloy Raman-active modes in terms of the parent system eigenmodes. Finally, we search for characteristic Raman signatures for impurities in MoS$_2$ in dilute concentrations.


Parallel Session 3

See titles in compact form

Physics of Materials and Condensed Matter II

Location: Hall 1
Time: March 7th, 11:00 - 12:30

Speech: March 7th, 11:00, Hall 1

FROM AB INITIO TO EXPERIMENTAL OBSERVABLE – MULTISCALE MODELLING OF ION RANGES IN TUNGSTEN

A. E. Sand1, R. Ullah2,3, A. A. Correa2

andrea.sand@helsinki.fi
1 Department of Physics, P.O. Box 43, FI-00014 University of Helsinki, Finland
2 Quantum Simulations Group, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
3 CIC nanoGUNE, Ave. Tolosa 76, 20018 Donostia-San Sebastián, Spain


Primary radiation damage formation from collision cascades has been simulated with molecular dynamics methods for several decades, yet despite early understanding that electronic effects may be significant in the highly non-equilibrium processes of particle irradiation events, such effects have proved difficult to incorporate into atomistic cascade simulations. Ion energies range over several orders of magnitude during the cascade process, with different physical models describing the ion-electron interactions in different energy regimes. The intermediate energy range is typically handled by a discreet cut-off imposed on the high-energy model of electronic stopping. This gives rise to a significant degree of empiricism in cascade simulations, where the treatment of energy losses in the intermediate ion energy range has a dominant effect on predictions of radiation damage.

A model for including electron-ion interactions in atomistic simulations, under a unified framework over the whole energy range relevant to cascade dynamics, has been suggested long ago [1], but only recently implemented by Tamm and Correa [2] with a parametrization for nickel. The model describes a smooth transition between the electronic stopping in the high-energy regime, and electron-phonon coupling in the low-energy regime, with the magnitude of the coupling varying due to the local electronic density experienced by the ion. As a first step towards realizing such a model in the fusion-relevant material tungsten (W), we have performed real-time time dependent density functional theory (TDDFT) calculations of the energy losses of a W projectile in W. The results of the computationally heavy energy loss calculations, explicitly accounting for a large number of semi-core electronic states, are then used to inform a molecular dynamics based model for calculations of ion ranges, providing good statistics of ion trajectories at experimental ion energies. We show by direct comparison to empirical data on ion implantation ranges, that the electronic stopping obtained in the <100> channel, predicted by TDDFT to be only a third of the value given by the state-of-the-art ion range software SRIM, leads to very good quantitative agreement with experiments of ion ranges. These results provide evidence of the importance of the local atomic environment, and of the validity of the TDDFT method even for the heavy ion W, hence opening the way for constructing an environmentally dependent model for electron-ion interactions with the goal of reducing the empiricism in atomistic radiation damage simulations in W.

[1] A. Caro and M. Victoria, Phys. Rev. A 40 (1989)
[2] https://github.com/LLNL/USER-EPH


Speech: March 7th, 11:15, Hall 1

HALF-QUANTUM VORTICES AND WALLS BOUNDED BY STRINGS IN TOPOLOGICAL SUPERFLUID 3HE

J. T. Mäkinen1, V. V. Dmitriev2, J. Nissinen1, J. Rysti1, G. E. Volovik1,3, A. N. Yudin2, K. Zhang1,4, V. B. Eltsov1

jere.makinen@aalto.fi
1 Aalto University
2 P. L. Kapitza Institute for Physical Problems of RAS
3 Landau Institute for Theoretical Physics
4 University of Helsinki


Symmetry and its breaking are central to the modern understanding of the physical world. In conjunction with a handful of judiciously chosen experiments and topological reasoning, they have guided us to formulating fundamental laws in the context of relativistic quantum field theory as well as the theory of possible states of matter, their phase transitions and universal behavior. Topological defects, encompassing monopoles, strings, and domain walls in various systems, affect the behavior at macroscopic scales. The range of possible defects is governed by the broken symmetries and topology of the system. Taking advantage of the broken symmetries in the phase diagram of superfluid $^3$He under nanoconfinement, we investigate half-quantum vortices (HQVs) – linear topological defects carrying half quantum of circulation – in the recently discovered polar-distorted A and polar-distorted B phases using nuclear magnetic resonance (NMR) techniques. Our results provide experimental evidence that HQVs – previously observed only in the polar phase [1] – survive the transitions to the superfluid phases with polar distortion. In the $p_x + ip_y$ polar-distorted A phase, HQV cores in 2D systems should harbor isolated Majorana modes. Moreover, isolated HQVs are topologically unstable in the fully gapped B phase but they nevertheless survive as composite defects – walls bounded by strings hypothesized decades ago in cosmology [2]. Our experiments [3] establish the superfluid phases of $^3$He in nanostructured confinement as a promising topological media for further investigations on a wide range of topics ranging from topological quantum computing to cosmology and grand unification scenarios.

[1] S. Autti, V. V. Dmitriev, J. T. Mäkinen et al, Phys. Rev. Lett. 117, 255301 (2016)
[2] T. W. B. Kibble, G. Lazarides, and Q. Shafi, Phys. Rev. D 26, 435 (1982)
[3] J. T. Mäkinen et al, Nature Communicationsvolume 10, 237 (2019)



Figure 1
Figure 1: Kibble-Lazarides-Shafi (KLS) wall configurations in the Polar-distorted B phase. Each HQV core terminates one spin soliton and one KLS wall. The orientation of the spin-vector is shown as arrows whose color indicates the angle $\theta$ w.r.t the easy plane. The order parameter is continuous across the virtual jumps. (a) The KLS wall is bound between a different pair of HQV cores as the spin soliton. (b) The spin soliton and the KLS wall are bound between the same pair of HQV cores.

Speech: March 7th, 11:30, Hall 1

EXPLORING THE RICH PHYSICS OF ROOM-TEMPERATURE ORGANIC POLARITON CONDENSATES.

Konstantinos Daskalakis1

konstantinos.daskalakis@aalto.fi
1 Aalto University, Department of Applied Physics


Polaritons in microcavities are hybrid light-matter bosonic quasi-particles formed when a semiconductor exciton is strongly coupled to a microcavity optical mode (photon). Owing to their bosonic nature, a macroscopic number of polaritons can occupy a single quantum state, and show wave-like properties and interference, similar to atomic Bose–Einstein condensates (BECs). Unlike atomic BECs, however, polaritons inherit a very small effective mass from their photonic component, thus allowing polariton condensation to be achieved at room temperature. The nonlinear character of polariton condensates makes microcavities one of the most versatile systems for realizing and studying a plethora of fascinating phenomena, such as superfluidity and the formation of dark solitons and vortices. However, the majority of studied microcavity systems have used CdTe- and GaAs-based semiconductors whose operation is limited to cryogenic temperatures due to their small exciton binding energy. Organic semiconductors have large exciton binding energy rendering them as viable materials for room-temperature polaritonic applications. Moreover, organics offer the advantage of a broad spectral range beyond that covered by inorganic semiconductors and can be easily fabricated without the need for epitaxial growth.

In this talk, I will present room-temperature polariton condensate in organic semiconductors [1] and discuss its nonlinear properties and dynamical processes of such a system. The samples consist of an oligofluorene thin film encapsulated in a dielectric Distributed Bragg reflector microcavity (SiO2/Ta2O5). Condensation in this system is realised by 200 fs pulsed optical excitation. On increasing the pump fluence, the nonlinear increase of the photoluminescence (PL) is accompanied by a simultaneous blueshift of the emission energy due to polariton interactions. We show that despite the very weak character of polariton interactions in tightly-bound Frenkel excitons, room-temperature superfluidity can be achieved due to large polariton densities attainable in organic microcavities. This suggests that even weak polariton-polariton interaction could play an important role. In addition, by using a Michelson interferometer in a retroreflector configuration, we study the emergence of spatial coherence and demonstrate several unique features stemming from the peculiarities of this material set and the unique dynamic-equilibrium character of this polariton condensate [3]. By imaging single-shot realizations of the organic polariton quantum fluid, we observe strong shot to shot fluctuations that are connected to the reservoir-mediated instability of nonequilibrium organic exciton-polariton condensates [4].

[1] K. S. Daskalakis, S. A. Maier, R. Murray and S. Kéna-Cóhen
"Nonlinear interactions in an organic polariton condensate"
Nature Materials 13, 271-278 (2014)

[2] G. Lerario, A. Fieramosca, F. Barachati, D. Ballarini, K. S. Daskalakis, L. Dominici, M. De Giorgi, G. Gigli, S. Kena-Cohen and D. Sanvitto
"Room-temperature superfluidity in a polariton condensate"
Nature Physics 13, 837-841 (2017)

[3] K. S. Daskalakis, S. A. Maier and S. Kéna-Cóhen
"Spatial coherence in a disordered organic polariton condensate"
Physical Review Letters 115, 035301 (2015)

[4] N. Bobrovska, M. Matuszewski, K. S. Daskalakis, S. A. Maier, S. Kéna-Cohen
"Dynamical instability of a non-equilibrium exciton-polariton condensate"
ACS Photonics 5, 111-118 (2018)


Speech: March 7th, 11:45, Hall 1

MEAN-FIELD THEORY FOR SUPERCONDUCTIVITY IN TWISTED BILAYER GRAPHENE

T. J. Peltonen1, R. Ojajärvi1, T. T. Heikkilä1

teemu.j.peltonen@student.jyu.fi
1 Nanoscience Center, Department of Physics, University of Jyväskylä, Finland


Recent experiments [1] show how bilayer graphene can be turned into a superconductor by relatively twisting the layers around a magic angle near $1^\circ$ and simultaneously doping it electrostatically. Most of the theoretical explanations so far [2,3] have concentrated on unconventional mechanisms for the superconductivity, perhaps mostly because of the similarities in the phase diagram with high-temperature superconductors. Here we instead show [4] that the conventional Bardeen-Cooper-Schrieffer (BCS) theory with $s$-wave pairing symmetry is able to capture the essentials of the superconducting behavior seen in the experiment, when viewed from the flat-band perspective.

By numerically solving the mean-field self-consistency equation, we show how the strongly peaked density of states, emerging due to the flat bands appearing near the magic angle, dictates the superconducting behavior. In addition to explaining some results of the experiment [1] we calculate new predictions about the superconducting state, such as the effect of the twist angle on the critical temperature. While our model cannot (yet) explain the observed insulating state, it may be possible in future studies by adding interactions on top of our simple model.

[1] Y. Cao, et. al., Nature 556, 43 (2018)
[2] H. C. Po, et. al., Phys. Rev. X 8, 031089 (2018)
[3] C. Xu and L. Balents, Phys. Rev. Lett. 121, 087001 (2018)
[4] T. J. Peltonen, R. Ojajärvi, T. T. Heikkilä, Phys. Rev. B 98, 220504 (2018)



Figure 1
Figure 1

Speech: March 7th, 12:00, Hall 1

SPIN-IMBALANCED PAIRING AND FERMI SURFACE DEFORMATION IN FLAT BANDS

K.-E. Huhtinen1, M. Tylutki1, P. Kumar1, T. I. Vanhala2, S. Peotta1, P. Törmä1

kukka-emilia.huhtinen@aalto.fi
1 Department of Applied Physics, Aalto University, Espoo, Finland
2 Faculty of Physics, Ludwig-Maximilians-Universität, München, Germany


Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states are superconducting states predicted to arise in large magnetic fields. They are characterised by a spatially modulated order parameter and Cooper pairs with a nonzero momentum. Despite experimental efforts to observe FFLO states, only indirect evidence for their existence has been found. It has been predicted that lattice systems may stabilise the FFLO state due to nesting of the Fermi surfaces [1,2], making them good candidates for the direct observation of spin-imbalanced superconductivity.

We study the attractive Hubbard model for two lattices featuring a flat band: the kagome and Lieb lattices. Using a mean-field model, we find FFLO like phases in both lattices. The stability of these phases is confirmed by dynamical mean-field theory calculations. Due to the presence of the flat band, the pairing mechanism is richer than just the Fermi surface shift conventionally responsible for FFLO states. Particle momentum distributions on the flat band can be modified at zero energy cost. This allows for multiband Cooper pair formation where the momentum distribution of the spin component residing on the flat band is deformed to mimic the distribution of the other spin component residing on a dispersive band. Our results highlight the profound effect of flat dispersion relations on Fermi surface instabilities and could provide a route to a direct observation of spin-imbalanced superfluidity [3].

[1] J. J. Kinnunen, J. E. Baarsma, J.-P. Martikainen and P. Törmä, Rep. Prog. Phys. 81, 046401 (2018).
[2] A. Cichy and A. Ptok, Phys. Rev. A 97, 053619 (2018).
[3] K.-E. Huhtinen, M. Tylutki, P. Kumar, T. I. Vanhala, S. Peotta and P. Törmä, Phys. Rev. B 97, 214503 (2018).


Speech: March 7th, 12:15, Hall 1

3+1D QUANTUM HALL EFFECT, ELASTICITY AND MIXED AXIAL-GRAVITATIONAL ANOMALIES

J. Nissinen1, G.E. Volovik1,2

jaakko.nissinen@aalto.fi
1 Low Temperature Laboratory, Aalto University, Finland
2 Landau Institute for Theoretical Physics, Russia


Prototypical topological states of matter are gapped systems with non-trivial gapless boundary degrees of freedom and responses to background fields, often protected by symmetries. Their existence is related to the similar appearance of various topological terms and associated quantum field theory anomalies, i.e. the failure of classical symmetries at the quantum level, in the effective action.

The quantum Hall effect is the archetype of topological response in 2+1 dimensions. We consider aspects of quasi-topological charge and energy (heat) transport in 3+1-dimensional topological insulators with anomalous quantum Hall effect protected by weak crystalline symmetries.

We describe the universal hydrodynamic response to background electromagnetic fields, temperature gradients and deformations of the crystal structure in terms of Chern-Simons like terms for electromagnetic, gravitational and elastic fields and how they represent the condensed matter analogue of field theoretic mixed axial-gravitational anomalies in odd spatial dimensions.

In particular, the variation of the Hall conductivity with respect to deformations is quantized in terms of topological momentum-space invariants. Similarly, we propose how thermal Hall conductivity could be consistently incorporated in the effective Chern-Simons description at the level of linear response. In the presence of crystal dislocations, the mixed responses satisfy consistent anomaly inflow with protected zero modes along dislocations in the bulk.

References:

J. Nissinen and G.E. Volovik, arXiv:1812.03175

J. Nissinen and G.E. Volovik, Tetrads in solids: from elasticity theory to topological quantum Hall systems and Weyl fermions, ZhETF 154, 1051 (2018), arXiv:1803.09234


Astrophysics and Space Physics

Location: Hall 13
Time: March 5th, 14:30 - 16:00

Speech: March 5th, 14:30, Hall 13

OBSERVATIONS OF THE NORTHERN LIGHTS BEYOND THE VISIBLE LIGHT

J. Sorri1

juha.sorri@oulu.fi
1 Sodankylä Geophysical Observatory, Tähteläntie 62, FIN-99600 Sodankylä, Finland


On behalf of KAIRA research community.

Sodankylä Geophysical Observatory (SGO) has been conducting geophysical measurements over a century [1]. Early observations were focused on earths magnetic field but towards the present day, observations of the near space and upper atmosphere have grown to be increasingly important tasks. Most notable being the resent studies of atmospheres radio opacity measurements in coincidence with the northern lights.

In a recent case study KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Ar- ray) [2] was used to observe strong pulsating auroras. Study combines the phased- array radio telescope methods with the ground based optical measurements and produces new information about evolution of the auroras and processes generating them [3].

A further study with improved sensitivity finds a direct relation between the visible auroras and cosmic noise absorption thus proofing that both are results from the same electron precipitation event [4].

Ability to indirectly estimate electron precipitation is a very important when the chemistry in the upper atmosphere is researched. As an example Sodankylä Ion Chemistry (SIC) model uses the electron precipitation as one of the main input parameters [5].

References
[1] H. Nevalinna, Sodankylän geofysiikan observatorio 1913-2013 - sata vuotta havaintoja ja tutkimusta (2017) .
[2] Derek McKay-Bukowski, IEEET RANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 53, NO. 3, MARCH 2015.
[3] Derek McKay, Pulsating aurora and cosmic noise absorption associated with growth-phase arcs, Ann. Geophys., 36, 59?69, 2018.
[4] Grandin, M, Observation of pulsating aurora signatures in cosmic noise absorption data, Geophys. Res. Lett., 44, 5292?5300, 2017.
[5] Verronen P, Diurnal variation of ozone depletion during the October-November 2003 solar proton events, J. Geophys. Res., 110, A09S32, 2005.


Speech: March 5th, 14:45, Hall 13

DISRUPTION OF THE EARTH'S FORESHOCK WAVE FIELD DURING SOLAR STORMS

L. Turc1, O.W. Roberts2, M.O. Archer3, M. Palmroth1,4, M. Battarbee1, T. Brito1, U. Ganse1, M. Grandin1, Y. Pfau-Kempf1, C.P. Escoubet5

1 Department of Physics, University of Helsinki, Helsinki, Finland
2 Space Research Institute, Austrian Academy of Sciences, Graz, Austria
3 Queen Mary University of London, London, UK
4 Finnish Meteorological Institute, Helsinki, Finland
5 ESA/ESTEC, Noordwijk, Netherlands


The foreshock is a region of near-Earth space characterized by intense electromagnetic wave activity. It extends upstream of the bow shock which slows down the supersonic solar wind flow before it impinges on the Earth's magnetic domain, the magnetosphere. Here we investigate how the foreshock wave field is modified when solar storms - giant clouds of solar particles and magnetic fields originating from tremendous eruptions in the Sun’s corona - interact with near-Earth space. These solar storms are one of the main sources of adverse space weather, which can damage key technologies in space and on the ground. Understanding how solar storms affect the different regions of near-Earth space is crucial to better forecast their effects.
In this study, we analyse observations from the four-spacecraft Cluster mission in the terrestrial foreshock during solar storms. We find that the usual foreshock quasi-monochromatic waves are replaced by a superposition of fast magnetosonic waves at different frequencies. The transverse extent of the wave fronts is also significantly reduced during solar storms, suggesting that the foreshock wave field is structured over smaller scales. Numerical simulations performed with the global hybrid-Vlasov Vlasiator model, developed at the University of Helsinki, further support that conditions associated with solar storms result in smaller wave fronts and less monochromatic wave activity. These modifications of the foreshock wave properties are likely to affect the regions closer to Earth, as the foreshock waves are advected earthward by the solar wind.



Figure 1
Figure 1: Global simulations of near-Earth space performed with the Vlasiator model, for regular solar wind conditions (left) and solar storm-like conditions (right). The Earth is located in the left-hand side of each panel, surrounded by the magnetosphere (in dark blue), while the solar wind flows from the right-hand side. Colour-coded is the proton density, using two colour schemes to highlight density variations both in the foreshock (centre to right) and the magnetosheath (downstream of the bow shock)

Speech: March 5th, 15:00, Hall 13

THEMIS OBSERVATIONS OF PARTICLE ACCELERATION BY A MAGNETOSHEATH JET-DRIVEN SHOCK

T. Liu1, H. Hietala2, V. Angelopoulos1, Y. Omelchenko3, V. Roytershteyn3

terryliuzixu@ucla.edu
1 University of California, Los Angeles
2 University of Turku
3 Space Science Institute, Boulder, CO


Shocks are important particle accelerator in the universe. They can reflect and accelerate upstream particles. In the downstream sheath region, localized sheath jets with high dynamic pressure are frequently observed due to shock ripples. When a sheath jet is supermagnetosonic relative to ambient plasma, a secondary shock has been observed to form. Such secondary shocks, in principle, could also reflect and accelerate particles, but evidence is needed. Using multipoint THEMIS observations in the Earth’s magnetosheath, we present that a shock driven by an intrinsically-formed magnetosheath jet can indeed reflect and accelerate particles up to tens of keV for ions and 100 keV for electrons. By analyzing the ion distributions, we set up a model to interpret how ions reach the spacecraft from the shock. Our study implies that particle acceleration at sheath jet-driven shocks could play an important role in shock acceleration.


Speech: March 5th, 15:15, Hall 13

SOLAR ELECTRONS AND ATMOSPHERIC OZONE

P. T. Verronen1, M. E. Andersson1, N. Kalakoski1

pekka.verronen@fmi.fi
1 Space and Earth Observation Centre, Finnish Meteorological Institute, Helsinki, Finland


Earth's atmosphere is continuously under the influence of a highly-varying flux of energetic particles from space. Driven by the solar wind, i.e. space weather, the magnitude of electron and proton bombardment is connected to the activity of Sun. For example, solar proton events are most frequent during solar maximum while high-energy electron precipitation tends to peak during declining phase of the solar cycle. Because of the protection provided by Earth's geomagnetic field, charged particles are guided into the atmosphere in the polar regions where their most widely-known effect is the Northern lights, i.e. aurora. With particle energies extending into the range of keVs and MeVs, precipitation leads also to increased ionization of atmospheric molecules, typically at altitudes from the upper stratosphere to mesosphere and thermosphere ($>$30 km). As a result, Earth's ionosphere is affected and neutral gas composition changes through ion-neutral chemistry. One of the most interesting chemical effects caused by particle precipitation is the decrease of ozone in the stratosphere and mesosphere. There is evidence of solar-driven decadal ozone variability modulating wintertime polar vortex dynamics, with a possible connection to regional climate variability. However, the exact mechanism connecting ozone and ground-level climate parameters is still under research.

Here we will present some highlights of the particle precipitation and ozone research conducted during the last ten years at the Finnish Meteorological Institute. We will particularly concentrate on the solar electrons that can directly affect mesospheric ozone during geomagnetic storms. We will utilize both observations and simulations, and discuss the variability of electron precipitation and ozone over time scales ranging from days to decades. We will show that in order to simulate polar ozone variability adequately, electron precipitation needs to be considered in climate models.


Speech: March 5th, 15:30, Hall 13

GLOBAL HYBRID MODELING OF THE SOLAR WIND INTERACTION WITH MERCURY: ULTRA-LOW FREQUENCY WAVES IN THE FORESHOCK

R. Järvinen1,2, M. Alho1, E. Kallio1, T.I. Pulkkinen3,1

riku.jarvinen@aalto.fi
1 Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University, Espoo, Finland
2 Finnish Meteorological Institute, Helsinki, Finland
3 Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, Michigan, USA


Mercury is the closest planet to the Sun and the smallest planet of the solar system. It has a moderate intrinsic magnetic field, which interacts with the magnetized solar wind flow of electrically charged particles. Mercury provides a unique opportunity to study space weather phenomena in a “pocket-sized” magnetosphere under the most extreme solar conditions of the solar system planets. The interplanetary magnetic field is dominated by a strong component in the direction of the solar wind flow on average. This means that the bow shock, the outermost boundary of a planet-solar wind interaction region, has a large quasi-parallel region, where the normal of the boundary surface and the magnetic field are closely aligned.

The ion foreshock forms ahead of the quasi-parallel bow shock in the region where the magnetic field is connected to the bow shock. A portion of incident solar wind particles is reflected by the quasi-parallel shock leading to enhanced ion temperature in the foreshock. The backstreaming ions are a source of free energy for the excitation of large-scale foreshock plasma waves. Here we study large-scale coherent foreshock waves in a three dimensional global hybrid simulation for the Mercury-solar wind interaction. In the simulation, ions are treated self-consistently as particles moving under the Lorentz force and electrons form a charge-neutralizing fluid. We find that the main wave mode in the ion foreshock is the oblique fast magnetosonic wave at about 5-second period in the ultra-low frequency range.

Mercury will be explored by the BepiColombo twin-orbiter mission launched in Oct 20, 2018. The Finnish scientists have been in the core team planning and building the spacecraft, making several hardware contributions to the mission. This modeling research sets the stage for the upcoming BepiColombo measurements, and our results highlight the need to examine the foreshock region to search for backstreaming ion populations and ultra-low frequency foreshock waves in plasma density and magnetic field.


Speech: March 5th, 15:45, Hall 13

PHOTOMETRIC REDSHIFT GALAXIES AS TRACERS OF THE FILAMENTARY NETWORK

M.Kruuse1, E.Tempel1, R.Kipper1, R.Stoica

maarja@to.ee
1 Tartu Observatory, University of Tartu, Observatooriumi 1, 61602 Tõravere, Estonia
2 Université de Lorraine, CNRS, IECL, F-54000 Nancy, France



The Javalambre Physics of the Accelerating Universe Astrophysical Survey will observe the photometric redshifts for galaxies up to redshift $1$. The development of observational technology has boosted the importance of using the photometric redshift galaxy data (these galaxies have rather uncertain distance estimations in redshift space).
The use of photometric galaxies would increase the number density of galaxies per volume of space, which would allow more detailed analysis of the intrinsic cosmic web, and vastly broaden the space in which cosmic web structure elements could be detected.

The mathematical framework of the Bisous model [1], developed to estimate the filamentary pattern from observed spectroscopic galaxy data (these galaxies have precise distances in redshift space), reveals a complex network of spines.
The aim of our paper [3] is to see whether the SDSS photometric redshift galaxies dataset [2] carries any information about the filamentary spine pattern detected by the Bisous model. We have applied summary statistics tools to analyse the eventual correlation between the photometric galaxies point pattern and the filamentary spine pattern. This statistics indicates possible inhibition or clustering between the observed sets.
Additionally we have analysed whether these galaxies locate inside or close to the filamentary spines physically. Our analysis views the datasets in distance tomography and also dependent on the line of sight orientation of the filamentary pattern. These aspects give us hints about the origin of the possible signal and the significance of the Finger-of-God effect compression on the filamentary structures parallel to the line of sight.

Our results show that the photometric redshift galaxies are vital building blocks in constructing the complex filamentary pattern of the cosmic web from observational data. We also see that photometric galaxies have a high probability in locating in groups with spectroscopic galaxies. The analysis also gave a rough estimate for the width of the filamentary spine. In addition to the filamentary web analysed in our paper the results give a basis for using the information embed in the photometric redshift galaxies in any structure element detection model.

[1] E. Tempel, R. S. Stoica, E. Saar, V. J. Martinez, L. J. Liivamägi and G. Castellan,
Detecting filamentary pattern in the cosmic web: a catalogue of filaments for the SDSS,
MNRAS (2014), 438, 3466

[2] R. Beck, L. Dobos, T. Budavári, A. S. Szalay and I. Csabai,
Photometric redshifts for the SDSS Data Release 12,
MNRAS (2016), 460, 1372

[3] M. Kruuse, E. Tempel, R. Kipper and R. S. Stoica,
Photometric redshift galaxies as tracers of the filamentary network,
A$\&$A submitted


Energy, Environment and Climate

Location: Hall 5
Time: March 7th, 11:00 - 12:30

Speech: March 7th, 11:00, Hall 5

MODELLING THE FRAGMENTATION OF IONIZED CLUSTERS IN API-TOF MASS SPECTROMETER

T. Zanca1, M. Passananti1, E. Zapadinsky1, H. Vehkamäki1

tommaso.zanca@helsinki.fi
1 University of Helsinki


The development of Mass Spectrometers (MS) as the Atmospheric Pressure interface Time Of Flight (APi-TOF) and the Chemical Ionization APi-TOF (CI-APi-TOF) has revolutionized the study of new atmospheric aerosol particle formation. These instruments are able to detect molecules and small clusters, which are involved in the first stages of new particle formation, even at environmental low concentration.
However, clusters’ binding energies are much weaker compared to molecules, hence they can undergo transformations (fragmentation and/or evaporation) inside a MS easier than molecules.
Here we present a model to describe the collision induced fragmentation of atmospheric clusters that may take place during their detection process inside APi-TOF. We consider each cluster individually and its trajectory is simulated as a random process.
The crucial part of the dynamics leading to fragmentation is the energy transfer to and between vibrational and rotational degrees of freedom. This is determined by the density of states of different modes and it is simulated by means of proper probability distribution functions.
After simulating the trajectory (and the fate) of a statistically significant number of clusters we calculate the proportion of the fragmented clusters. The results are in good agreement with the experiments.


Speech: March 7th, 11:15, Hall 5

DEFECT ENGINEERING OF ATOMIC LAYER DEPOSITED TITANIUM DIOXIDE FOR PHOTOCATALYTIC APPLICATIONS

J. Saari1, H. Ali-Löytty1, M. Hannula1, L. Palmolahti1, B.D. Bhuskute1, K. Lahtonen1, M. Valden1

jesse.saari@tuni.fi
1 Surface Science Group, Photonics Laboratory, Tampere University, P.O.B. 692, FI-33014 Tampere University, Finland


Photoelectrochemical (PEC) water splitting is one of the potential methods of storing solar energy into chemical form as hydrogen. A major issue with the method and a challenge of renewable energy production is the development of efficient, chemically stable and cost-effective semiconductor photoelectrodes. Crystalline TiO$_2$ as such is extremely stable and capable of unassisted photocatalytic water splitting but the efficiency is limited by the bandgap (3.0–3.2 eV) to harvest photons only in the UV range. Recently, otherwise unstable semiconductor materials that can harvest the full solar spectrum has been successfully stabilized by amorphous titanium dioxide (am.-TiO$_2$) coatings grown by atomic layer deposition (ALD) [1]. However, the stability of am.-TiO2 without additional co-catalyst has remained unresolved [2].

In our recent studies, we have reported means to thermally modify the defect structure of ALD grown am.-TiO$_2$ thin film under oxidative [3] and reductive [4] conditions. TiO$_2$ films were grown on silicon and fused quartz substrates by ALD at 200 $^{\circ}$C using tetrakis(dimethylamido)titanium (TDMAT) and deionized water as precursors. Based on the results, the as-deposited am.-TiO$_2$ is chemically unstable and visually black exhibiting both enhanced absorbance in the visible range and exceptionally high conductivity due to the trapped charge carriers (Ti$^{3+}$). Heat treatment in air at 200 $^{\circ}$C induces oxidation of Ti$^{3+}$, decrease in absorbance and conductivity but has only a minor effect on the stability. However, a reasonable stability is obtained after oxidation at 300 $^{\circ}$C, simultaneously with the crystallization of TiO$_2$ into rutile. Furthermore, oxidation at 500 $^{\circ}$C results in stable rutile TiO$_2$ that produces the highest photocurrent for water oxidation. In contrast, reductive heat treatment in ultra-high vacuum (UHV) at 500 $^{\circ}$C retains the amorphous phase for TiO$_2$ but enhances the stability due to the formation of O$^–$ species via electron transfer from O to Ti.



[1] S. Hu, M.R. Shaner, J.A. Beardslee, M. Lichterman, B.S. Brunschwig, N.S. Lewis, ”Amorphous TiO$_2$ Coatings Stabilize Si, GaAs and GaP photoanodes for Efficient Water Oxidation”, Science 344, pp. 1005–1009, 2014. DOI: 10.1126/science.1251428.
[2] K. Sivula, ”Defects Give New Life to an Old Material: Electronically Leaky Titania as a Photoanode Protection Layer”, ChemCatChem 6, pp. 2796–2797, 2014. DOI: 10.1002/cctc.201402532.
[3] H. Ali-Löytty, M. Hannula, J. Saari, L. Palmolahti, B.D. Bhuskute, R. Ulkuniemi, T. Nyyssönen, K. Lahtonen, M. Valden, ”Diversity of TiO$_2$: Controlling the Molecular and Electronic Structure of Atomic-Layer-Deposited Black TiO$_2$”, ACS Appl. Mater. Interfaces In press, 2019. DOI: 10.1021/acsami.8b20608.
[4] M. Hannula, H. Ali-Löytty, K. Lahtonen, E. Sarlin, J. Saari, M. Valden, ”Improved Stability of Atomic Layer Deposited Amorphous TiO$_2$ Photoelectrode Coatings by Thermally Induced Oxygen Defects”, Chemistry of Materials 30, pp. 1199–1208, 2018. DOI: 10.1021/acs.chemmater.7b02938.



Figure 1
Figure 1: The as-deposited am.-TiO$_2$ is chemically unstable and visually black. Heat treatment in air at 500 $^{\circ}$C results in stable rutile TiO$_2$ that produces the highest photocurrent for water oxidation.

Speech: March 7th, 11:30, Hall 5

ISOTOPE EFFECT ON THE DETACHMENT ROLLOVER DENSITY IN JET OHMIC PLASMAS IN HORIZONTAL DIVERTOR PLASMA CONFIGURATION

V. Solokha1, M. Groth1, S. Brezinsek2, M. Brix3, G. Corrigan3, C. Guillemaut4, D. Harting3, S. Jachmich5, U. Kruezi6, S. Marsen7, S. Wiesen2, and JET contributors8

vladimir.solokha@aalto.fi
1 Aalto University, P.O. Box 14100, FI-00076, Aalto, Espoo, Finland
2 Institute of Energy and Climate Research - Plasma Physics, Forschungszentrum Julich GmbH, Julich, Germany
3 EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK
4 Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Lisbon, Portugal
5 Ecole Royale Militaire School, Av de la Renaissance 30, Brussels, Belgium
6 ITER Organization Route de Vinon sur Verdon, Saint Paul-lez-Durance, France
7 Max-Planck-Institute for Plasma Physics, Greifswald, Germany
8 See the author list of "X. Litaudon et al 2017 Nucl. Fusion 57 102001"


The research of the isotope effect on divertor plasma conditions at JET with carbon wall (JET-C) revealed that rollover density scales inversely with the ion mass in vertical target configurations of JET-C L-mode discharges at both divertor targets [1].

We explored the isotope effect in Ohmic plasma discharges in JET with ITER-like walls (JET-ILW). Experiments were carried out with a vertical-horizontal divertor plasma configuration at I$_p$ = 2MA and B$_T$ = 2T. The measured ion currents by divertor Langmuir probes array showed that the detachment rollover density difference is as small as 10% between hydrogen and deuterium plasma. The isotope effect was observed at outer divertor target only. The local electron density at outer target measured by Langmuir probes scales as m$_i^{0.5}$.

Simulations of the edge plasma using EDGE2D/EIRENE with subdivertor included qualitatively reproduced isotope effect on the rollover density at the outer target, but the simulated subdivertor pressures are up to 10x higher than experimental values [2, 3]. The discrepancy could be reduced by increasing cryopump capture coefficient.

Standalone EIRENE simulations of neutral particles transport indicate that isotope effect could be caused by the molecular flow of neutrals to the subdivertor. In steady state conditions, with fixed gas injection rates and molecular temperature, hydrogen molecules have higher thermal velocity than deuterium. Therefore, the molecular density scales as m$_i^{0.5}$ to match currents. It is consistent with the experimental data. The absence of isotope effect at the inner divertor target caused by divertor hardware geometry and ballistic flow of recycled molecules from strike points. The simulations suggest the presence of the louvre (radiation shield) at the pumping plenum reduces the conductivity of the system and the effectiveness of cryopump in JET-ILW.


Speech: March 7th, 11:45, Hall 5

COMPARISON AND VALIDATION OF TUNGSTEN TRANSPORT SIMULATIONS IN THE SCRAPE-OFF LAYER OF JET L-MODE PLASMAS

H. Kumpulainen1, M. Groth1, M. Fontell1, A. Järvinen2, G. Corrigan3, D. Harting3, A. Meigs3

henri.kumpulainen@aalto.fi
1 Aalto University, Finland
2 Lawrence Livermore National Laboratory, USA
3 Culham Centre of Fusion Energy, UK


Tungsten impurity transport in JET tokamak low-confinement plasmas is simulated using the quasi-kinetic Monte Carlo code DIVIMP and the edge-fluid, kinetic neutral code EDGE2D-EIRENE. The simulation results of the two codes are compared to assess the level of agreement between Monte Carlo and fluid treatment of tungsten and to analyze the reasons to any significant discrepancies. Synthetic diagnostics of W I and W II spectral line radiation are used to validate the simulations against experimental measurements with spectroscopy in the visible wavelength range.

The simulation cases are based on earlier impurity studies [1]. The power crossing through the plasma is 2.2 MW, split equally between the ion and electron channel, and the electron density is varied. The impurity sources are determined by physical sputtering at the divertor tiles, mainly due to intrinsic beryllium ions and charge-exchange deuterium neutrals, consistent with previous work [2]. The background plasma solution and tungsten ionization source obtained from EDGE2D-EIRENE are used as inputs in DIVIMP to limit the comparison to tungsten ion transport.

DIVIMP and EDGE2D-EIRENE agree within 5% on tungsten deposition at each target and wall surface. The density distributions are qualitatively similar, but the DIVIMP predictions for total W content are around 50% higher for low and intermediate density cases compared to EDGE2D-EIRENE. The bundling effect of W ionization stages in EDGE2D-EIRENE [3] results in up to 33% lower average tungsten charge in the main chamber outer scrape-off layer, which in turn leads to weaker W trapping and thus lower W density in the upstream. The highest upstream W density was 3.5 · 10$^{14}$ m$^{-3}$ in EDGE2D-EIRENE and 8.1 · 10$^{14}$ m$^{-3}$ in DIVIMP. At the target boundaries W density in EDGE2D-EIRENE reaches values up to one order of magnitude higher than in DIVIMP due to the prescribed boundary condition for parallel W velocity. Additionally, the impurity pressure gradient force in EDGE2D-EIRENE was found to scale with the impurity temperature, while its DIVIMP equivalent, parallel diffusion, applies a different model based on the background ion temperature, density and impurity charge, yielding slightly more localized peaks in tungsten density near ionization sources.

[1] M. Groth et al., Nucl. Fusion 53 (2013) 093016.
[2] D. Harting et al., J. Nucl. Mater. 438 (2013) S480-S483
[3] J.D. Strachan et al., J. Nucl. Mater. 415 (2011) S501-S504



Figure 1
Figure 1: Tungsten density distribution in the JET tokamak according to an EDGE2D-EIRENE simulation.

Speech: March 7th, 12:00, Hall 5

EFFECT OF CHARGE EXCHANGE REACTIONS ON FAST ION CONFINEMENT IN FUSION DEVICES

P. Ollus1, J. Varje1, T. Kurki-Suonio1, R. Akers2, J.F. Rivero-Rodriquez3, Samuel Ward4

patrik.ollus@aalto.fi
1 Aalto University, Espoo, Finland
2 United Kingdom Atomic Energy Authority, Culham Science Centre, Abingdon, United Kingdom
3 University of Seville, Spain
4 University of York, ITER organization, Culham Science Centre, Abingdon, United Kingdom


In magnetic confinement fusion, ions in a hot plasma are confined in a torus-shaped magnetic bottle to produce energy through fusion reactions. By receiving electrons in atomic reactions, these ions can be neutralized and break the magnetic confinement. For fast ions in fusion-relevant plasmas, which are significantly more energetic than the thermal bulk ions and have energies greater than 10-100 keV, recombination with electrons is negligibly unlikely. However, charge exchange (CX) reactions between ions and neutral atoms have considerably higher cross-sections, and remain significant at these energies.

Per its significance in fusion-relevant plasmas, the CX fast ion losses are an important topic in fusion research. For example in today's large tokamaks, rough estimates place the increase in fast ion losses due to CX at 1-10%. Detailed understanding of the effect of CX on fast ion confinement is needed for the successful design and operation of current and future fusion devices.

Complement to experiment in various devices, computational modelling is an integral part of the effort to understand CX in fusion. The Monte Carlo fast ion simulation code ASCOT, with its atomic reactions module, is used to study the effect of CX on fast ion confinement in fusion devices. This particular work focuses on spherical tokamaks. Because of their high wall area to volume -ratio, these compact devices are especially prone to CX effects, since the CX reaction counter parts, the neutrals, are predominantly recycled from the device wall. First simulations suggest that CX losses of fast ions reduce net heating power, torque and current drive, and can increase power loads to the plasma-facing wall.


Speech: March 7th, 12:15, Hall 5

EN ROUTE TO FUSION POWER PLANT: CONFINING ENERGETIC IONS IN WENDELSTEIN 7-X STELLARATOR

T. Kurki-Suonio1, S. Äkäslompolo and the Wendelstein 7-X Team2, J. Kontula1

taina.kurki-suonio@aalto.fi
1 Aalto University
2Max Planck institute, Greifsald


While the tokamak concept represents the most mature line of fusion energy research, and the first actual fusion reactor, ITER, is being built based on it, an older concept, the stellarator, has recently appeared as a very competitive concept. The reason for the enormous headstart of tokamaks in the 60's to 90's lies in their symmetry properties which, according to Noether's theorem, lead to conservation laws and, thus, to better confinement of the fuel plasma. However, the symmetry is a result of transformer action and, thus, tokamaks are inherently pulsed devices.

While tokamaks fight to find a way for continuous operation, a stellarator is inherently a steady-state device and, thus, directly power plant compatible. The re-appearance of stellarators was made possible only with the advent of super-computers that allowed a numerical optimization of the true 3-dimensional equilibrium of a stellarator. The first such optimized stellarator, Wendelstein 7-X (W7-X between friends), started operating at Max Planck institute in Greifswald in 2015, and so far it has not only met but exceeded many of its goals.

One of the primary goals of W7-X is to demonstrate confinement of not just the bulk plasmas but even of the energetic (fast) ions necessarily present in a burning plasma due to fusion reactions. Since W7-X is not a reactor, its way of demonstrating this is to use the fast ions produced by external heating. The energy of these ions is tuned with respect to the machine size so that they represent 3.5 MeV fusion alpha particles in a foreseen Helias-type reactor. Fast ions are very collisionless and, thus, highly susceptible to any bumps and twists in the magnetic field - and a stellarator provides plenty of those. Escaping fast ions not only provide a loss of heating but can even jeoparize the integrity of the vessel wall.

Therefore extreme care was taken when introducing the neutral beams that provide the fast ions heating the W7-X plasma. In the initial phase, only electrons were heated with electromagnetic fields, and only in the late summer 2018 the beams were started. This campaign was preceded by years of predictive beam simulations using the ASCOT code, developed and maintained at Aalto University, to identify possible hot spots on unprotected wall components. The simulations served to develop the most benign plasma configuration from the fast ion confinement point of view. Not only was the commissioning of the beams successful, but the analysis of the first experimental results related to fast ion losses shows how reliable careful simulations can be.

In this contribution, we shall briefly introduce W7-X, the neutral beams, the ASCOT code, and show how well the fast ion power loads can be reproduced (or, in this case: predicted) by simulations by comparing the infrared camera images of the W7-X wall to corresponding synthetic diagnostics in ASCOT.


High Energy and Nuclear Physics II

Location: Hall 12
Time: March 7th, 11:00 - 12:30

Speech: March 7th, 11:00, Hall 12

INCLUSIVE AND DIFFRACTIVE DIJETS IN ULTRA-PERIPHERAL COLLISIONS AT THE LHC WITH PYTHIA 8

I. Helenius1, C.O. Rasmussen2

ilkka.m.helenius@jyu.fi
1 University of Jyväskylä
2 Lund University


Pythia 8 [1] is a general-purpose Monte-Carlo event generator which main focus has been on simulations of high-energy proton-proton collisions at the LHC. Recently it has been extended also for other collision systems, such as heavy-ion collisions and processes involving intermediate photons. In this talk we will focus on the new photoproduction framework which is validated against data from electron-proton collisions at HERA [2]. As a new application, we will show predictions for jet production in ultra-peripheral heavy-ion collisions at the LHC and discuss about their sensitivity to nuclear PDFs. In these collisions the beam particles pass each other with a large impact parameter without a strong interaction but the quasi-real photons emitted by one beam interacts with the another. First preliminary analysis for such a measurement in heavy-ion collisions at the LHC has been recently published by the ATLAS collaboration.

In addition, we will introduce our new hard-diffraction model for photoproduction [3]. The underlying idea is that the particles formed due to additional partonic interactions between a resolved photon and a proton in the same event may shroud the diffractive signature of the event by filling up the rapidity gap and thus supress the rate of observed diffractive events. The same idea has been previously used in Pythia 8 to explain the observed factorization breaking for diffractive dijets in proton-proton collisions at the Tevatron and the LHC. The new model provides now a natural explanation for the suppression of diffractive dijets in photoproduction regime observed by HERA experiments. We will present predictions also for diffractive dijets in ultra-peripheral collisions at the LHC and show that such measurements would be ideal to fill the gap between the mild factorization-breaking effects seen at HERA and the substantial, order-of-magnitude, suppression seen in usual proton-proton collisions. These kind of measurements at the LHC would provide strong constraints for the factorization-breaking models and provide valuable information of the underlying physics.


[1] T. Sjöstrand et al., An Introduction to PYTHIA 8.2,Comput. Phys. Commun. 191 (2015)159.
[2] I. Helenius, Photon-photon and photon-hadron processes in Pythia 8, arXiv:1708.09759 [hep-ph].
[3] I. Helenius and C. O. Rasmussen, Hard diffraction in photoproduction, arXiv:1901.05261 [hep-ph].


Speech: March 7th, 11:15, Hall 12

JET FRAGMENTATION TRANSVERSE MOMENTUM DISTRIBUTIONS IN PROTON-LEAD COLLISIONS MEASURED BY THE ALICE EXPERIMENT AT LHC

T. Snellman1,2, D.J. Kim, B.K. Kim, S. Räsänen, J. Rak

tomas.snellman@cern.ch
1 University of Jyväskylä
2 Helsinki Institute of Physics


The main goal of ultra relativistic heavy ion collisions and the ALICE experiment [1] at the LHC is the study of nuclear matter under extreme conditions and its deconfined phase known as Quark Gluon Plasma (QGP), where quarks and gluons are no longer bound to hadrons.

Partons with high transverse momentum traversing through the QGP medium are a sensitive probe of QGP. The medium induces energy loss on the partons and thus alters how they fragment into jets [2]. We study the fragmentation in proton-proton (p-p) and proton-lead (p-Pb) collisions. It is expected that no QGP is created in p-Pb collisions, but interactions with cold nuclear matter can still cause some modification. Vacuum conditions in p-p collisions provide a reference for p-Pb and Pb-Pb collisions.

Jet production in Quantum Chromodynamics (QCD) can be thought of as two separate stages [3]. After being produced in the hard scattering, partons reduce their high virtuality through emitting gluons. Since the transverse momentum scale ($Q^2$) is large during the showering, perturbative QCD calculations can be applied. When $Q^2$ becomes of the order of $\Lambda_{\mathrm{QCD}}$, partons hadronize into final-state particles through a non-perturbative process.

In my presentation I will discuss the jet fragmentation transverse momentum ($j_T$) distributions in $\sqrt{s_{NN}}=5.02\,\mathrm{TeV}$ p-Pb collisions at the LHC as measured by the ALICE experiment using full jet reconstruction. In a dihadron correlation analysis it has been established that $j_T$ distributions can be divided into two components; a narrow component that represents the hadronisation stage of jet formation and a wide component representing the showering process. [4] This study shows that the same separation of components is seen when using jet reconstruction.

[1] K. Aamodt et al. [ALICE Collaboration], JINST 3 (2008) S08002. doi:10.1088/1748-
0221/3/08/S08002

[2] M.Gyulassy, P. Levai and I.Vitev, Nucl. Phys. B {\bf 571} (2000) 197, doi:10.1016/S0550-3213(99)00713-0, [hep-ph/9907461].

[3] A. Buckley et al., Phys. Rept. 504, 145 (2011), 1101.2599.

[4] S.~Acharya et al. [ALICE Collaboration], [arXiv:1811.09742 [nucl-ex]].


Speech: March 7th, 11:30, Hall 12

FLAVOUR-DEPENDENT JET ENERGY CORRECTIONS AT D0 AND TOP QUARK MASS

T. Mäkelä1,2,3, M. Voutilainen1,3, H. Siikonen1,3

toni.makela@cern.ch
1 Helsinki Institute of Physics
2 Aalto University
3 University of Helsinki


The top quark is the heaviest elementary particle in the current standard model of particle physics. It is even massive enough to cause significant corrections to the behaviour of the Higgs potential. If no new physics exists below the Planck scale and if the interpretation of the relation between contemporary measurements and theory is right, these corrections imply that the electroweak vacuum -- and consequently the Universe as we know it -- lies in a metastable state.

The most accurate single measurements of the top quark mass $m_t$ performed by the CMS and ATLAS collaborations at the LHC differ from a similarly precise measurement of the D0 experiment at the Tevatron by about 2.5 GeV. This amounts to almost 3 standard deviations, so that the D0 result lifts the world average top mass value to $m_t \approx 173.3$ GeV, towards the direction of vacuum instability.

Since the top decays most commonly (99 percent of the time) into a bottom quark $b$, the measured values of $m_t$ depend on the reconstruction of $b$-jets, sprays of particles originating from a bottom quark. The very high accuracy of the D0 measurement relies on a unique and very precise calibration of the flavour-dependent jet energy scale corrections. In particular, the D0 correction factor for $b$-jets is notably different from those used by other experiments. As the moratorium for the D0 collaboration's internal information necessary for understanding their calibration techniques expired in 2018, we have recently been able to perform a first-in-the-world accurate reproduction of the D0 $b$-jet energy scale corrections using standalone Monte Carlo simulations. We investigate the sensitivity of the $b$-jet correction to various assumptions, which may then have direct impact on the measured top quark mass.



Figure 1
Figure 1: Comparison of the flavour-dependent jet energy correction factors as reported and used by the D0 collaboration and our standalone Monte Carlo simulations, reproduced following D0 papers and analysis notes.

Speech: March 7th, 11:45, Hall 12

SEARCHING FOR NEW ISOTOPES USING MARA AND JUROGAM 3

H. Tann1

hotann@student.jyu.fi
1 University of Jyväskylä


MARA is a vacuum-mode recoil separator, commissioned in 2016, primarily used for proton drip-line studies around N$\approx$Z nuclei based at the University of Jyväskylä. In this work MARA was used in conjunction with a versatile focal plane detection system and the JYtube charged particle detector at the target position. A search was conducted for new isotopes $^{133}$Gd and $^{132}$Eu by utilising their expected beta-delayed proton emission properties. The analysis of these results is in progress and as a side product many new isomers were identified. To develop upon these results, the next step will be to add in-beam measurements to our studies at the target position, using JUROGAM 3- a movable detector array consisting of 24 clover detectors and 15 phase one detectors. JUROGAM 3 can be positioned at the target positions of both the RITU gas-filled recoil separator and MARA. It is proposed to use the array in conjunction with MARA to measure excited states in the highly deformed proton emitter $^{131}$Eu. In addition it is proposed to use newly identified isomers from the earlier experiment for isomer tagging to identify states feeding these isomers.



Speech: March 7th, 12:00, Hall 12

PRECISION MASS MEASUREMENTS ON NEUTRON-RICH RARE-EARTH ISOTOPES AT JYFLTRAP

M. Vilén1, J.M. Kelly2, A. Kankainen1, M. Brodeur2, A. Aprahamian2, L. Canete1, T. Eronen1, A. Jokinen1, I.D. Moore1, M.R. Mumpower2,3, D.A. Nesterenko1, H. Penttilä1, I. Pohjalainen1, M. Reponen1, S. Rinta-Antila1, A. de Roubin1, R. Surman2, J. Äystö1

markus.k.vilen@student.jyu.fi
1 University of Jyväskylä, P.O. Box 35, FI-40014 University of Jyväskylä, Finland
2 University of Notre Dame, Notre Dame, Indiana 46556, USA
3 Theory Division, Los Alamos National Lab, Los Alamos, New Mexico 87544, USA


The astrophysical rapid neutron capture process ($r$-process) is responsible for the production of around half of elements heavier than iron. The $r$-process path lies far away from the valley of stability on the neutron-rich side of the nuclear chart making many $r$-process nuclei challenging to reach via experiments. Several neutron-rich rare-earth isotopes relevant for the formation of the rare-earth abundance peak in the $r$-process have already been measured at the JYFLTRAP double Penning trap mass spectrometer at IGISOL, see [1]. In this contribution, precision mass measurements will be presented that extend results of the previous measurement campaign towards even more neutron-rich nuclei. Phase-imaging ion-cyclotron-resonance (PI-ICR) technique was utilized to study nuclei with low-lying isomeric states, providing sufficient resolving power to measure isomeric and ground states separately. Additionally, the time-of-flight ion-cyclotron-resonance (TOF-ICR) technique was used to study cases where only one state was assumed to be present. Altogether, eight previously experimentally unknown atomic masses were measured and the precision of literature values were improved for several isotopes [2].

References:
[1]
M. Vilen, J.M. Kelly, A. Kankainen, M. Brodeur, et al., Precision mass measurements on neutron-rich rare-earth isotopes at JYFLTRAP: Reduced neutron pairing and implications
for r -process calculations. Physical Review Letters, 120(26), jun 2018.

[2]
M. Vilen, J.M. Kelly, A. Kankainen, M. Brodeur, et al. To be submitted.


Speech: March 7th, 12:15, Hall 12

ON-LINE PRODUCTION OF RADIOACTIVE ACTINIDE ION BEAMS BY LIGHT-ION FUSION-EVAPORATION AT IGISOL

I. Pohjalainen1, O. Beliuskina1, L. Canete1, C. Delafosse1, Ch.E. Düllmann2,3,4, T. Eronen1, S. Geldhof1, R.P. de Groote1, R. Haas2,3, M. Hukkanen1, A. Jokinen1, A. Kankainen1, I.D. Moore1, D.A. Nesterenko1, H. Penttilä1, D. Renisch2, M. Reponen1, S. Rinta-Antila1, A. de Roubin1, M. Vilen1, V. Virtanen1, A. Zadvornaya1

ilkka.pohjalainen@jyu.fi
1 University of Jyvaskyla, Department of Physics, P.O. Box 35, 40014 Jyväskylä Finland
2 Johannes Gutenberg-Universität Mainz, Institut für Kernchemie, Fritz Strassmann Weg 2, 55128 Mainz, Germany
3 Helmholtz-Institut Mainz, Staudinger Weg 18, 55128 Mainz, Germany
4 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstraße 1, 64291 Darmstadt, Germany


The production of actinide ion beams has become a focus of recent efforts at the IGISOL facility of the Accelerator Laboratory, University of Jyväskylä, aimed at the measurement of nuclear properties of heavy elements. In addition to the double Penning trap mass spectrometer of JYFLTRAP for high precision measurement of atomic masses [1], the high-resolution collinear laser spectroscopy facility at IGISOL can be used to obtain nuclear properties of charge radii, spins, electric dipole and quadrupole moments [2].

Currently, the isotope in the actinide region that has received significant attention in recent years is $^{229}$Th. Due to its exceptionally low-lying isomeric state of only 7.8(5) eV [3], $^{229}$Th is the basis of many envisioned novel applications such as a highly precise nuclear-based frequency reference [4, 5]. The recent detection [6] and measurement of the hyperfine structure of $^{229m}$Th [7] has prompted new questions of the properties of $^{229m}$Th such as the life time of the singly-charged ion.

To study the properties of the isomer at IGISOL, the production of $^{229}$Th, together with other actinide isotopes, has been investigated with proton-induced fusion-evaporation reactions during several days of on-line beam time. Using 50 MeV and 60 MeV proton beams impinging on $^{232}$Th targets, several radioactive actinide isotopes including $^{226, 227}$Pa and $^{226}$Th were produced with considerable yields. One of the key requisites for successful beam production was the target durability, which was observed to be poor for thin metallic~$^{232}$Th targets. Therefore tests were performed on several new $^{232}$Th targets that were produced by a novel drop-on-demand inkject printing method [8] provided by the Nuclear Chemistry Institute of Johannes Gutenberg-Universität Mainz. The possibility of using this novel target manufacturing method to produce exotic targets from long-lived actinide isotopes in order to extend the ion beam production to neutron-deficient actinide isotopes is extremely attractive for the future study of heavy elements at IGISOL.

[1] T. Eronen et al., Eur. Phys. J. A 48, 46 (2012)
[2] A. Voss et al., Phys. Rev. A 95, 032506 (2017)
[3] B. R. Beck et al., Proc. of 12th Int. Conference on Nucl. React. Mech., LLNL-PROC, 415170 (2009)
[4] E. Peik and C. Tamm, Europhysics Letters (EPL), 61(2), 181 (2003)
[5] C. J. Campbell, et al., Phys. Rev. Lett., 108 (2012)
[6] L. von der Wense et al., Nature, 533, 47 (2016)
[7] J. Thielking et al. Nature 556 321 (2018)
[8] R. Haas et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 874, 43 (2017)


Nanoscale Physics

Location: Hall 13
Time: March 7th, 11:00 - 12:30

Speech: March 7th, 11:00, Hall 13

DNA ORIGAMI NANOSTRUCTURES: ON THEIR STABILITY AND BIOMEDICAL APPLICATIONS

V. Linko1

veikko.linko@aalto.fi
1 Biohybrid Materials, Department of Bioproducts and Biosystems, Aalto University, Finland


DNA origami structures [1] can serve as nanorulers [2], dynamic devices [3] and templates for e.g. nanoparticles [4] and nanoplasmonic/-photonic components [5]. Recently, our group has studied/improved the following bionanotechnological properties/functions of DNA origami; drug loading [6], protein coating [7,8], bio- and immunocompatibility [7,9], stability [9,10], nuclease resistance [7,11], cellular delivery [7,8] and controlled encapsulation and display of molecular cargo [12].

Here I focus on the endonuclease-DNA nanostructure interaction and DNA origami stability, both of them are essential for biomedical applications. We have monitored digestion patterns and pathways of structurally distinct DNA origami, and we also assess the effect of the solid-liquid interface on DNA origami digestion by comparison with experiments in bulk solution. We show that DNA origami digestion is strongly dependent on its superstructure and flexibility and also on the local topology of the individual structure [11].

References
[1] S. Nummelin, J. Kommeri, M. A. Kostiainen, and V. Linko, Adv. Mater. 30, 1703721 (2018).
[2] E. Graugnard, W. L. Hughes, R. Jungmann, M. A. Kostiainen, and V. Linko, MRS Bull. 42, 951 (2017).
[3] H. Ijäs, S. Nummelin, B. Shen, M. A. Kostiainen, and V. Linko, Int. J. Mol. Sci. 19, 2114 (2018).
[4] S. Julin, S. Nummelin, M. A. Kostiainen, and V. Linko, J. Nanopart. Res. 20, 119 (2018).
[5] B. Shen, M. A. Kostiainen, and V. Linko, Langmuir 34, 14911 (2018).
[6] F. Kollmann, S. Ramakrishnan, B. Shen, G. Grundmeier, M. A. Kostiainen, V. Linko, and A. Keller, ACS Omega 3, 9441 (2018).
[7] H. Auvinen, H. Zhang, Nonappa, A. Kopilow, E. H. Niemelä, S. Nummelin, A. Correia, H. A. Santos, V. Linko, and M. A. Kostiainen, Adv. Healthcare Mater. 6, 1700692 (2017).
[8] V. Linko, J. Mikkilä, and M. A. Kostiainen, Methods Mol. Biol. 1776, 267 (2018).
[9] C. Kielar, Y. Xin, B. Shen, M. A. Kostiainen, G. Grundmeier, V. Linko, and A. Keller, Angew. Chem. Int. Ed. 57, 9470 (2018).
[10] S. Ramakrishnan, H. Ijäs, V. Linko, and A. Keller, Comput. Struct. Biotechnol. J. 16, 342 (2018).
[11] S. Ramakrishnan, B. Shen, M. A. Kostiainen, G. Grundmeier, A. Keller, and V. Linko, under review (2019).
[12] H. Ijäs, I. Hakaste, B. Shen, M. A. Kostiainen, and V. Linko, under review (2019).


Speech: March 7th, 11:15, Hall 13

DEEP CORE PHOTOIONIZATION OF IODINE IN CH3I AND CF3I MOLECULES: HOW DEEP DOWN DOES CHEMICAL SHIFT REACH?

N. Boudjemia1, K. Jänkälä1, T. Geo2, K. Nagaya3, K. Tamasaku4, M. Huttula1, M. N. Piancastelli, M. Simon6, O. Masaki7

nacer.boudjemia@oulu.fi
1 University of Oulu
2 RIKEN SPring-8 center/ University of Hyogo
3 RIKEN SPring-8 center/ Kyoto University
4 RIKEN SPring-8
5 RIKEN SPring-8 center/ Sorbonne Université/ Uppsala University
6 Sorbonne Université/ RIKEN SPring-8 center
7 RIKEN SPring-8 center


Deep core photoionization of iodine in CH$_3$I and CF$_3$I molecules: how deep down does chemical shift reach?

The availability of hard X-ray synchrotron radiation has opened up new avenues in gas-phase molecular photoelectron spectroscopy. The possibility to create a very deep core hole via single photon excitation or ionization enables studies of multitude of interesting phenomena like ultrafast electronic and molecular relaxation dynamics.

The present work aims to answer to what extend the created deep core hole feels its molecular environment. Is there still an observable chemical shift in the binding energy of iodine 1s and 2s levels in two different molecules, CH$_3$I and CF$_3$I? Indeed, a chemical shift is observed, indicating that deep core levels are not purely atomic in nature, thus widespread concepts such as electronegativity and charge distribution inside a molecule extend down to very deep levels.

The subsequent Auger spectra are also studied. It is observed that CH$_3$I and CF$_3$I have virtually identical Auger spectra and the overall spectral features and their relative intensities are close to the recently reported corresponding Auger spectrum in Xe. High-level theoretical simulations including QED effects have been performed for neutral iodine atom and Xe-like iodine negative ion, and it is found that the experimental spectrum lies in-between these two extremes. Using Z+1 and Z+2 approximations, a charge distribution analysis was carried out, and it was observed that the slightly positive iodine in neutral molecule gains more negative charge (withdraws charge from the rest of the molecules) as a function of the excess positive charge in the core.

This study reports the first time a chemical shift in a very deep core level. The experimental observations are supported by a combination of molecular and atomic calculations taking into account different relativistic corrections.



Figure 1
Figure 1

Speech: March 7th, 11:30, Hall 13

NC-AFM RECOGNITION OF MOLECULAR ADSORPTION AND ON-SURFACE REACTION OF SI AND TRIPLE BOND CONTAINING MOLECULE

Shigeki Kawai1, Ondřej Krejčí2, Adam S.Foster2,3,4, Rémy Pawlak5, Feng Xu6, Lifen Peng6, Akihiro Orita6, Ernest Meyer5

ondrej.krejci@aalto.fi
1 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan
2 Department of Applied Physics, Aalto University School of Science, P.O. Box 11100, FI-00076 Aalto, Finland
3 WPI Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
4 Graduate School Materials Science in Mainz, Staudinger Weg 9, 55128 Mainz, Germany
5 Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
6 Department of Applied Chemistry and Biotechnology, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005, Japan


In this work we are focusing on identification of reactants and products of Cu(111) mediated on-surface reaction. We used Non-Contact Atomic Force Microscopy (NC-AFM) experiments and simulations with microscope tip modified by CO molecule [1] for this purpose:
The reactant is adorbed trimethelsilyl molecule. Since non-planar parts of this molecule are flexible, an enhanced model [2] of frequently used Probe Particle code [3,4] and extensive density functional theory (DFT) calculations were employed to recognise the atomistic model of the reactant, and to understand the experimentally measured contrast. The resolved model shows an unforeseen binding of small, partially aromatic molecule to the copper surface.
The examined reaction was one-shot desilylative homocoupling performed for the first time on Cu(111) surface and which resulted in diacetylene linked anthracene oligomers.
The NC-AFM force measurement combined with theoretical calculations revealed the chemical nature at the centre of products anthracene unit [5].

[1] L. Gross, F. Mohn, N. Moll, P. Liljeroth, and G. Meyer, The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy, Sci. 325, pp. 1110-1114, $\bf{2009}$.

[2] M. Di Giovannantonio, J. I. Urgel, U. Beser, A. V. Yakutovich, J. Wilhelm, C. A. Pignedoli, P. Ruffieux, A. Narita, K. Mllen, and R. Fasel, J. Am. Chem. Soc. 140, 3532, $\bf{2018}$.

[3] P. Hapala, G. Kichin, C. Wagner, F. S. Tautz, R. Temirov, and P. Jelínek, Mechanism of high-resolution STM/AFM imaging with functionalized tips, Phys. Rev. B 90, 085421, $\bf{2014}$.

[4] P. Hapala, R. Temirov, F. S. Tautz, and P. Jelínek, Origin of High-Resolution IETS-STM Images of Organic Molecules with Functionalized Tips, Phys. Rev. Lett. 113, 226101, $\bf{2014}$.

[5] S. Kawai, O. Krejčí, A. S. Foster, R. Pawlak, F. Xu, L. Peng, A. Orita, and E. Meyer, Diacetylene Linked Anthracene Oligomers Synthesized by One-Shot Homocoupling of Trimethylsilyl on Cu(111)}, ACS Nano 12, 8791−8797, $\bf{2018}$.



Figure 1
Figure 1: On-surface desilylative homocoupling reaction and CO-tip employed NC-AFM image of its product. [5]

Speech: March 7th, 11:45, Hall 13

DETECTING STABLE SURFACE ADSORBATES WITH BAYESIAN OPTIMIZATION AND ATOMIC FORCE MICROSCOPY

J. Järvi1, B. Alldritt1, M. Todorovic1, P. Liljeroth1, P. Rinke1

jari.jarvi@aalto.fi
1 Aalto University


Atomic force microscopy (AFM) has considerable resolution for imaging and characterization of stable surface adsorbates. However, interpreting images of complex 3-dimensional adsorbates can be difficult. Simulations are often used to clarify experiments, but employing accurate quantum mechanical (QM) methods, such as density-functional theory (DFT), to calculate the structure of complex adsorbates is prohibitively expensive. This requires calculating the potential energy surface (PES) of the system, i.e. the energy of all possible atomic configurations. Instead, traditional structure search methods have relied on chemical intuition, focusing on the likely minimum-energy structures and thus exploring only a small fraction of the PES. With complex materials, this human intuition is difficult to apply and can lead to biased and incorrect results.

We combine AFM experiments, QM simulations and artificial intelligence (AI) to resolve stable surface adsorbate structures. We apply the Bayesian Optimization Structure Search (BOSS) method [1] with DFT to study the adsorption of camphor (C$_{10}$H$_{16}$O) on a Cu(111) surface. BOSS is a brand new AI tool, which accelerates the structure search via an intelligent and unbiased sampling of the PES, minimizing the number of expensive energy computations with DFT. As a result, BOSS acquires the complete PES for a clear identification of the most stable minimum-energy structure.

In this study, we first analyze camphor conformers with a 3-dimensional search of methyl group rotations (see Figs. 1a and 1b). We then employ the identified minimum-energy structure of the molecule to investigate its adsorption on a Cu(111) surface as a function of molecular orientation and translations (see Fig. 1c). By combining QM simulations and AI, we have developed unique fingerprints to determine stable adsorbate structures in AFM images.

[1] M. Todorovic, M. Gutmann, J. Corander, and P. Rinke, submitted to NPJ Comput. Mater., arXiv:1708.09274 (2017).



Figure 1
Figure 1: a) Camphor conformer search with 3 methyl group rotations, and b) the resulting energy landscape calculated with BOSS, showing the global energy minimum. c) Orientational and translational search for a stable adsorption structure of camphor on Cu(111).

Speech: March 7th, 12:00, Hall 13

INTERFERENCE PHENOMENA OF FLEXURAL MODES IN GRAPHENE MECHANICAL RESONATORS

J. Manninen1, A. Laitinen1, F. Massel2, M. T. Haque1, A. Isacsson3, P. Hakonen1

juuso.manninen@aalto.fi
1 Department of Applied Physics, Aalto University, Finland
2 Department of Physics and Nanoscience Center, University of Jyväskylä, Finland
3 Department of Physics, Chalmers University of Technology, Sweden.


We are able to identify several experimentally observed mechanical resonances of a system with two micromechanical gold beams coupled to a Corbino-shaped monolayer graphene disk. The resonances are detected using a signal that is frequency mixed by the graphene Corbino disk.

The mixing current is used to identify different types of mechanical modes, i.e. bare graphene disk resonances and combined gold-graphene modes, by considering several of its properties. The response function given by the mixing current in frequency space is related to an additional phase difference in the mechanical amplitude of graphene with respect to drive. This phase difference can originate from the gold resonators interacting with the graphene disk, analogous to a simple case with coupled mass and spring resonators. The gate voltage dependence of the resonant frequencies is used to support these identifications.

We observe that the phase of the mixing current behaves similarly for adjacent modes within a certain frequency domain after which it experiences a sudden $\pi$ phase flip that remains consistent for a set of modes before abruptly flipping back to the original phase. Analytical calculations of a Corbino disk experiencing pumping from the edges can be used to explain this phenomenon. In this model, the graphene disk experiences $\pi$ phase flips with respect to the drive at the resonant frequencies of the disk. Thus, the phase flips in the mixing current allow us to identify the modes of the bare graphene disk.

The identification of the modes is supported by finite element simulations of the system performed using COMSOL Multiphysics. The frequencies of the simulated modes match the experimental ones quite well and the mode shapes of these resonances coincide with the predictions made on the basis of the shape of the mixing current. Analytical calculations of a driven Corbino system together with the COMSOL simulations are used to study the strain and the wave velocity of transverse waves in graphene.

Altogether, by considering the phase and shape of the measured mixing currents, the gate voltage dependence of the modes, and the simulated mode shapes as well as analytical calculations, we are able to set mixed gold-graphene resonances apart from pure graphene oscillations. The results of this work are presently being employed to analyze de Haas - van Alphen effect in monolayer graphene.


Speech: March 7th, 12:15, Hall 13

RECENT ADVANCE IN NANOPARTICLE BEAM SOURCE: MAGNETRON SPUTTERING INERT GAS CONDENSATION & MATRIX ASSEMBLY CLUSTER SOURCE

J. Zhao1, K. Nordlund1, F. Djurabekova1, J. Vernieres2,3, P. Grammatikopoulos2, R. E. Palmer3, M. Sowwan2

junlei.zhao@helsinki.fi
1 Department of Physics and Helsinki Institute of Physics, University of Helsinki
2 Nanoparticles by Design Unit, Okinawa Institute of Science and Technology Graduate University, Okinawa, Japan
3 College of Engineering, Swansea University, Swansea, United Kingdom


There is an increasing interest in the generation of well-defined nanoparticles (NPs) not only because of their size-related particular properties, but also because they are promising building blocks for more complex materials in nanotechnology. Compared with colloidal synthesis of nanoparticles, the physical approach is green - it involves no solvents and no effluents; particles can be size-selected as desired; and challenging combinations of metals (nanoalloys) can readily be produced. Here, we will shortly introduce two physical synthesis technologies: magnetron sputtering inert gas condensation and matrix assembly cluster source. We will give an exemplary study of the formation mechanisms of iron and iron-gold nanoparticles grown by magnetron sputtering inert gas condensation. Our results emphasize the decisive kinetics effects that give rise specifically to cubic morphologies. Our experimental results, as well as computer simulations carried out by two different methods, indicate that the cubic shape of Fe [1, 2] and FeAu [3] NPs is explained by basic differences in the kinetic growth modes of {100} and {110} surfaces, rather than surface formation energetics. Both our experimental and theoretical investigations show that the final shape is defined by the combination of the condensation temperature and the rate of atomic deposition onto the growing nanocluster. Adding Au into the system, we find that the cluster corners are decorated with Au atoms, with different possible outcomes depending on temperature. In the second part, we study the mechanisms of growth of Ag nanoclusters in a solid Ar matrix and the emission of these nanoclusters from the matrix assembly cluster source[4]. Our computer simulations show that the cluster growth mechanism can be described as “thermal spike-enhanced clustering” in multiple sequential ion impact events. We further describe the mechanism of emission of the metal nanocluster that, at first, is formed in the cryogenic matrix due to multiple ion impacts, and then is emitted as a result of the simultaneous effects of interface boiling and spring force.

References:
[1] J. Zhao, E. Baibuz, J. Vernieres, P. Grammatikopoulos, V. Jansson, M. Nagel, S. Steinhauer, M. Sowwan, A. Kuronen, K. Nordlund, and F. Djurabekova, “Formation Mechanism of Fe Nanocubes by Magnetron Sputtering Inert Gas Condensation”, ACS Nano 10, 2684 (2016).
[2] J. Vernieres, S. Steinhauer, J. Zhao, A. Chapelle, P. Menini, N. Dufour, R. E. Diaz, K. Nordlund, F. Djurabekova, P. Grammatikopoulos, and M. Sowwan, “Gas Phase Synthesis of Multifunctional Fe-based Nanocubes”, Adv. Funct. Mater. 27, 1605328 (2017).
[3] J. Vernieres, S. Steinhauer, J. Zhao, P. Grammatikopoulos, R. Ferrando, K. Nordlund, F. Djurabekova, and M. Sowwan, “Growth of Phase-separated Fe-Au Nanocubes with Complex Morphologies”, submitted for publication.
[4] J. Zhao, L. Cao, R. E. Palmer, K. Nordlund, and F. Djurabekova, “Formation and Emission Mechanisms of Ag Nanoclusters in the Ar Matrix Assembly Cluster Source”, Phys. Rev. Mater. 1, 066002 (2017).


New Methods for Experimental Research

Location: Hall 10
Time: March 5th, 14:30 - 16:00

Speech: March 5th, 14:30, Hall 10

LABORATORY-SCALE X-RAY ABSORPTION SPECTROMETER WITH VERSATILE DETECTION MODES

A.-P. Honkanen1, A.-J. Kallio1, S. Ollikkala1, M. Blomberg1, S. Huotari1

ari-pekka.honkanen@helsinki.fi
1 University of Helsinki


X-ray absorption spectroscopy (XAS) is a widely used tool for materials research. It is an element-selective probe of local atomic structure around a selected element. In XAS the probability for x-ray absorption in the vicinity of a binding energy of an inner core shell electron is measured, and the method probes the density of states above the Fermi level in the presence of a core hole. The spectra yield information on the oxidation state, coordination number and geometry, as well as the type of nearest atomic neighbours. The method can be used in ordered and disordered systems; solids, liquids and gases alike. Typically experiments are done at synchrotron light sources.

In this contribution, we describe a novel experimental home laboratory-based experimental apparatus that we have designed and built for XAS and installed at the University of Helsinki [1]. It is based on spherically-bent crystal optics. The designed energy range of the instrument is 4-20 keV, which covers well the K-shell absorption edges of 3d transition metals and L-shell edges of lanthanides and 5d transition metals. In addition to the direct absorption mode, our instrument can operate also in fluorescence mode which allows the study of non-transparent samples such as thin films on thick substrate or commercial Li-ion battery cells in operando. Our instrument is also equipped with a position-sensitive detector, which allows us to do monochromatic absorption imaging and computed tomography that can be utilized to obtain element-sensitive contrast. A showcase of real-life applications is given to demonstrate its potential in the study of battery materials and electrochemistry [2], nuclear waste and other radioactive materials [3], long-term in situ catalysis experiments [4], soil chemistry, and life sciences.

[1] Ari-Pekka Honkanen, Sami Ollikkala, Taru Ahopelto, Antti-Jussi Kallio, Merja Blomberg and Simo Huotari, Johann-type laboratory-scale X-ray absorption spectrometer with versatile detection modes, (2019), arXiv:1812.01075

[2] Wenhai Wang, Long Kuai, Wei Cao, Marko Huttula, Sami Ollikkala, Taru Ahopelto, Ari‐Pekka Honkanen, Simo Huotari, Mengkang Yu and Baoyou Geng, Mass‐Production of Mesoporous MnCo$_2$O$_4$ Spinels with Manganese (IV)‐and Cobalt (II)‐Rich Surfaces for Superior Bifunctional Oxygen Electrocatalysis, Angewandte Chemie (2018) 129 pp. 15173-15177

[3] Rene Bes, Taru Ahopelto, Ari-Pekka Honkanen, Simo Huotari, Gregory Leinders, Janne Pakarinen, Kristina Kvashnina, Laboratory-scale X-ray absorption spectroscopy approach for actinide research: Experiment at the uranium L3-edge, Journal of Nuclear Materials (2018) 507 pp. 50-53

[4] José G Moya‐Cancino, Ari‐Pekka Honkanen, Ad MJ van der Eerden, Herrick Schaink, Lieven Folkertsma, Mahnaz Ghiasi, Alessandro Longo, Frank MF de Groot, Florian Meirer, Simo Huotari and Bert M Weckhuysen, In‐situ X‐Ray Absorption Near Edge Structure Spectroscopy of a Solid Catalyst using a Laboratory‐Based Set‐up, ChemCatChem (2018), Early view, doi:10.1002/cctc.201801822


Speech: March 5th, 14:45, Hall 10

ULTRAFAST MULTIDIMENSIONAL LAPLACE NMR FOR A RAPID AND SENSITIVE CHEMICAL ANALYSIS

O. Mankinen1, V.V. Zhivonitko1, S. Ahola1, V.-V. Telkki1

otto.mankinen@oulu.fi
1 NMR Research Unit, Faculty of Science, University of Oulu, Oulu, Finland


NMR relaxation and diffusion measurements provide versatile information about dynamics and structures of e.g. porous materials, and reveal interactions of nuclei within their microscopic environment. Since relaxation and diffusion data comprise exponentially decaying components, the processing requires a Laplace inversion in order to extract the diffusion coefficient and relaxation time distributions. Thus, these methods are referred to as Laplace NMR (LNMR). [1]

Multidimensional approach increases the chemical resolution of an NMR experiment. Multidimensional and even some 1D experiments are really time consuming, since the experiment needs to be repeated several times with varying evolution delay or gradient strength to gain proper multidimensional data. This restricts the applicability of multidimensional LNMR methods and is considered general problem of multidimensional NMR. Also in many cases it prevents the use of hyperpolarized substances for signal amplification. Problem of a long experimental time can be tackled by introducing spatial encoding of two-dimensional data, as was originally done in ultrafast NMR spectroscopy [2,3] and later in ultrafast LNMR [4-6]. The price to pay is reduced sensitivity. However, the single-scan approach enables the use of hyperpolarization methods (e.g. PHIP, DNP [5] and SEOP [6]), which provide much higher sensitivity boost than the loss due to spatial encoding.

In this presentation we describe the concept of multidimensional ultrafast LNMR and principles of various ultrafast multidimensional Laplace NMR methods developed by us. We also demonstrate that these methods can utilize modern hyperpolarization methods to increase sensitivity by many orders of magnitude in various systems such as porous medium. The ultrafast LNMR methods are applicable by mobile NMR instruments, lowering the price gap to utilize NMR methods for sensitive analysis. Mobile NMR methods also provide interesting new areas to explore with that would serve also industrial side. [4-9]

References

[1] Y. Q. Song, J. Magn. Reson. 229, 12-24, 2013.
[2] L. Frydman, T. Scherf and A. Lupulescu, P. Natl. Acad. Sci. 99,15858-15862, 2002.
[3] A. Tal and L. Frydman, Prog. Nucl. Mag. Res. Sp 57, 241–292, 2010.
[4] S. Ahola and V.-V. Telkki, ChemPhysChem. 15, 1687-1692, 2014.
[5] S. Ahola, V.V. Zhivonitko, O. Mankinen, G. Zhang, A.M. Kantola, H.-Y. Chen, C. Hilty, I.V. Koptyug and V.-V. Telkki. Nat. Commun. 6, 8363, 2015.
[6] O. Mankinen, J. Hollenbach, S. Ahola, J. Matysik and V.-V. Telkki, Microporous Mesoporous Mater, 269 75–78, 2018.
[7] J.N. King, V.J. Lee, S. Ahola, V.-V. Telkki, and T. Meldrum, Angew. Chem. Int. Ed., 55, 5040-5043, 2016.
[8] J.N. King, A. Fallorina, J. Yu, G. Zhang, V.-V. Telkki, C. Hilty and T. Meldrum, Chem. Sci., 9, 6143, 2018.
[9] G. Zhang, S. Ahola, M. Lerche, V.-V. Telkki and C. Hilty, Anal. Chem. 90, 18, 11131-11137, 2018.


Speech: March 5th, 15:00, Hall 10

TOWARDS A 3D ACOUSTOJET

Tuukka Kekkonen1, Joni Mäkinen1, Daniel Veira Canle1, Jere Hyvönen1, Antti Kuronen1, Tapio Kotiaho2,3, Ari Salmi1, Edward Hæggström1

tuukka.o.kekkonen@helsinki.fi
1 Department of Physics, University of Helsinki, Finland
2 Department of Chemistry, University of Helsinki, Finland
3 Division of Pharmaceutical Chemistry and Technology, Faculty of Pharmacy, University of Helsinki, Finland


The Rayleigh criterion states that the lateral resolution of an optical or acoustic system is limited by diffraction [1]. According to classical wave theory, it is impossible to focus energy to a spot smaller than λ/2. In optics, however, it has been shown that it is possible to circumvent this limitation, resulting in super resolution jets [2].

In our previous study, we showed that these super resolution jets can be created in the acoustic realm by using a cylinder immersed in liquid [3]. Using a cylinder resulted in a line focus, which in practice meant that the experiment was done in pseudo-2D.

In this study, we present a finite element method (FEM) simulation of an improved geometry to generate a 3D jet. The simulation was done in the frequency domain using COMSOL Multiphysics® (ver. 5.4). The materials used in the simulation were chosen such that the geometry could be realized in practice. We achieved a full width at half maximum (FWHM) of 0.466λ at 1 mm distance from the focusing lens (Fig. 1). The geometry of the lens is designed such that it could be attached to an acoustic immersion microscope increasing its lateral resolution thus resulting to higher quality images. We also discuss our experimental efforts to realize this construct.


[1] Rayleigh, F. R. S. Investigations in optics, with special reference to the spectroscope. London, Edinburgh, Dublin Philos. Mag. J. Sci. 8, 261-274 (1879).
[2] Chen, Z., Taflove, A. & Backman, V. Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique. Opt. Express 12, 1214 (2004).
[3] Veira Canle, D. et al. Breaking the acoustic diffraction limit across length scales. submitted, (2019).



Figure 1
Figure 1: Results of our FEM simulations. (a) The intensity map of the focused acoustic field. The maximum intensity is at about 1 mm from the focusing structure. (b) Normalized intensity along r axis for the distance of the maximum intensity (blue). We fitted a Gaussian (red) to the data and calculated the full width of the half maximum to be 0.466λ.

Speech: March 5th, 15:15, Hall 10

X-RAY TOMOGRAPHY – 3D TARGETED ANALYSIS FOR BETTER 2D RESULTS

J. Kuva1, AR. Butcher1, Y. Lahaye1, PWSK. Botha2

jukka.kuva@gtk.fi
1 Geological Survey of Finland GTK, P.O. Box 96, 02150 Espoo, Finland
2 Hippo Geoscience, Brisbane, Australia


Analysis of geological samples is commonly done with 2D surface methods applied directly to the sample surface or the surface of a carefully prepared thin section. Automated analysis tools, such as QEMSCAN (Quantitative Evaluation of Minerals by SCANning electron microscopy) with SEM-EDS (Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy), have become quite efficient and convenient, while more manual tools, such as EPMA (Electron Probe MicroAnalyzer) or LA-ICP-MS (Laser Ablation – Inductively Coupled Plasma Mass Spectroscopy) provide unparalleled accuracy.

The main drawback with 2D surface methods is knowing a priori where to section a 3D sample in order to obtain the best representative analysis. Contextual knowledge of the sample material can help a lot, but the most interesting parts of a sample often nevertheless remain unexamined. We have therefore developed a workflow which takes away most of the subjectivity and chance of preparing 2D slices from 3D objects, such as rocks.

The process begins with an XCT (X-ray Computed Tomography) scan of a complete sample, which is often an uncut drill core sample. Efficiently scanning samples at the drill core scale and larger (up to 30 cm in diameter) has only recently been made possible in Finland with the installation of GTK’s new XCT equipment. The resulting tomographic image shows the internal structure, along with the heterogeneities, and can be used to locate areas of specific interest to the analyst. Thin sections, both optical and polished, are then prepared so as to deliberately intercept the features of interest observed within the 3D volumes.

The first step of the workflow, XCT, is non-destructive and can later be used as a map for all subsequent analyses. This workflow has been successfully applied to geological samples, combining XCT with QEMSCAN, and is now being steered towards other types of samples as well, such as batteries, where other types of analysis, such as trace element analysis with LA-ICP-MS, can be of use. From geochemical analysis covering over 90% of the periodic table, using LA-ICP-MS, the economic evaluation of the sample improves exponentially from a surface to a volume.

GTK has a unique facility in Finland with XCT and advanced 2D analysis systems in the same lab, with the researchers working in close collaboration. We present results where this workflow has been used to great effect in the discovery of minerals in exploration samples of commercial importance (see Fig. 1), as well as preliminary results from other studies.



Figure 1
Figure 1: Left: Cross-section from the 3D tomographic image, showing an area of interest (light grayscale value equals high density). Middle: Optical thin-section of the same location, preparation aided by the tomographic image. Right: QEMSCAN analysis of the thin-section, identifying the interesting fracture-filling mineral as barite.

Speech: March 5th, 15:30, Hall 10

PILOT NBR MEASUREMENTS AT A BSUIN UNDERGROUND LABORATORY: CALLIO LAB

J. Puputti1, T. Enqvist1, P. Jalas1, J. Joutsenvaara1, H-M. Karjalainen1, K. Loo2, J. Kisiel3, K. Polaczek-Grelik3, K. Szkliniarz3, A. Walencik-Łata3, A. Djakonow4, K. Jedrzejczak4, M. Kasztelan4, W. Marszał4, J. Orzechowski4, J. Szabelski4, V. Gostilo5, S. Pohuliai5, A. Sokolov5

Hannah.Puputti@oulu.fi
1 University of Oulu, Finland
2 University of Jyväskylä, Finland
3 University of Silesia, Katowice, Poland
4 National Center for Nuclear Research, Astrophysics Division in Lodz, Poland
5 Baltic Scientific Instruments, Riga, Latvia


Baltic Sea Underground Innovation Network (BSUIN) is striving to make underground laboratories (ULs) more accessible for science and innovation [1]. A vital part of the feasibility of ULs for different types of research is the characteristic natural background radiation (NBR). As of July 2018, several pilot NBR measurements have been conducted in Callio Lab at the Pyhäsalmi Mine in Finland [2,3].

BSUIN consists of six underground laboratories and 14 partner organizations. Through thorough geophysical, structural, organizational and NBR characterization, BSUIN wants to provide a methodologically consistent technical characterization of each underground laboratory.

Due to differences in the geology of bedrock, each underground environment has unique NBR [4]. The characterization measurements consist of gamma ray spectrum, neutron flux, and radon measurements. These are conducted as pilot cases in specific halls and not throughout the underground laboratory facilities. The measurement methodology is consistent lab-specifically as well as from lab to lab. Emphasis has been placed on ensuring that measurement setups can be easily recreated. This not only provides an opportunity to compare results between laboratories, but also enables the testing and development of different technologies.

NBR measurements help assess the need for protective measures (especially in the case of problematic radon) and better develop the working environment [5]. The required accuracy of NBR measurements and the accepted level of NBR can vary greatly. These are determined by the intended use of the underground facilities e.g. requirements for low-background experiments differ significantly from the requirements of general-purpose operations [6].

We present the measurement technology and procedures used, as well as preliminary results. For demonstration, the setup at Callio Lab will be used as a working example of the methodology BSUIN is piloting.

BSUIN is funded by the EU´s Interreg Baltic Sea Region Programme



[1] Baltic Sea Underground Innovation Network, www.bsuin.eu, 17 Jan 2019

[2] Callio Lab – Underground Center for Science and R & D, www.calliolab.com, 17 Jan 2019

[3] L. B. Bezrukov et al., New Low-Background Laboratory in the Pyhäsalmi Mine, Finland, Physics of Particles and Nuclei, 49, 769-773, 2018

[4] G. Cinelli et al., Digital version of the European Atlas of natural radiation, Journal of Environmental Radioactivity, 196, 240-252, 2019

[5] E. E. K. Nang et al., Review of the potential health effects of light and environmental exposures in underground workplaces, Tunnelling and Underground Space Technology 84, 01-209, 2019

[6] J. Joutsenvaara, Deeper understanding at Lab 2: the new experimental hall at Callio Lab underground centre for science and R & D in the Pyhäsalmi Mine, Finland, (Master´s thesis), University of Oulu, Finland, 2016


Speech: March 5th, 15:45, Hall 10

SCANNING ACOUSTIC MICROSCOPE WITH EXTENDED WORKING DISTANCE

Antti Meriläinen1, Jere Hyvönen1, Ari Salmi1, Edward Hæggström1

antti.merilainen@helsinki.fi
1 Electronics Research Laboratory, Physics Department, Faculty of science, University of Helsinki


Scanning acoustic microscopy (SAM) can measure both the topology and the mechanical properties of a sample by employing an ultrahigh frequency (300 – 500 MHz) focused ultrasonic transducer, [1]. The lateral resolution is in the µm range. However, such high frequencies are attenuated in the coupling media, typically ion-exchanged water, which limits the working distance. Even more, the image-in-focus range is limited only to 10 µm due to the tight focus of the acoustic lens. This restricts what samples can be measured: typically, they should be flat, with a maximum roughness of ±5 μm across their top surface in the imaging area (for traditional SAM techniques).

We built a custom-made SAM system that employs coded excitation, which gives higher signal-to-noise ratio than any traditional method, and which translates into a longer working distance, [2]. We show that by using the synthetic aperture focusing technique (SAFT), the narrow focus range can be increased by post-processing the measured signals.

We measured an USAF 1951 resolution test sample, both in optimal focus and 50 µm off from the optimal focus distance. By utilizing the SAFT technique, the focus of image was moved to the target plane creating a sharp image, Figure 1. The results demonstrate that the sharp focus range can be extend from 10 µm least up to 50 µm and that also the working distance is increased the same 50 µm. This shows that SAFT algorithm makes the microscope more robust for surface roughness of the sample.

Ref 1: Raum et al. “Multilayer analysis: Quantitative scanning acoustic microscopy for tissue characterization at a microscopic scale,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 50, no. 5, pp. 507-516, May 2003
Ref 2: Meriläinen A. I. et al. ”Solid state switch for GHz coded signal ultrasound microscopy”, Electronics Letters. vol 49, no. 3, pp. 169-170 Jan. 2013



Figure 1
Figure 1: Figure 1, SAM image of USAF 1951 resolution sample, groups 6 & 7 are imaged. Left, 50 µm off-focus image. Right, Based on same data the image after SAFT refocusing.

Photonics & Optics

Location: Hall 12
Time: March 5th, 14:30 - 16:00

Speech: March 5th, 14:30, Hall 12

VECTOR PROPERTIES OF RADIALLY SELF-ACCELERATING BEAMS

M. Ornigotti1,2, A. Szameit2

marco.ornigotti@tuni.fi
1 Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland
2 Institut für Physik, Universität Rostock, Rostock, Germany


Accelerating beams, i.e., electromagnetic fields, that propagate along curved trajectories in free space, without the aid of an external force, have been the subject of an extensive research in the last years. Among the different classes of accelerating beams [1-3], radially self-accelerating beams (RSABs) propagate along spiraling trajectories around their propagation direction [4]. Because of this special property, RSABs have potential impactful applications in different areas of physics, such as sensing [5], material processing [6], and particle manipulation [7]. Despite their high potential, RSABs have only been studied within the scalar electromagnetic theory, and a consistent theory for vector RSABs has not been presented yet. Here, we present a complete theory of vector RSABs, in terms of their linear and angular momentum content. Moreover, we also study under which conditions, the self-accelerating character of RSABs is preserved under focussing.
Scalar RSABs are defined in terms of superpositions of Bessel beams, where each single component is characterized by an angular velocity, proportional to its orbital angular momentum [4]. To generalise their definition to vector beams, we use the Hertz method of potentials to construct electric and magnetic fields, i.e., $\textbf{E}(\textbf{r},t)=-\partial_t \left[\nabla\times\left(\psi_{RSAB}e^{-I\omega t}\textbf{u}\right)\right]$, and $\textbf{B}(\textbf{r},t)=\nabla\times\left[\nabla\times\left(\psi_{RSAB}e^{-I\omega t}\textbf{u}\right)\right]$, where $\psi_{RSAB}$ is the scalar RSAB, given explicitly in Ref. [4], and $\textbf{u}$ accounts for the initial polarisation of the vector beam. To check, whether or not these vector fields are self-accelerating, we apply the criteria developed in Ref. [4], and in particular, we check, whether or not there exists a suitable co-moving reference frame, in which the electric and magnetic fields defined above appear propagation invariant, i.e.,$\partial_z\textbf{E}(\textbf{r},t)=0=\partial_z\textbf{B}(\textbf{r},t)$ [8]. By introducing the co-rotating coordinate$\Phi=\theta+\Lambda z$, where $\Lambda$ is the angular velocity of the scalar RSAB, it turns out, that the only possibility for the vectorialisation procedure to conserve the self-accelerating character of RSABs, is to choose $\textbf{u}=(\textbf{x}\pm I\textbf{y})/\sqrt{2}$, i.e., that in order to maintain their self-accelerating character, vector RSABs must possess circular polarisation.
For paraxial vector RSABs, moreover, we calculate their spin (SAM) and orbital (OAM) angular momenta, according to the usual separation $\textbf{J}=\textbf{r}\times(\textbf{E}\times\textbf{B}^*)=\textbf{L}+\textbf{S}$. [9]. Our calculations reveal, that while the global (i.e., integrated) SAM gives a result consistent with the vector beam being circularly polarised (i.e., the helicity of the beam is $\sigma=\pm 1$), the SAM density shows regions of negative value, thus meaning a local inversion of the helicity axis. The OAM, instead, contains, as expected, both intrinsic and extrinsic components. The former is related to the intrinsic OAM carried by Bessel beams, and both its longitudinal and transverse component are proportional to the standard spin-orbit interaction term $m(m+\sigma)$. The latter, on the other hand, has a hybrid nature, since the transverse component of the extrinsic OAM depends only on the angular velocity $\Lambda$ of the RSAB, as it should be for a field rotating about an axis, while its longitudinal component is proportional to the product between the intrinsic OAM carried by Bessel beams composing the RSAB, and its actual angular velocity, i.e., $m\Lambda$.

References
[1] G.A. Siviloglou, et al. “Observation of Accelerating Airy Beams”, Phys. Rev. Lett. 99, 213901 (2007).
[2] M.A. Bandres, “Accelerating Parabolic Beams”, Opt. Lett. 33, 1678 (2008).
[3] M.A. Bandres, et al. “Three-Dimensional Accelerating Electromagnetic Waves”, Opt. Express 21, 13917 (2013).
[4] C. Vetter, et al. “Generalised Radially Self-Accelerating helicon Beams”, Phys. Rev. Lett. 113, 183901 (2014).
[5] P. Polynkin, et al. “Curved Plasma Channel Generation Using Ultraintense Airy Beams”, Science 324, 5924 (2009).
[6] A. Jesacher, et al. “Parallel Direct Laser Writing in Three Dimensions with Spatially Dependent Aberration Correction”, Opt. Express 18, 21090 (2010).
[7] D. McGloin, et al. “Interfering Bessel Beams for Optical Micromanipulation”, Opt. Lett. 28, 657 (2003).
[8] M. Ornigotti, et al. “Vector Properties of Radially Self-Accelerating Beams”, J. Opt. 20, 125601 (2018).
[9] D.L. Andrews, and M. Babiker, “The angular Momentum of Light” Cambridge University Press (2013).


Speech: March 5th, 14:45, Hall 12

EMISSION AND PROPAGATION OF LIGHT IN SPATIALLY DISPERSIVE NANOMATERIALS

M. Nyman1, A. Shevchenko1, V. Kivijärvi1, M. Kaivola1

markus.nyman@aalto.fi
1 Department of Applied Physics, Aalto University


Optical metamaterials and metasurfaces can provide extraordinary control over the propagation and emission of light, with applications including arbitrary phase and polarization transformations and enhanced fluorescence. The elementary units of these structures are shaped nanoparticles and nanoresonators. These units are subwavelength in size, but are not negligibly small, allowing multipoles of higher order than electric dipoles to be excited in them. These can be utilized to design useful nanomaterials.

When designing spatially dispersive nanomaterials, we describe them in terms of two effective wave parameters: refractive index and wave impedance determined for each plane-wave mode in the material. Together with the angular spectrum representation, they describe the propagation of light in nanostructured media [1,2]. We design a metamaterial that separately controls the phase and energy propagation of an optical beam. This material consists of a lattice of tilted silver nanorods in glass. Figure 1(a) shows the intensity distribution of a Gaussian beam propagating in the material. Outside of the metamaterial the beam propagates normally, but inside the material its energy propagates at an angle. The corresponding phase distribution is shown in Figure 1(b) which shows that the wavefronts are not tilted inside the material. The effect combines strong anisotropy and spatial dispersion.

To model optical emission in a spatially dispersive nanomaterial, we present an electric-current decomposition method that utilizes the wave parameters [3]. We use it to study how a diffraction-compensating metamaterial modifies a quantum emitter's radiation. This material is also made of silver nanorods in glass. A dipole emitter in the material produces the intensity distribution shown in Figure 1(c). Normally the dipole would radiate in all directions, but inside the material it instead creates a narrow beam. With the method in hand we also design metasurfaces that enhance the far-field radiation intensity of fluorescent films [4].


[1] A. Shevchenko, M. Nyman, V. Kivijärvi and M. Kaivola, ``Optical wave parameters for spatially dispersive and anisotropic metamaterials,'' Opt. Express 25, 8550 (2017).

[2] V. Kivijärvi, M. Nyman, A. Karrila, P. Grahn, A. Shevchenko and M. Kaivola, ``Interaction of metamaterials with optical beams'', New J. Phys. 17, 063019 (2015).

[3] M. Nyman, V. Kivijärvi, A. Shevchenko and M. Kaivola, ``Generation of light in spatially dispersive materials,'', Phys. Rev. A 95, 043802 (2017).

[4] M. Nyman, A. Shevchenko and M. Kaivola, ``Fluorescence enhancement and nonreciprocal transmission of light waves by nanomaterial interfaces,'', Phys. Rev. A 96, 053828 (2017).



Figure 1
Figure 1: Left: propagation of an optical beam in a silver-rod-based nanomaterial. The intensity distribution is shown in (a) and the phase distribution in (b). Right: figure (c) shows the intensity distribution created by a dipole emitter (black dot) in a diffraction-compensating nanomaterial.

Speech: March 5th, 15:00, Hall 12

FAST AND AUTOMATED OVARIAN TISSUE CLASSIFICATION USING MULTIPHOTON MICROSCOPY AND DEEP LEARNING

M. J. Huttunen, A. Hassan2, C. W. McCloskey3,4, S. Fasih2, J. Upham2, B. C. Vanderhyden3,4, R. W. Boyd2,5, S. Murugkar6

mikko.huttunen@tuni.fi
1 Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland
2 Department of Physics, University of Ottawa, Ottawa, Canada
3 Department of Cellular and Molecular Medicine, University of Ottawa, Ottawa, Canada
4 Centre for Cancer Therapeutics, Ottawa Hospital Research Institute, Canada
5 The Institute of Optics and Department of Physics and Astronomy, University of Rochester, Rochester, USA
6 Department of Physics, Carleton University, Ottawa, Ontario, Canada


Histopathological image analysis of stained tissue slides is routinely performed by a pathologist to detect and classify tumor. Although effective, the approach is not without problems. First, tissue slides are prepared and stained for imaging contrast, which is a labor-intensive, slow and costly process. The subsequent image analysis is also labor-intensive and can even contain a risk for bias or human error. Therefore, development of faster and more cost-effective approaches could be beneficial. Nonlinear microscopy can alleviate the problems associated with sample preparation, because nonlinear optical processes occur intrinsically and can thus be used for label-free imaging [1]. In addition, recent leaps in machine learning and artificial intelligence are enabling unforeseen possibilities for fast and automated image analysis [2]. Here, we perform combined multiphoton microscopy and deep-learning based automated image analysis and demonstrate how many of the problems in tissue diagnosis can be effectively alleviated [3].

We image unstained murine reproductive tissues by using nonlinear imaging modalities of two-photon emission fluorescence (TPEF) and second-harmonic generation (SHG) [see Fig. 1(a)-1(d)]. Using around 200 images, we fine-train four openly available convolutional neural networks (AlexNet, VGG-16, VGG-19 and GoogLeNet) to perform binary image classification (healthy vs. cancerous), and compare their performance in terms of classification accuracy. We show that over 95% accuracy can be reached when combined TPEF and SHG data are used [see Fig. 1(f)].

[1] W. R. Zipfel R. M. Williams and W. W. Webb, "Nonlinear magic: multiphoton microscopy in the biosciences," Nat. Biotechnol., 21, 1369-1377 (2003).
[2] A. Krizhevsky, I. Sutskever, and G. E. Hinton, "Imagenet classification with deep convolutional neural networks," In Advances in neural information processing systems," Adv. Neural Inf. Process. Syst., 25, 1097-1105 (2012).
[3] M. J. Huttunen et al., "Automated classification of multiphoton microscopy images of ovarian tissue using deep learning," J. Biomed. Opt., 23, 1-7 (2018).



Figure 1
Figure 1: (a) and (b) Representative brightfield images of murine reproductive tissues stained with picrosirius red. (c) and (d) Corresponding combined TPEF (red) and SHG (green) images from adjacent unstained tissue sections. Scale bars are 50 $\mu$m. (f) Calculated classification accuracies for four openly available pre-trained convolutional neural networks. Data augmentation by increasing the number of image patches $N$ improved performance by almost 10%.

Speech: March 5th, 15:15, Hall 12

VALOMC – A MONTE CARLO SOFTWARE FOR SIMULATING LIGHT PROPAGATION IN BIOLOGICAL TISSUE

Aleksi Leino1, Aki Pulkkinen1, Tanja Tarvainen1

1 Department of Applied Physics, University of Eastern Finland, Post-office box 1627, FIN-70211, Kuopio, Finland


We present a simulation software, ValoMC, for modelling light transport in biological tissue using triangular or tetrahedral meshes to describe the simulation geometry [1]. The implementation follows the photon packet method introduced by Prahl et al. [2] and later developments. The simulation code is written C++, while the main benefit over existing implementations is in its MATLAB (Mathworks Inc., Natick, MA) interface, which enables a quick adoption and fast prototyping. The interface simplifies setting up light sources, boundary conditions and analyzing the results. Furthermore, it offers functions that enable seamless use with other software, for example, with k-Wave [3] to simulate the photoacoustic effect and with NETGEN to import meshes [4]. The main outcome from the simulations are the fluence distribution in the computation domain and exitance at the boundaries. To validate the computations, we show that they are in good agreement with other simulation software and analytical solutions to radiative transfer equation. ValoMC is open source and distributed under the MIT license.

Source code: https://github.com/InverseLight/ValoMC
Home page and documentation: https://inverselight.github.io/ValoMC/

[1] A. A. Leino, A. Pulkkinen and T. Tarvainen, “Monte Carlo software for simulating light propagation in biological tissue”, submitted to OSA Continuum, 2019

[2] S.A. Prahl, M. Keijz and A.J. Welch, “A Monte Carlo Model of Light Propagation in Tissue,” SPIE Proc. Dosim. Laser Radiat. Med. Biol. IS 5, pp. 102–111, 1989

[3] B. E. Treeby and B. T. Cox, “k-Wave: MATLAB toolbox for the simulation and reconstruction of photoacoustic wave-fields”, J. Biomed. Opt. 15, p. 021314, 2010.

[4] J. Schöberl, “NETGEN An advancing front 2D/3D-mesh generator based on abstract rules”, Comput. Vis. Sci. 1, pp. 41–52, 1997


Speech: March 5th, 15:30, Hall 12

IMPROVED DNA-ASSISTED LITHOGRAPHY

P. Piskunen, B. Shen, V. Linko

petteri.piskunen@aalto.fi
1 Biohybrid Materials, Department of Bioproducts and Biosystems, Aalto University, Finland


The self-assembling properties and addressability of DNA can be exploited in the fabrication of fully tailored, nanoscale macromolecules called DNA origami [1]. These DNA origami can be used in the manufacturing of metallic nanostructures by utilizing lithographical techniques in a process called DNA-assisted lithography (DALI). In DALI, DNA origami are used in the creation of a negative-pattern lithographical mask, which can subsequently be used to transfer the shape of the origami onto a substrate material as metallic nanostructures. After pattern transfer, the mask layers are removed with hydrofluoric acid (HF) etching, leaving only the metallic nanostructures on the substrate. [2, 3] The method has been proven capable of creating nanostructures with plasmonic functionality, like that illustrated in figure 1, right [3].


A modification to the original DALI protocol, first proposed by Shen [4] (figure 1, left), is attempted here by adding a sacrificial layer of polymethylmethacrylate (PMMA) onto the substrate as the first step. As a result, the PMMA and all layers above it can be removed by using acetone, bypassing the need for more aggressive lift-off procedures such as HF etching. This has the potential to significantly expand the utility of DALI, as it will allow the process to be performed on substrates that are more common: for example silica-based glass instead of sapphire. It would also allow previously sensitive materials (Ag, Ti and Cu) to be used for the resulting nanostructures. The proposed modification is tested by applying a spin-coated PMMA layer on quartz glass chips before proceeding with the rest of the DALI process. After the final deposition step, lift-off is performed with heated acetone instead of HF. Resulting nanostructure quality is then compared to the original DALI protocol. If successful, this improved process will bring DALI one step closer to the batch manufacturing of inexpensive metamaterials.


References
[1] V. Linko, M.A. Kostiainen, Nature Biotechnology 34, 826 (2016)
[2] B. Shen, et al., Nanoscale 7, 11267 (2015)
[3] B. Shen, et al., Science Advances 4,
eaap8978 (2018).
[4] B. Shen, University of Jyväskylä, Dept. of Physics Research Report, 3 (2018).


 


 



Figure 1
Figure 1: Left - Scheme of the modified DALI protocol as proposed by B. Shen [4].
Right – Illustration of Bowtie-shaped antennas with plasmonic hotspots fabricated using DALI [3].

Speech: March 5th, 15:45, Hall 12

FOURIER TRANSFORM PHOTOACOUSTIC SPECTROSCOPY WITH SUPERCONTINUUM AND OPTICAL FREQUENCY COMB LIGHT SOURCES

Tommi Mikkonen1, Ibrahim Sadiek2, Caroline Amiot1,3, Antti Aalto1, Kim Patokoski1, Markku Vainio1,4, Goëry Genty1, Aleksandra Foltynowicz2, Juha Toivonen1

tommi.mikkonen@tuni.fi
1 Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland
2 Department of Physics, Umeå University, Umeå, Sweden
3 Institut FEMTO-ST, UMR 6174 CNRS, Université Bourgogne Franche-Comté, Besançon, France
4 Department of Chemistry, University of Helsinki, Finland


In contrast to conventional absorption spectroscopy methods, photoacoustic spectroscopy (PAS) measures the absorbed light power directly, which makes the technique highly sensitive and also enables small sample volumes [1]. These properties are important in multicomponent trace-gas analysis, which is possible in PAS using a broadband light source together with a Fourier transform spectrometer (FTS) as a light modulator. A cantilever-enhanced photoacoustic spectroscopy (CEPAS) is a non-resonant photoacoustic technique enabling sensitive detection over a wide range of frequencies [2]. The CEPAS technique together with FTS has so far been used only with conventional infrared radiators, which suffer from poor spectral power density and low spatial coherence [3].

In this work, we demonstrate broadband Fourier transform photoacoustic spectroscopy using two supercontinuum light sources at 1–3.5 µm and an optical frequency comb at 3.1–3.5 µm wavelength range. Compared to conventional infrared radiators, these light sources provide larger spectral power density and therefore larger photoacoustic signal. In addition, high spatial coherence of the sources enables the use of an FTS with longer delay range for high resolution measurements. We measured rovibrational photoacoustic spectra of water vapor in room air around 1.9 µm and methane (400 ppm) in nitrogen around 3.3 µm at atmospheric pressure using an infrared radiator and the supercontinua as light sources (Fig. 1a and Fig. 1b). We also measured the same absorption band of methane (100 ppm) with the optical frequency comb and a high-resolution FTS (Fig. 1c). All measured spectra are in good agreement with a model based on HITRAN (inverted).

We achieved a 20- and 70-fold increase in photoacoustic signal with the supercontinua compared to the infrared radiator, demonstrating the advantage of larger spectral power density of the supercontinuum sources [4]. Furthermore, we achieved the best resolution reported with broadband PAS (400 MHz) using the optical frequency comb and the longer FTS [5]. Thus, the combination of supercontinuum or optical frequency comb light source, FTS and CEPAS detection enables broadband multi-gas detection using small sample volumes.

[1] J. Li, W. Chen, B. Yu, Appl. Spectrosc. Rev. 46, 440-471 (2011).
[2] T. Kuusela, J. Kauppinen, Appl. Spectrosc. Rev. 42, 443-474 (2007).
[3] C. B. Hirschmann et al., Appl. Spectrosc. 64, 293-297 (2010).
[4] T. Mikkonen et al., Opt. Lett. 43, 5094-5097 (2018).
[5] I. Sadiek et al., Phys. Chem. Chem. Phys. 20, 27849-27855 (2018).



Figure 1
Figure 1: Photoacoustic spectra of a) water vapor and b) methane measured using two supercontinua and a broadband infrared source (magnified), and of c) methane measured using an optical frequency comb and a longer FTS. Reference spectra from HITRAN are also shown (inverted).

Physics Education & Outreach

Location: Hall 12
Time: March 6th, 11:00 - 12:30

Speech: March 6th, 11:00, Hall 6

SITUATIONAL ACADEMIC EMOTIONS OF THE FIRST-YEAR UNIVERSITY PHYSICS STUDENTS: STUDYING IN DIFFERENT SIZED GROUPS

L. Timonen1, K. Juuti2, S. Harmoinen3

laura.timonen@oulu.fi
1 Nano- and molecular systems research unit: Physics didactics research group, University of Oulu, Oulu, Finland
2 Faculty of Education, University of Helsinki, Helsinki, Finland
3 Faculty of Education, University of Oulu, Oulu, Finland


We present a study, which enlightens the different emotions first year higher education physics students have, when studying in different sized groups. It is suggested in theories behind optimal learning moment (OLM), that in an optimal situation for learning, students are feeling engagement towards what they are doing. Besides engagement, there are other emotions, which either enhance, detract or act as accelerants for the arising of an OLM. [1]

The knowledge about situational emotions composing an OLM is vital for designing courses and coursework so that there would be plenty of opportunities for OLMs. In our study, we are currently concentrating on the group size variable and how it is connected to different academic emotions. The categories for groups in this study are: alone, together with someone, group of 3-10 people, or a crowd of over 10 people. Traditionally in higher education it is up to the students to decide, whether they study alone or in a group of varying size. With well-designed coursework, the teacher has the possibility to steer this choice to an optimal direction.

The data for this study was collected during the academic year 2016-2017. Our context is a Finnish university and its first-year physics students. In the year of the data collection, the curriculum of the students consisted mostly of basic studies of physics, mathematics and chemistry, as well as some general studies. The students had some choice over the courses they enrolled in. The teaching methods during that year varied from traditional lecturing in large auditoriums to working in small groups or with partner when doing laboratory work, and voluntary participating in tutored exercise events. Therefore, the students had the possibility to study in varying size groups or alone, to seek tutoring or not.

Of the 72 students that commenced their studies in autumn 2016, 36 participated voluntarily in our research and 27 of them filled a background questionnaire, containing questions about e.g. educational background and the reasons to study physics. The method of data collecting was experience sampling method (ESM) [2], which is superior in its capability to collect situational data. Therefore, the data is assumed to contain less e.g. retrospective bias than when collected with more traditional methods.

The data collection was executed with PACO-application [3] that students installed to their smartphones. The application sent a prompt to answer to a questionnaire several times a week at random times, beginning in September 2016 and ending in May 2017. In the questionnaire, students answered to open ended or multiple-choice questions about their whereabouts, what they were doing and how many people they were with. Then followed a series of Likert-scaled questions about the emotions described in the theories behind OLM.

At the moment, the ongoing analysis focuses on ESM-data collected in September and October 2016 and the results so far consist mostly of descriptive statistics. For example, there seems to be less accelerant and detractor emotions, when studying together with some one or in a group, than when studying alone or in a crowd.

[1] B. Schneider, M. Krajcik, J.Lavonen, K. Salmela-Aro, M. Broda, J. Spicer, J. Bruner, J. Moeller, J. Linnansaari, K. Juuti, and J. Viljaranta; J Res Sci Teach 53 (3), 2016
[2] S. Zirkel, J. A. Garcia, and M. C. Murphy; Educational Researcher 44 (1), 2015
[3] http://www.pacoapp.com


Speech: March 6th, 11:15, Hall 6

PHYSICIST'S WORDLINE - INITIATION OF NEW STUDENTS AIMING TO HIGHER AGENCY

S.S. Räsänen1, J. Maunuksela1, P. Koskinen1

sami.s.rasanen@jyu.fi
1 University of Jyväskylä


Regarding new students’ initiation, the Physics department at the University of Jyväskylä has long had two main practices. First, the department has assigned staff members as tutor teachers (omaopettaja) to groups of 8-12 students once they start their studies. The new students have made their study plans for the Bachelor's Degree with the tutor teacher, who have also provided counselling and advice for the first three years of studies and helped in the search of a topic for the thesis. Second practice in the initiation of the new students has been the course Flying Start to Physics, a two-week intensive period, during which students have heard info and presentations on research at the department.

At the beginning of year 2018, the tutor teacher system had been running for roughly ten years at the department and, thus, it was time to gather experiences from both teachers and students to find out the best practices and see how one could develop the tutor teacher activities. To this end, we formed a group from experienced tutor teachers and newly selected peer tutors willing to commit to the development work that began in spring 2018 with a SWOT analysis on the activity and continued in several meetings.

One of the first observations from the SWOT analysis was that tutor activities and Flying Start to Physics had made students to feel secure in the beginning. However, once the two-week initiation period was over, some of the tutor groups broke and transition to every day study routines was not always smooth. It became evident that we also need to renew the Flying Start and integrate it better with the tutor teacher activities.

To this end, we integrated the initiation practices into a single model named "Physicist's Worldline" (Fyysikon maailmanviiva) that we implemented with students that started their studies in fall 2018. Physicist's Worldline starts with a get-together -retreat outside Jyväskylä where the main goal is to further nourish grouping. Then there are seven so-called Theme Fridays along the first year of studies. During Theme Fridays the students interview personnel, hear about exchange possibilities and meet physics alumni. The Theme Fridays are organized such that students do not "merely sit" in the lecture hall and listen but instead participate actively. This increases the agency of the students. Most importantly, we aim to strengthen the development of the academic identity of students such that they can make reasoned choices in their studies.

The first feelings from Physicist's Wordline are very positive. The group working in the retreat was a success. Participation to Theme Fridays has been active and tutor groups seem to have held together, as we hoped. We have obtained some critical feedback on the role of the peer tutors and arrangements of Theme Fridays. In this presentation, we would like to share what was done in Physicist's Wordline and the first experiences on how it has been received.


Speech: March 6th, 11:30, Hall 6

OPEN DATA IN EDUCATION - BRIDGING THE GAP BETWEEN SCHOOLS AND CUTTING EDGE RESEARCH

P. Veteli1

peitsa.veteli@helsinki.fi
1 Helsinki Institute of Physics


There are some universally acknowledged problems in school sciences. Across the developed countries worldwide, young people are not interested in studying STEM-subjects. Whether that is because of perceived lack of personal relevance, disconnect from the actual fields of study, "sanitized" school practices or other factors is up to debate, but it is eminently clear that as educators we have to do our best in combatting this trend.

In this speech we present the CMS Open Data project as a freely available digital toolbox for that purpose and some feedback we've gotten from teachers and students. Our aim is to provide the teachers with the means to incorporate some methods and findings of contemporary scientific research in their everyday teaching with a minimal amount of hassle. Though most of the ready materials are built around openly available particle physics data from the CMS experiment at CERN, the tools are not limited in subject and can be used in cross-disciplinary fashion with other contexts very easily. They also form a very adaptable and beginner friendly template for less educational purposes, such as outreach by professional researchers.

This approach has several benefits:

- Setup is trivial and won't take valuable time from the learning session.
- The tools used are not specific to one subject but form a solid basis for broader skills that many, especially Finnish, school curriculums encourage.
- Using freely available data sets or analyzing those produced by the students themselves allows for wide contextual variety, increasing the likelihood of interest and motivation.
- Moving from the classroom limits of maybe ten smooth data points on a neat line to the ”real world” situation with hundreds of thousands of measurements, errors and all, allows us to give them a peek into the exciting world of scientific research.


Speech: March 6th, 11:45, Hall 6

REFORMING INTRODUCTORY QUANTUM MECHANICS: SPIN FIRST APPROACH AND THE ROLE OF MATHEMATICS

Elina Palmgren1, Inkeri Kontro1, Kimmo Tuominen1

elina.palmgren@helsinki.fi
1 University of Helsinki


Quantum mechanics is a challenging topic to learn, not only because of its conceptual subtleties but also the heavy mathematical machinery behind the physical theory. Traditionally quantum mechanics has been introduced to students starting with wave functions and making explicit relations between classical and quantum phenomena (Position First approach). While wave functions are often thought to be more intuitive quantum objects than bra-ket vectors, the mathematical analysis grows quickly very heavy. Moreover, analogies between classical and quantum phenomena can often be somewhat misleading.

The Spin First approach [1] starts by discussing experiments on spin-1/2 particles in terms of Dirac notation. While the mathematics is kept as simple as possible, students are introduced to basic two-state systems and encouraged to contemplate the interpretations of the mathematically represented phenomena. The paradigmatic shift in teaching quantum mechanics, transfer from the Position First to the Spin First approach, aims at providing students with a better understanding of the underlying theory.

During the last two academic years, the first-year physics curriculum at the University of Helsinki has been reformed to give more time to learning mathematical skills. Previously, first-year mathematics was covered before quantum mechanics topics were introduced, but currently, circa half of the first year students study linear algebra at the same time as the course Basics of Quantum Physics. This year, also the Basics of Quantum Physics course was restructured in alignment with the mathematics courses, and the Spin First instructive approach was adapted.

In this talk, we will present the results of our study from the first round of the reformed Basics of Quantum Physics course regarding the interplay between the students’ mathematical skills and their (self-efficacy) beliefs regarding learning quantum mechanics. In addition, we will discuss advantages and disadvantages of the Spin First approach, and the preliminary findings from this year’s course.


References

[1] H. Sadaghiani and J. Munteanu, Spin First instructional approach to teaching quantum mechanics in sophomore level modern physics courses. Paper presented at Physics Education Research Conference 2015, College Park, MD, (2015).


Speech: March 6th, 12:00, Hall 6

BEYOND THE FCI – CONCEPTUAL LEARNING IN INTRODUCTORY MECHANICS

I. Kontro1, I.G. Bearden2

inkeri.kontro@helsinki.fi
1 Department of Physics, University of Helsinki
2 Niels Bohr Institute, Faculty of Science, University of Copenhagen


Physics lecturers often use diagnostic tests developed in the United States to measure conceptual learning. These tests are frequently used to assess learning and to gather longitudinal data on the level of new students. The Force Concept Inventory (FCI) [1] is perhaps the most widely used diagnostic test for introductory mechanics. However, like most other introductory mechanics assessments, the FCI is problematic for students in the Nordic countries.

Students who have studied physics more extensively in high school tend to saturate the FCI already when entering the university. For example, the average pre-test score of physics students from the University of Helsinki, Finland, was 22.9 (76.4%), with 27% of students achieving >90% (N = 195). For Niels Bohr Institute in Denmark, the corresponding numbers are 19.7 (65.7%) and 15% (N = 779). Student interviews also reveal that high-achieving students tend to interpret some questions wrong, yielding false negatives.

We have developed and are testing a new instrument, Beyond-FCI (BFCI). BFCI includes parts of the FCI for comparability, and extends to more difficult questions and concepts the FCI does not cover. First results indicate that even students who achieve very high scores on the FCI and thus seem to have a very good grasp of force-related concepts are not able to apply these in questions that are more complex. Hence, there is value in measuring conceptual learning of forces also in high-achieving student populations. The BFCI is currently in its beta version, and seems a promising diagnostic tool for evaluating learning on introductory mechanics courses.

[1] D. Hestenes, et al. (1992) The Physics Teacher 30 141.


Physics of Living Systems

Location: Hall 6
Time: March 5th, 14:30 - 16:00

Speech: March 5th, 14:30, Hall 6

VISUALIZATION OF MICRONUTRIENT ACCUMULATION IN 3D IN PLANTA WITH HIGH RESOLUTION SYNCHROTRON X-RAY IMAGING METHODS

H. Help-Rinta-Rahko1, M. Lusa2, A.P. Honkanen1, H. Suhonen1, S. Huotari1

1 University of Helsinki, Department of Physics, Division of Materials Physics, X-ray Laboratory
2 University of Helsinki, Department of Chemistry, Division of Radio-chemistry


This abstract presents our current project on nm-scale resolution 3D cryogenic synchrotron x-ray imaging of chemical element composition, electron density and cell anatomy of plant root tissues.

Soil-borne bacteria can modify the chemical landscape around them altering the availability of various chemical elements for plant roots. Bacteria can also promote the uptake of hazardous radioactive compounds or heavy metals into plants, and plant-microbe interactions can thus be utilized both in bioremediation and biomining. Considerable amounts of selenium enter the environment via activities such as coal combustion and mining. The biological effects of Se make it a particular hazard for environmental releases. While bacterial Se reduction is an environmentally important process [1], only a few SeO32- respiring bacteria have been isolated, including Pseudomonas sp. strain T5-6-I used in our previous studies [2]. We have recently shown that bacteria can change root anatomy & Se speciation and enhance plant Se-uptake & accumulation both in root and shoot tissues [3]. This is an extremely interesting finding from the perspective of plant-microbe based bioremediation. Even though Se related plant literature is quite comprehensive [4], it has not yet been inconclusively shown in which cellular organ and in what chemical species Se is deposited in fresh intact plant tissues (e.g. roots) leaving room for speculation. Taken together our previous data and the gaps in selenium literature, our aim was to visualize and quantify Se uptake and translocation in planta in a non-invasive manner with the highest achievable resolution (nm scale) in young fresh Arabidopsis thaliana lateral root tips. We achieved this by utilizing three parallel cryogenic high brilliance synchrotron imaging methods: 3D cryo-ptychography, 3D cryo-holotomography and 2D/3D cryo-fluorescence tomography (providing information on root sample electron density, tissue and cell anatomy and chemical element composition, respectively). Observed features in root tissue anatomy and electron density were compared and matched to fluorescence verified Se localization hot-spots both on tissue and cellular level (incl. aggregation). We could also detect specific changes in other micronutrient levels in response to bacteria and Se additions. Our nm resolution 3D synchrotron data also verified our previous observations on bacteria co-culturing affecting root morphology.

Apart from bioremediation of nuclear waste sites and other contaminated areas, the methods we used will shed new light on the basics of micro- and non-nutrient compound uptake and promote understanding on bio-fortification of crops. Understanding the mechanisms of how soil bacteria can promote the uptake of various substances and quantifying it on whole plant level will benefit various scientific fields including soil microbiology, ecology, developmental biology, plant breeding and forest- & agricultural biotechnology. This will improve our agricultural practices, especially in eroded or contaminated areas where soil health would benefit tremendously from the introduction of beneficial soil microbes.


References
[1] Lusa M, Bomberg M, Aromaa H, Knuutinen J, Lehto J, 2015. The microbial impact on the sorption behaviour of selenite in an acidic, nutrient-poor boreal bog. Journal of Environmental Radioactivity, 147: 85-96.
[2] Lusa M, Knuutinen J, Bomberg M, 2017. Uptake and reduction of Se(IV) in two heterotrophic aerobic Pseudomonads strains isolated from boreal bog environment. AIMS Microbiology, 4: 798-814.
[3] Lusa M, Help-Rinta-Rahko H, Honkanen AP, Bomberg M, 2019. Unpublished, manuscript in progress.
[4] White P J, 2016. Selenium accumulation by plants. Annals of Botany, 117 (2): 217-35.



Figure 1
Figure 1: Figure: Left – holotomogram of Se supplemented root, Right – 2D fluorescence map taken from the same position showing calcium, iron and selenium, Middle – overlay of holotomogram and fluorescence maps

Speech: March 5th, 14:45, Hall 6

MULTISCALE SIMULATION APPROACHES TO UNDERSTAND BIOLOGICAL ENERGY CONVERSION IN MITOCHONDRIAL MEMBRANES

Vivek Sharma1, Marco Reidelbach1, Jonathan Lasham1, Amina Djurabekova1, Outi Haapanen1

vivek.sharma@helsinki.fi
1 Department of Physics, University of Helsinki, Finland


The fundamental process of biological energy conversion in mitochondria, bacteria and plants depends on the tight interplay between massive membrane-bound proteins and their lipidic environments. In the bioenergetic membranes of mitochondria, respiratory complexes catalyze the redox-reactions that are tightly coupled to proton pumping, which leads to the synthesis of ATP [1]. One of the key enzymes in this ATP-generating pathway is respiratory complex I, a ~1 MDa nanomachine, whose mechanism and function remains elusive despite recent surge in structural data from X-ray and cryo-EM techniques [2]. By applying physics-based multi-scale computations on the entire structure of bacterial complex I, we provided first putative insights into the mechanism of complex I [3,4], and found electrostatics and short-to-long ranged conformational dynamics central to the coupling mechanism. In this talk, I will present our recent results on the redox-chemistry and dynamics of quinone substrate in bacterial and mitochondrial complex I. The data from hybrid QM/MM calculations and classical atomistic molecular dynamics simulations provide a mechanistic picture that is congruent with recent proposals [1,2]. Moreover, encompassing dimensions, I will show
how combined coarse-grained and atomistic simulation approaches reveal importance of cardiolipin-
protein interactions in complex I stability and function. Understanding lipid-protein-solvent dynamics by means of physics-based computational approaches has potential in answering subtle questions on mitochondrial dysfunction, which remains a key cause for several metabolic and neuro-degenerative disorders with limited treatment options.

Acknowledgments: This work is supported by the research grants from the University of Helsinki,
Academy of Finland and the Sigrid Jusélius Foundation. CSC – IT Center for Science, Finland is
acknowledged for computing time.

[1] M. Wikström, V. Sharma, V. Kaila, J. Hosler, and G. Hummer, Chem. Rev., 115, 2196–2221 (2015).
[2] O. Haapanen and V. Sharma, Biochim. Biophys. Acta Biomembr., 7, 510–523 (2018).
[3] V. Sharma et al., Proc. Nat. Acad. Sci. USA, 112, 11571–11576 (2015).
[4] O. Haapanen and V. Sharma, Sci. Rep, 7, 7747 (2017).



Figure 1
Figure 1

Speech: March 5th, 15:00, Hall 6

ROLE OF SEIPIN IN LIPID DROPLET MATURATION

X. Prasanna1, V.T. Salo2,3, E. Ikonen2,3, I. Vattulainen1,4

xavierprasanna.anthonyraj@helsinki.fi
1 Department of Physics, University of Helsinki, 00014 Helsinki, Finland
2 Department of Anatomy, Faculty of Medicine, University of Helsinki, Helsinki, Finland
3 Minerva Foundation, Institute of Medical Research, Helsinki, Finland
4 Laboratory of Physics, Tampere University of Technology, 33101 Tampere, Finland



Speech: March 5th, 15:15, Hall 6

HEART RATE, HEART RATE VARIABILITY AND SKIN CONDUCTIVITY DURING COGNITIVE AND PHYSICAL CHALLENGES REQUIRING SISU

Johanna Närväinen1, Bart van der Sanden1, Pentti Henttonen2, Ilmari Määttänen2

johanna.narvainen@vtt.fi
1 VTT Technical Research Centre of Finland Ltd
2 University of Helsinki


The Finnish concept sisu refers to extraordinary tenacity and ability to push through challenges. However, it lacks a direct equivalent in other languages and has remained elusive and understudied. We have taken this challenge and developed a sisu questionnaire, studied sisu in the lab and will continue with sisu at work. This presentation is about the psychophysiological responses during a controlled induction of sisu. It is known that stress, load and emotions are reflected in biosignals [1] but the protocols used in previous research vary and the data are not usually combined to parameters characterizing the subjective appraisals and personality. The aim of this work was two-fold. First, we wish to gain understanding of the interrelations between personality, cognitive performance and the reflections of stress, load and use of sisu on the biosignals. Second, we want to acquire reference data for recognition of such events in peoples’ everyday life, based on limited and lower quality data from different wearable devices.
The sisu induction protocol consisted of baseline, anticipation and recovery periods, as well as physical (enduring the discomfort of hand immersion in ice water) and cognitive challenges (5 min of anagrams and 7 min of verbal puzzles, of which some subtasks were insolvable, and monitoring an eventless video for events). We also used Maastricht stress test (MAST) which interleaves 45-90 s periods of cold immersion and mental arithmetics (counting down from 2043 in steps of 17 with time pressure and penalty for mistakes). The tasks were run in randomized order using Presentation software. The aim of the protocol was to induce various types of stress but also to create situations where negative sisu (stubbornness, inability to move on) might present itself. We also logged the task execution, including the time spent on impossible tasks, as well as evaluations of emotional state and own performance. The subjects filled the novel sisu questionnaire, resulting in a sisu score, as well as several other personality/behavior questionnaires. The participants (n=24) were right-handed healthy young adults. EEG, EOG, ECG and skin conductivity EDA were recorded continuously @ 1000 Hz using 64 EEG +16 bipolar channel NeurOne acquisition system (Bittium, Finland). ECG was measured along the long axis of heart (electrodes below the left collarbone and on the right lower back) and EDA from the left hand (between index and middle fingers) which was kept immobile for the whole protocol.
All data analysis was done in Matlab, using house-made functions, Ledalab (www.ledalab.de) and HRVTool v1.02 (marcusvollmer.github.io/HRV/). For heart rate and variability (HR and HRV), a sliding window of 15 s (10 s overlap) was applied, and for EDA, the phasic component of downsampled (20 Hz) signal was extracted by the CDA method. Only HR(V) and EDA results are discussed here.
The HR, HRV and EDA averages over all subjects are shown in graphs A-F. The difference between rest and tasks throughout the whole protocol is evident in all parameters (A-C). HR decelerates and HRV and EDA recover during rest, as expected [1,2]. Also the switching between physical and cognitive stress in MAST is clear (D-F): the counting rounds elevate HR, indicating loading, and increase EDA and decrease HRV, both indicative of stress. Interestingly, during counting there seems to be a trend in HR and HRV suggesting that the loading takes place only 20-30 s after the task start, which coincides with typical task performance: the first numbers are often remembered from the previous rounds and the actual counting starts later. Perhaps surprisingly, the cold immersion of hand seems to have a relaxing effect, despite the discomfort and even pain that the subjects feel and express.
The data and results in this abstract are very preliminary and need to be referenced to baseline periods and then analyzed further to include EEG, detailed task performance data, subjective evaluations and sisu scores. Rather than looking at separate tasks, a bigger picture of individual responsivity and its relation to personality will be constructed using machine learning methods. Yet, already now it is evident that the type of the cognitive challenge or stressor is reflected in biosignals. These findings will help in interpreting the data acquired “in the wild”, and using that information to motivate and guide people to achieve a more balanced lifestyle and possibly learn to employ their good sisu more efficiently.
[1] Kreibig and Gross, Biological Psychology 84 (2010) 394–421; [2]Castaldo et al, Biomedical Signal Processing and Control 18 (2015) 370–377; [3] Bakker et al, 2011 IEEE 11th International Conference on Data Mining Workshops



Figure 1
Figure 1: Averages over all subjects of HR (beats-per-minute), HRV (rms of the standard deviations of inter-beat intervals, s) and the phasic component of EDA (a.u.). The colored blocks are the tasks: violet = challenges; yellow = 60 s anticipation periods before challenges; pink = baselines. The block averages are shown in A-C and the time-locked MAST time series are shown in D-F. In MAST, the grey blocks are cold immersion and the green blocks mental arithmetics.

Speech: March 5th, 15:30, Hall 6

INTRINSIC COMPLEXITY OF HEARTBEAT DYNAMICS AT THE CELLULAR LEVEL

J. Kim1, I. Potapov1, D. Shah2, K. Aalto-Setälä3,4, E. Räsänen1

jiyeong.kim@tuni.fi
1 Computational Physics Laboratory, Tampere University
2 Faculty of Medicine and Life Sciences, Tampere University
3 Faculty of Medicine and Life Sciences, Tampere University, Tampere, Finland
4 Heart Hospital, Tampere University Hospital


It has been well established that a healthy heart exhibits long-range correlated, or fractal, fluctuations in heart rate [1]. The complexity of heart rate variability is considered as an important measure in cardiac health, as it reflects the heart's ability to adapt to sudden perturbations. QT intervals also exhibit complex variability, which is useful in monitoring increased risks of fatal ventricular arrhythmias [2]. We assessed the intrinsic complexity of RR and QT intervals at the cellular level by studying inter-beat intervals (IBIs) and field potential durations (FPDs) measured directly from clusters of spontaneously beating human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) [3] under healthy and diseased conditions.

In this study, complexity is quantified by the sample entropy (SampEn) [4] of a time series. We used the multiscale sample entropy (MSE) method [5], in which SampEn is obtained from a set of time series that are coarse-grained with a range of scales. The MSE analysis provides a physiologically valid measure of complexity that reflects the dynamics of long-range correlated signals.

In a healthy condition, SampEn of RR and QT intervals retained constant values over most scales. SampEn of QT intervals peaked at the smallest scale but reduced to a constant within the first few scales. Similar patterns were observed for hiPSC-CMs. The hiPSC-CMs with symptomatic long QT syndrome exhibited monotonically decreasing SampEn throughout the scale, showing a clear difference from the healthy cases.

RR and QT interval time series of a healthy heart exhibits scale-invariant SampEn, which is a characteristic of long-range correlation. The complexity is intrinsic at the cellular level, as a similar multiscale property, especially in the large scale (>10) regime, is present in IBI and FPD time series of healthy hiPSC-CMs. The intrinsic complexity is noticeably altered in hiPSC-CMs with hereditary cardiac diseases; their MSE patterns are similar to those of uncorrelated white noise, indicating the loss of complexity.


[1] A. L. Goldberger, et al. Fractal dynamics in physiology: alterations with disease and aging. Proc. national academy sciences 99. suppl 1 (2002): 2466-2472.

[2] M. Baumert, et al. QT interval variability in body surface ECG: measurement, physiological basis, and clinical value: position statement and consensus guidance endorsed by the European Heart Rhythm Association jointly with the ESC Working Group on Cardiac Cellular Electrophysiology. Europace 18. 6 (2016): 925-944.

[3] S. Yamanaka. Induced pluripotent stem cells: past, present, and future. Cell stem cell 10 (2012): 678-684.

[4] J. S. Richman, et al. Physiological Time-Series Analysis Using Approximate Entropy and Sample Entropy. American Journal of Physiology Heart and Circulatory Physiology 278(6) (2000): H2039-H2049.

[5] M. Costa, et al. Multiscale Entropy Analysis of Complex Physiologic Time Series. Physical Review Letters 89(6) (2002): 068102.


Speech: March 5th, 15:45, Hall 6

COMMISSIONING OF THE NUBEAM BNCT SOURCE AT HELSINKI UNIVERSITY HOSPITAL CANCER CENTER

L. Porra1, T. Seppälä1, L. Vaalavirta1, H. Koivunoro, P. Eide, B. Park, N. Smick, M. Tenhunen1

liisa.porra@hus.fi
1 Comprehensive Cancer Center, Helsinki University Hospital, Finland
2 Neutron Therapeutics Finland Oy, Helsinki, Finland
3 Neutron Therapeutics Inc., Danvers, MA, USA


Radiation therapy is one of the most important treatments for cancer, and its need is constantly increasing as people age and due to the population growth. Radiation therapy is an effective treatment, but it has side effects because it affects both normal and cancerous tissue. With the advancement of technology and drug treatment, new targeted forms of radiation therapy have been developed where the radiation dose is directed directly to the tumor or tumor cells and thus reduces the adverse effects in healthy tissue. One form of biologically targeted radiotherapy is Boron Neutron Capture Therapy (BNCT), which has been used successfully to treat patients with advanced cancer in Finland in 1999-2012 [1]. The safety and efficacy of L-BPA-F mediated BNCT was evaluated in the clinical protocols of head and neck carcinomas and malignant gliomas using the epithermal neutron beam at research reactor FiR 1 in Otaniemi. After the shutdown of FiR 1 reactor, BNCT treatments have not been available in Finland. In overall, the lack of neutron sources suitable for hospital environment has limited the adoption of BNCT in the word.

HUS Helsinki University Hospital (HUS) and Neutron Therapeutics (NT) have recently started a joint project to install a nuBeam neutron source BNCT in the Helsinki Cancer Center. NT’s nuBeam is the first accelerator-based neutron source of its kind to operate in the hospital environment.

HUS is currently building a new radiation therapy facility for BNCT treatments inside the hospital area. The facility consists of an accelerator room, treatment room, control room, patient preparation room, boron laboratory and dosimetry rooms. For radiation safety reasons, the accelerator room, beamline room and treatment room are built with heavy concrete. In addition, the treatment room walls, doors and floor are covered with lithium plastic to slow down and absorb the neutrons. Doors are shielded with lead and equipped with “last-man-out” sweep switches. The treatment room is equipped with a patient positioning system and a CT for image-guided BNCT treatment.

The project aims at a clinically functional BNCT treatment. The starting point and the goal are based on the experience accumulated during the Finnish research reactor-based BNCT project. The current project has three phases: the installation and testing of the neutron accelerator (step 1), clinical BNCT trials with ethically approved protocols (step 2), and established use of BNCT through clinical protocols (step 3). Neutron source is currently being installed at the facility, and preliminary measurements of the beam are undergoing.

This presentation focuses in the first step of the project from a physicist point of view, where the accelerator is installed. This includes radiation safety arrangements and measurements, set-up of a dosimetry laboratory, and beam characterization measurements for dose planning. When the patient safety of the BNCT system required for clinical use is confirmed, clinical BNCT trials can start.

[1] Savolainen et al. Boron neutron capture therapy (BNCT) in Finland: Technological and physical prospects after 20 years of experiences. Phys Med. 2013 May;29(3):233-48.


Quantum Devices & Information

Location: Hall 6
Time: March 7th, 11:00 - 12:30

Speech: March 7th, 11:00, Hall 6

ULTRASENSITIVE GRAPHENE BOLOMETER

J-Ph Girard1, R. Kokkoniemi1, J. Govenius2, V. I. Vesterinen2, R. Lake1, K. Y. Tan1, D. Hazra1, I. Sallinen1, A. Laitinen1, P.J. Hakonen1, M. Möttönen1

jean-philippe.girard@aalto.fi
1 Aalto University
2 VTT




Single-microwave-photon detection would provide a versatile tool for circuit quantum electrodynamics, which is a promising field to implement quantum computing. Currently, single-photon detectors are readily available at optical wavelengths. However, the required sensitivity of thermal detectors for single microwave photons has not been achieved yet.
To address this issue, we develop a bolometer based on a graphene Josephson junction. Due to its unusual thermal properties, graphene is a promising material to implement high-sensitivity microwave photon detection.
We couple the graphene Josephson junction to on-chip capacitors forming a temperature dependent LC oscillator. Radiation incident on the graphene modifies the resonance frequency of the oscillator, which server as our thermometer.
Our preliminary results suggest a noise equivalent power of $\rm{NEP} = 20\, \rm{zW}/\sqrt{\rm{Hz}}$ and an energy resolution in the yJ range.


Speech: March 7th, 11:15, Hall 6

UNIVERSAL EQUATION OF MOTION FOR OPEN QUANTUM SYSTEMS: ROLE OF CORRELATION

S. Alipour1, A. T. Rezakhani2, A. P. Babu1, K. Mølmer3, M. Möttönen4, T. Ala-Nissila1,5

sahar.alipour@aalto.fi
1 QTF Center of Excellence, Department of Applied Physics, Aalto University, P. O. Box 11000, FI-00076 Aalto, Espoo, Finland
2 Department of Physics, Sharif University of Technology, Tehran 14588, Iran
3 Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000, Aarhus C, Denmark
4 QCD Labs, QTF Center of Excellence, Department of Applied Physics, Aalto University, P. O. Box 13500, FI-00076 Aalto, Espoo, Finland
5 Interdisciplinary Centre for Mathematical Modelling and Department of Mathematical Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK


The recent rise in high-fidelity quantum technological devices has necessitated detailed understanding of open quantum systems and how their environment influences their performance. However, it has remained unresolved how to include explicitly known correlations between a system and its environment in the dynamical evolution. In the standard weak-coupling (Born-Markov) regime, the explicit dependence of the dynamics on correlations are neglected. Beyond this regime, more general equations have been obtained, yet without explicit dependence on correlations. Here, we propose a correlation picture which allows to derive an exact dynamical equation with system-environment correlations included. We show that systematic approximations to this equation yield conveniently solvable master equations. Our formalism provides a powerful approach to describe open systems and may give new insight in fundamental processes such as thermalization.


Speech: March 7th, 11:30, Hall 6

ASPECTS OF CAPACITY OF ENTANGLEMENT

Jan De Boer1, Jarkko Järvelä2,3, Esko Keski-Vakkuri2,3

esko.keski.vakkuri@helsinki.fi
1 University of Amsterdam
2 University of Helsinki
3 Helsinki Institute of Physics


Many quantum information theoretic quantities are similar to and/or inspired by thermodynamic quantities, with entanglement entropy being a well-known example. In this work, we study a less well-known example, capacity of entanglement, which is the quantum information theoretic counterpart of heat capacity. It can be defined as the second cumulant of the entanglement spectrum and can be loosely thought of as the variance in the entanglement entropy.

We review the definition of capacity of entanglement and its relation to various other quantities such as fidelity susceptibility and Fisher information. We then calculate the capacity of entanglement for various quantum systems, conformal and non-conformal quantum field theories in various dimensions, and examine their holographic gravity duals. Resembling the relation between response coefficients and order parameter fluctuations in Landau-Ginzburg theories, the capacity of entanglement in field theory is related to integrated gravity fluctuations in the bulk.
We address the question of measurability, in the context of proposals to measure entanglement and R\'enyi entropies by relating them to U(1) charges fluctuating in and out of a subregion, for systems equivalent to non-interacting fermions.

From our analysis, we find universal features in conformal field theories, in particular the area dependence of the capacity of entanglement appears to track that of the entanglement entropy. This relation is seen to be modified under perturbations from conformal invariance. In quenched 1+1 dimensional CFTs, we compute the rate of growth of the capacity of entanglement. The result may be used to refine the interpretation of entanglement spreading being carried by ballistic propagation of entangled quasiparticle pairs created at the quench.


Speech: March 7th, 11:45, Hall 6

TOWARDS MICROWAVE OPTOMECHANICS USING A SUPERCONDUCTING CARBON NANOTUBE WEAK LINK

M. Will1, J.-P. Kaikkonen1, T.S. Abhilash1, D. Golubev1, P. Hakonen1

marco.will@aalto.fi
1 School of Science, Low Temperature Laboratory, Aalto University, FI-00076 AALTO, Finland


Clean, in-situ-grown single-walled carbon nanotubes (SWCNT) facilitate creation of a variety of high-sensitivity sensors. In contact with superconducting metals, high-quality SWCNTs can be employed to sustain proximity-induced supercurrents that are extremely sensitive to charge induced on it by tiny electric fields. Such superconducting devices provide excellent sensors to probe the quantum ground state of a mechanical resonator and the related macroscopic quantum phenomena.

Reproducible and reliable fabrication of suspended superconducting SWCNT devices, however, is still to be demonstrated because of the challenging demands on high temperature stable materials that the SWCNT growth requires and the crucial role the contact resistance plays for inducing superconductivity into the SWCNT. We approach the challenge with suspended, 300 nm long SWCNT contacted on MoRe leads. Good transparency of the superconductor-nanotube contacts allows observation of proximity-induced supercurrents of up to 45 nA, tunable by gate induced charge. Under rf irradiation the SWCNT display clear Shapiro steps which depend on the rf frequency and power. Furthermore, mechanical resonances of around 1.5 GHz with quality factors up to $\textit{Q}$=$15000$ are observed. In the superconducting regime the mechanical mode can be observed by inducing Shapiro steps resonantly with the mechanical mode. Using such weak links in an optomechanical microwave setting (cavity frequency ~7GHz), coupling energies on the order of 700 kHz can be reached between the mechanical resonator and the electrical cavity. Provided that the quality factor $\textit{Q}$ of the microwave cavity can be kept above $10000$ (as well as the mechanical $\textit{Q}$ $> 1000$), an optomechanical setting with ultra strong coupling can be reached.



Figure 1
Figure 1: a) Typical suspended SWCNT and array of devices. b) Mechanical resonance frequency of suspended SWCNT in dependence of gate voltage.

Speech: March 7th, 12:00, Hall 6

SUPREMACY OF INCOHERENT SUDDEN CYCLES

Jukka P. Pekola1, Bayan Karimi1, George Thomas1, Dmitri V. Averin2

george.thomas@aalto.fi
1 QTF centre of excellence, Department of Applied Physics, Aalto University School of Science, P.O. Box 13500, 00076 Aalto, Finland
2 Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, NY 11794-3800, USA


We investigate theoretically a refrigerator based on a two-level system (TLS) coupled alternately to two different heat baths [1]. Modulation of the coupling is achieved by tuning the level spacing of the TLS as shown in Fig. 1. We find that the TLS, which avoids quantum coherences, creates finite cooling power for one of the baths in sudden cycles, i.e. acts as a refrigerator even in the limit of infinite operation frequency. By contrast, the cycles that create quantum coherence in the sudden expansions and compressions lead to heating of both the baths. We propose a driving method that avoids creating coherence and thus restores the cooling in this system. We also discuss a physical realization of the cycle based on a superconducting qubit coupled to dissipative LC-resonators.


Reference:
1. J. P. Pekola, B. Karimi, G. Thomas and D. A. Averin, arXiv:1812.10933 [quant-ph] (2018).



Figure 1
Figure 1: (a) The cooling cycle where the TLS couples alternately to one of the baths at a time. The interaction of the TLS with each bath is controlled by the level separation: for small (large) splitting it exchanges energy with the cold (hot) bath. The green arrows depict the abrupt expansion (compression). (b) The driving protocol in time, demonstrating one cycle of the process in (a). In a sudden cycle, $\delta t \rightarrow 0$. (c) Potential experimental realization.

Speech: March 7th, 12:15, Hall 6

MAJORANA REPRESENTATION OF SUPERADIABATIC PROCESSES

S. Dogra1, A. Vepsäläinen1,2, M. Haataja1, G. S. Paraoanu1

shruti.dogra@aalto.fi
1 Aalto University
2 Massachusetts Institute of Technology


The Majorana representation is a geometrical realization of the state of a spin-$s$ by $2s$ points on a unit sphere [1]. It is a good tool to study the dynamics of a spin-$s$ and develop an intuitive picture. We use the Majorana representation to observe spin-$1$ dynamics under the effect of superadiabatic processes.
There have been recent advancements that exploit the Stimulated Raman Adiabatic Passage (STIRAP) to achieve non-trivial quantum operations with good fidelities, that lead to population exchange between the energy levels adiabatically [2,3], and thus further used to develop a NOT-gate [4]. A circuit quantum electodynamics-based implementation first benchmarked the protocol for STIRAP pulses, where it is experimentally realized using first three energy levels of a transmon, forming a qutrit [2]. It is well-known that adibaticity requirements require ideally infinitely long operation times, and therefore for realistic finite-time experimental conditions one expects a tradeoff between time and fidelity. A smart solution is obtained using superadiabatic(sa)-STIRAP pulses [3], which introduces a counter-adiabatic drive to correct for any non-adiabaticity in the dynamics.
This work is focused on the geometrical representation of sa-STIRAP pulses on a three-level quantum system on Majorana sphere. Contrary to STIRAP pulses, sa-STIRAP pulses confine the single qutrit dynamics to a plane, which is also analogous to the characteristic parametric-evolution of a single-qutrit canonical state on a Majorana sphere [5]. Dynamics under STIRAP versus sa-STIRAP pulses on the Majorana sphere will also be discussed.

References:
1. Atomi orientati in campo magnetico variable, E. Majorana, Nuovo Cimento, 9, 43 (1932).
2. Stimulated Raman adiabatic passage in a three-level superconducting circuit, K.S. Kumar, A. Vepsäläinen, S. Danilin, and G.S. Paraoanu, Nature Communications, 7, 10628 (2016).
3. Superadiabatic population transfer by loop driving and synthetic gauges in a superconducting circuit, A. Vepsäläinen, S. Danilin, and G.S. Paraoanu, quant-ph/arXiv:1709.03731 (2017).
4. Optimal superadiabatic population transfer and gates by dynamical phase corrections, A. Vepsäläinen, S. Danilin, and G.S. Paraoanu, Quantum Sci. Technol., 3, 024006 (2018).
5. Majorana representation, qutrit Hilbert space and NMR implementation of qutrit gates, S. Dogra, K. Dorai, and Arvind, J. Phys. B: At. Mol. Opt. Phys., 51, 045505 (2018).


Scientific Computing, Machine Learning, Big Data: New Tools

Location: Hall 1
Time: March 6th, 11:00 - 12:30

Speech: March 6th, 11:00, Hall 1

CHEMICAL DIVERSITY IN MOLECULAR ORBITAL ENERGY PREDICTIONS WITH KERNEL RIDGE REGRESSION

Annika Stuke1, Milica Todorović1, Matthias Rupp2, Christian Kunkel3,1, Kunal Ghosh1,4, Lauri Himanen1, Patrick Rinke1

annika.stuke@aalto.fi
1 Department of Applied Physics, Aalto University, P.O. Box 11100, Aalto FI-00076, Finland
2 Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany
3 Chair for Theoretical Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstr. 4, D-85747 Garching, Germany
4 Department of Computer Science, Aalto University, P.O. Box 15400, Aaalto FI-00076, Finland


The predictive power of machine learning methods for instantaneous molecular property predictions is commonly tested on well-established benchmark datasets like QM9 [1]. However, it remains uncertain how conclusions from such studies extend to more chemically diverse settings. We therefore apply machine learning to two lesser-known, diverse datasets of technological relevance: a set of amino acid and dipeptide conformers [2], referred to as AA, and a set of crystal-forming, optically-active molecules, referred to as OE. The molecular size distribution in panel a) of Figure 1 and the $t$-Distributed Stochastic Neighbor Embedding ($t$-SNE) similarity analysis in panel b) illustrate the diversity of the three datasets.
Our machine learning model is based on kernel ridge regression and our target property is the highest occupied molecular orbital (HOMO) energy, for feasibility computed at the level of density-functional theory. Two different representations to encode molecular structures are studied: the Coulomb matrix (CM) [3] (shown in panel c)) and the many-body tensor representation (MBTR) [4]. We find that kernel ridge regression performance inherently depends on the chemical complexity of the underlying dataset, in addition to the training set size and molecular representation [5]. For two of the three datasets, MBTR outperforms the CM, predicting HOMO energies with a mean absolute error (MAE) as low as 0.09 eV. The learning curves for MBTR shown in panel d) reflect the different chemistries in the three employed datasets. The OE dataset features the largest chemical and structural diversity among all three datasets and is therefore much more difficult to learn than QM9 and AA, resulting in slow learning rates and considerably high MAEs.

[1] R. Ramakrishnan, P. O. Dral, M. Rupp, and O. A. von Lilienfeld, Sci. Data 1 (2014)
[2] M. Ropo, M. Schneider, C. Baldauf, and V. Blum, Sci. Data 3 (2016)
[3] M. Rupp, A. Tkatchenko, K.-R. Müller, and O. A. von Lilienfeld, Phys. Rev. Lett. 108 (2012)
[4] H. Huo and M. Rupp, arXiv:1704.06439 [cond-mat, physics:physics] (2017)
[5] A. Stuke, M. Todorović, M. Rupp, K. Gosh, L. Himanen, and P. Rinke, arXiv:1812.08576 [physics.chem-ph] (2018)



Figure 1
Figure 1: a) Distribution of molecular size within the three different datasets QM9 (green), AA (blue) and OE (red). b) 2D-map of the three datasets visualized by the dimensionality reduction technique $t$-SNE. OE molecules are widely spread out in both dimensions, while AA and QM9 molecules form their own groups. c) CM representation of the molecule 2-chloro-5-nitropyridin-4-amine taken from the OE dataset. d) Learning curves for all three datasets, where MBTR is used as molecular representation.

Speech: March 6th, 11:15, Hall 1

SCIENTIFIC COMPUTING, MACHINE LEARNING, BIG DATA: NEW TOOLS, INDEED!

Pekka Manninen1

pekka.manninen@csc.fi
1 CSC - IT Center for Science Ltd, PO Box 405, FI-02101 Espoo, Finland.


Finland will procure a new supercomputing infrastructure that will provide Finnish researchers with European leading computing capacity and all-new capabilities. The new supercomputer will provide Finnish researchers six times more the capacity than currently and enable researchers to, e.g., predict climate change and its effects, go deeper into the quantum mechanical world, solve problems around renewable and fusion energy, utilize Artificial Intelligence (AI) to develop new medical treatments, and solve other computationally intensive problems. New solutions will be introduced for handling large and complex scientific data. The setup will also include a solution dedicated to advanced AI research. In this presentation, the new infrastructure and its installation timeline will be presented, hardware specifications discussed, and the related service portfolio introduced.


Speech: March 6th, 11:30, Hall 1

MACHINE LEARNING MODELS FOR CU SURFACE KINETIC MONTE CARLO

J. Kimari1, V. Jansson1, S. Vigonski1,2, V. Bazaliy1, J. Määttä1, T. Roos1, V. Zadin2, F. Djurabekova1

jyri.kimari@helsinki.fi
1 University of Helsinki
2 University of Tartu


Atomistic Kinetic Monte Carlo (KMC) is an efficient simulation method for studying slow processes such as diffusion. The cost of efficiency is the requirement to calculate the energy barriers of all possible transition events in the system. Depending on the number of these events, which in turn depends on the complexity of the system and the desired accuracy of the simulation, compiling and storing the list of energy barriers beforehand may be infeasible.

Machine learning offers an alternative approach to the problem of numerous barrier calculations. Given a way to describe the atomic environments associated with each energy barrier, a database of inputs and outputs can be constructed and used to train a machine learning function that can be called to predict barriers at KMC runtime.

We have calculated a database of energy barriers for Cu surface migration processes with the nudged elastic band (NEB) method, and trained different machine learning models, such as artificial neural networks (ANN), Gaussian processes (GP) and radial basis functions (RBF), to fit the data. The models were also tested in KMC simulations. Based on the simulation results, sufficient accuracy for barrier predictions in this system has been achieved.



Figure 1
Figure 1: From left to right: a local atomic environment of a migration process, some radial basis function prototypes learned from the barrier data, the prediction performance of the machine learning model, and a KMC simulation using the barriers predicted by the model.

Speech: March 6th, 11:45, Hall 1

DEEP NEURAL NETWORKS IN QUARK AND GLUON JET IDENTIFICATION

Kimmo Kallonen1

kimmo.kallonen@cern.ch
1 Helsinki Institute of Physics


Quarks and gluons are elementary particles, which produce collimated sprays of particles, called jets, when protons are collided together in the Large Hadron Collider particle accelerator. These jets are present in practically every collision event of interest. The resulting particles from a collision event are measured by huge particle detectors, such as the Compact Muon Solenoid (CMS). A major challenge faced by the CMS experiment is distinguishing between jets produced by gluons and the lightest three quarks, since on a superficial level they appear very similar. Subtle differences between the two types of jets are caused by the fact that, on average, gluons induce more additional radiation. Traditional algorithms dedicated to the identification of these jets have been simple likelihood-based discriminators. This kind of discriminator is built on a few theoretically motivated high-level variables. However, as the event reconstruction algorithm of CMS allows the jets to be studied as a collection of individual particles, it is possible to construct more complex representations of the jets. Deep neural networks (DNNs) can make use of these representations and find more nuanced discriminating features from them. Several DNN-based approaches have been developed in hopes of achieving higher level of performance in the task in comparison to the traditional approach. Here three different DNN models for quark and gluon jet identification are presented. The results of the comparative study are shown with a likelihood-based discriminator used as the benchmark model.


Speech: March 6th, 12:00, Hall 1

CHAOTIC CAVITY BASED ACOUSTIC LOCATION DETECTOR AUGMENTED WITH ARTIFICIAL INTELLIGENCE

T. Sillanpää, T. Rauhala2, C. Rajani3, K. Longi3, A. Klami3, A. Salmi, E. Hæggström

tom.sillanpaa@helsinki.fi
1 Department of Physics, P.O.B 64, FIN-00014, University of Helsinki, Finland
2 Altum Technologies
3 Department of Computer Science, P.O.B 68, FIN-00014, University of Helsinki, Finland


Detection of internal structures inside containers that are opaque to light is challenging. The applications for such techniques are wide, e.g. detection of location of humans or animals inside rooms or detection of location of defects in otherwise homogeneous media. One approach for this is ultrasonic imaging. Traditionally ultrasonic imaging requires many transducers. In addition, the inverse problems arising from the unknown number of defects and their unknown locations are challenging.

To tackle this problem, we employ chaotic cavities, that spread the beam of transducers, and artificial intelligence (AI) methods based on machine learning, that allow interpreting complex ultrasonic signals. A simple setup (Fig. 1A) was built from an acrylic cylinder with one internal structure, a Ø=25 mm acrylic pipe with water inside it. A 1.3 MHz transducer was attached to a chaotic cavity (Fig. 1B), which was glued to the container wall. Data for training the AI method, a deep neural network, was gathered by moving the pipe to randomly selected locations and the ultrasonic signals measured were saved together with the locations into a database. After teaching the AI, a series of test measurements was performed to quantify the accuracy of the location detection. In this contribution, we discuss the accuracy of the AI-based location prediction compared to measured data.



Figure 1
Figure 1: A) Measurement setup consisting of motorized arm translating the acrylic pipe (D = 25 mm) to randomized positions inside the acrylic cylinder (D = 290 mm). B) Close up view of the transducer attached to the chaotic cavity, which was glued to the container wall.

Speech: March 6th, 12:15, Hall 1

OPTICAL PARAMETER ESTIMATION FROM PRESSURE TIME SERIES IN QUANTITATIVE PHOTOACOUSTIC TOMOGRAPHY

Aki Pulkkinen1, Tanja Tarvainen1,2

Aki.Pulkkinen@uef.fi
1 Department of Applied Physics, University of Eastern Finland
2 Department of Computer Science, University College London


Photoacoustic tomography (PAT) is an imaging technique based on ultrasonic emission followed by absorption of a short light pulse in tissue. Quantitative photoacoustic tomography (QPAT) seeks to estimate optical absorption and scattering distributions in imaged medium based on the acoustic pressure time series [1].

Conventionally the inverse problem in QPAT is approached in two folded manner: first, an acoustical inverse problem is solved to estimate an initial pressure distribution describing the absorption of light; second, an optical inverse problem is solved to estimate the optical parameters based on the initial pressure distribution. In this work, a direct estimation approach is utilized [3]. This enables estimation of optical parameters without need to estimate intermediate parameters.

The optical forward model utilized in this work is based on the diffusion approximation (DA) of the radiative transfer equation (RTE). For the acoustical model, a homogeneous linear wave-equation is used. The equations are coupled via the photoacoustic effect. The DA is approximated using finite element method (FEM) and the wave-equation by using numerical approximations of Green's function. The models are combined to form a single forward model mapping the optical parameters into the pressure time series. The inverse problem is expressed using a Bayesian approach [2], where the unknown parameters are expressed as random variables and their statistics is inferred based on the measurements and prior information. In addition to point estimates of the unknowns, the approach enables evaluation of the credibility intervals of the estimates. For details of the approach, see Ref. [3].

The approach is studied in acoustically full and limited view measurement settings both in two and three dimensions. The results demonstrate, that QPAT should be feasible in even acoustically limited view measurement configurations.

[1] B. T. Cox, S. R. Arridge, K. P. Köstli, and P. C. Beard, Two-dimensional quantitative photoacoustic image reconstruction of absorption distributions in scattering media by use of a simple iterative method, Applied Optics 45 (8), 1866—1875 (2006).
[2] J. P. Kaipio, and E. Somersalo, Statistical and Computational Inverse Problems, Springer, 2005.
[3] A. Pulkkinen, B. T. Cox, S. R. Arridge, H. Goh, J. P. Kaipio, and T. Tarvainen, Direct estimation of optical parameters from photoacoustic time series in quantitative photoacoustic tomography, IEEE Transactions on Medical Imaging, 35 (11), 2497—2508 (2016).


Academy of Finland session: Novel Applications of AI in Physical Sciences and Engineering research

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Academy of Finland session: Novel Applications of AI in Physical Sciences and Engineering research

Location: Hall 6
Time: March 6th, 17:00-18:00

Speech: March 6th, 16:00, Hall 6

CATAFM - RECOGNISING MOLECULES IN ATOMIC RESOLUTION FORCE MICROSCOPY

Adam Foster1

adam.foster@aalto.fi
1 Department of Applied Physics, Aalto University


Atomic Force Microscopy (AFM) lies centrally within developments in nanotechnology without material restrictions and is increasingly being used for nanoscale characterization in a wide variety of physical, biological and chemical processes. In general for AFM, the tip itself has often been the barrier to translating atomic (and beyond) resolution into physical understanding, with many images and processes ultimately being identified as a convolution with the tip structure. Recently, the use of a relatively inert functionalized tip means it can approach very close to the object of interest, allowing the interaction to be dominated by extremely short-range Pauli repulsion between atoms in the sample and at the tip apex - this provides the very high resolution at the heart of this new approach. However, once the technique moves into the much wider world of three-dimensional molecular structures, the link between image and interpretation becomes much more complex, and cannot be elucidated from current modelling approaches. This is a significant barrier to the wider adoption of AFM in molecular characterisation, and prevents its obvious potential being fully realised.

The CATAFM project targets an opportunity to develop a systematic software approach to understand and predict AFM images for molecules of any size, configuration or orientation. We use the latest modelling approaches to build a database of 3D AFM images for a wide variety of molecular structures - effectively a computational tomographic approach. This is integrated into a machine learning infrastructure that can then predict molecular structure directly from an arbitrary AFM experimental image, without any of the current constraints on dimensionality and shape. This opens the door to apply this powerful technique to a huge variety of systems where routine atomic and chemical structural resolution can be a major breakthrough.



Figure 1
Figure 1: Schematic showing the database generation and machine learning infrastructure of the CATAFM project.

Speech: March 6th, 16:15, Hall 6

MACHINE LEARNING ANALYSIS OF EXTREME EVENTS IN OPTICAL FIBRE MODULATION INSTABILITY

L. Salmela1, M. Närhi1, J. Toivonen1, C. Billet2, J. M. Dudley2, G. Genty1

lauri.salmela@tuni.fi
1 Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland
2 Institut FEMTO-ST, Université Bourgogne Franche-Comté CNRS UMR 6174, Besançon, France


Modulation instability (MI) is a universal process in nonlinear physics that describes the exponential growth of weak perturbations or noise on top of a continuous wave input signal. When seeded by noise, MI leads to the emergence of high intensity localized breather structures that show complex dynamics with random statistics. It has been also suggested that MI may be linked with the formation of rogue waves on the surface of the ocean [1]. Despite the recent developments in real-time measurement techniques both in the spectral and temporal domain [2], the study of instabilities in nonlinear fibre optics still remains limited in various ways. For example, in addition to limitations on measurement power and bandwidth, the limited dynamic range of the spectral measurements restricts the type of dynamics that can be observed [3]. Here, we overcome these restrictions by combining techniques of machine learning with a novel high-dynamic range ($>$50 dB) real-time spectral intensity measurement scheme to yield qualitative information about the temporal characteristics of a random MI field.

Figure 1 shows a schematic of the experimental setup along with a selection of recorded single-shot spectra when pulses (3 ps duration, 175 W peak power) from a Ti:Sapphire laser at 825 nm are injected into a 68 cm long photonic crystal fibre (PCF) in the anomalous dispersion regime to generate a random MI field. By using a rapidly-rotating mirror, sequential pulses are focused on different vertical positions of the entrance slit of a Czerny-Turner (C-T) spectrograph. Spectral windowing and differential attenuation are used to take advantage of the full dynamic range of an EMCCD camera, yielding a single-shot dynamic range exceeding 50 dB and with 1 nm spectral resolution. The experimental spectra are then analyzed using a neural network (NN) trained for relating the temporal properties of the noisy MI field with the corresponding spectral intensity characteristics. The NN trained from generalized nonlinear Schrödinger equation simulations parameterized to our experiments is capable of accurately predicting the temporal shot-to-shot statistics of the maximum temporal intensity solely based on spectral intensity measurements. Additionally, an unsupervised clustering analysis was used for classifying the spectra into subsets with distinct temporal structures (not shown here). Our results are highly significant since they are the first demonstration of spectral measurements combined with machine learning to predict the occurrence extreme events in a nonlinear optical system.

References
[1] D.R. Solli, C. Ropers, P. Koonath and B. Jalali, "Optical rogue waves", Nature 450, 1054-1057 (2007).
[2] P. Ryczkowski, et al., "Real-time full-field characterization of transient dissipative soliton dynamics in a mode-locked laser", Nat. Photonics 12, 221–227 (2018).
[3] N. Akhmediev, et al., "Rogue wave early warning through spectral measurements?", Phys. Lett. A 375.3, 541- 544 (2011).



Figure 1
Figure 1: (a) Schematic of the experimental setup. Ti:Sa: Titanium-Sapphire mode-locked laser, AOM: acousto-optic modulator, PCF: photonic crystal fibre, ND: neutral density. (b) Example of recorded single-shot spectra with $>$50 dB dynamic range (blue) and the mean spectrum (black).

Speech: March 6th, 16:30, Hall 6

STRUCTURE PREDICTION OF HYBRID NANOPARTICLES VIA ARTIFICIAL INTELLIGENCE

Antti Pihlajamäki1, Joakim Linja2, Joonas Hämäläinen2, Paavo Nieminen2, Sami Malola1, Tommi Kärkkäinen2, Hannu Häkkinen1,3

antti.e.pihlajamaki@student.jyu.fi
1 University of Jyväskylä, Department of Physics, Nanoscience Center
2 University of Jyväskylä, Faculty of Information Technology
3 University of Jyväskylä, Department of Chemistry, Nanoscience Center


In the fields of imaging, catalysis and medicine hybrid nanoparticles or
monolayer protected clusters (MPCs) have shown great results and promises
[1, 2, 3]. The structure of these particles consist of three parts: metallic
core, metal-ligand interface and protecting ligands [4]. In order to truly
understand the properties and behaviour of them, both experiments and
theoretical computations are needed. The usual choice of computational
method is density functional theory (DFT), which is known to be accurate
but also demanding. Our goal is utilize artificial intelligence and machine
learning methods to predict and model the properties of MPCs. Ultimately
these methods should reduce the computational burden.

Currently we are developing methods to predict the positions of sulfur
atoms in the metal-ligand interface of the gold clusters [5]. The positions of
the sulfur atoms are determined by the local chemical environments. This
chemical information is extracted from the known structures and in the cur-
rent algorithm it is used to define the rules of sulfur atom positions. The
algorithm uses these rules to predict where are the most probable positions
of sulfur atoms for the structure of interest. The versions under development
are relaying on machine learning and heuristic algorithms.

References
[1] Y. Zhu, H. Qian, M. Zhu, and R. Jin, Thiolate-protected $\text{Au}_n$ nanoclus-
ters as catalysts for selective oxidation and hydrogenation processes, Adv.
Mater. 22, 17 (2010), doi:10.1002/adma.200903934
[2] Y.-C. Shiang, C.-C. Huang, W.-Y. Chen, and P.-C. Chen, H.-T. Chang,
Fluorescent gold and silver nanoclusters for the analysis of biopolymers
and cell imaging, J. Mater. Chem. 22, (2012), doi:10.1039/c2jm30563a
[3] Z. Luo, K. Zheng, and J. Xie, Engineering ultrasmall water-soluble gold
and silver nanoclusters for biomedical applications, Chem. Commun. 50,
(2014), doi:10.1039/c3cc47512c
[4] T. Tsukuda, H. Häkkinen, Protected Metal Clusters: From Fundamentals
to Applications, Elsevier, (2015)
[5] S. Malola, P. Nieminen, J. Hämäläinen, T. Kärkkäinen, and H. Häkkinen,
Generalized algorithm for structure prediction of metal-ligand interfaces,
manuscript in preparation


Speech: March 6th, 16:45, Hall 6

BAYESIAN INFERENCE FOR ORGANIC-INORGANIC INTERFACES

Patrick Rinke1

patrick.rinke@aalto.fi
1 Department of Applied Physics, Aalto University, Finland


Quantum mechanical accuracy is required to simulate hybrid organic-inorganic interfaces. Here we focus first on the structures and properties of molecules on surfaces. A problem we face is that accurate calculations are costly and extensive sampling is prohibitive, so studies into molecular assembly and surface-supported processes like diffusion are guided by human intuition. To promote unbiased studies into molecular surface structures and phenomena, we have combined atomistic simulations with Bayesian optimisation - an artificial intelligence (AI) technique designed for complicated optimisation tasks [1]. We demonstrate how the AI was adapted to learn surface and property landscapes of molecules on surface with minimal computational sampling [2], delivering most stable surface structures with favorable designer properties. Energy landscapes can be further data-mined for low energy paths and associated trajectories to reveal the atomistic mechanisms behind key processes. We showcase the capability of AI to infer complex properties on several examples of atomic and molecular surface adsorbates (see Figure 1).

[1] M. U. Gutmann, J. Corander, J. Mach. Learn. Res. 17, 1 (2016).
[2] M.Todorović, M. U. Gutmann, J. Corander and P. Rinke, arXiv:1708.09274 (2017).



Figure 1
Figure 1: AI applied to different organic/inorganic interface problems: a) complex oxide adsorption energy lanscape for coronene/Cu(110)-O c(6x2); b) simultaneous inference of adsorption energy and charge transfer landscapes for benzene/Cu(100).

IoP Finland Chapter session

See the title in compact form

IOP Finland Chapter session

Location: Hall 13
Time: March 5th, 16:00 - 17:00

Speech: March 5th, 16:00, Hall 13

USING CONTEXT TO ENGAGE STUDENTS

Tajamal Bhutta




The Institute of Physics (IOP) has developed a number of context-based physics activities for science clubs and outreach. This workshop will introduce hands-on activities that illustrate two areas: - how physics applies to aircraft design. - the search for habitable planets around other stars. Participants of this workshop will be provided with copies of relevant resources produced by the IOP.


Poster Session A

See titles in compact form

First Floor

Time: March 5th, 16:00 - 18:00

Poster: A101, March 5th, 16:00 - 18:00, 1st floor

TISSUE CHARACTERIZATION BY FAST GENERATION OF SHOCKWAVES

D.Veira Canle1, P.Ihalainen1, A.Salmi1, A.Kontiola2, E.Hæggström1

daniel.veiracanle@helsinki.fi
1 Department of Physics, Division of Materials Physics, University of Helsinki, Finland
2 Photono Oy, Helsinki, Finland


Fast generation of guided shockwaves is a unique actuation mechanism for the generation of surface waves on membranes. Such a generation mechanism would allow to study the mechanical properties of biological samples that are susceptible to change. One could characterize the mechanical properties of tissue by studying surface wave propagation whilst diminishing the temporal variation, such as heartbeat.
To achieve this goal, we propose a circuit board design that switches an inductor and charges a 100 nF capacitor to 2 kV. A secondary spark triggers the release of the energy stored in the capacitor which is guided through a tube as a shockwave. By means of wave guiding one can direct the acoustic energy produced by the sparks into the area of study. We reached a repetition rate of 3.3 Hz for 160 mJ electric sparks.
We believe that this kind of mechanical actuation will find application in non-contact characterization of tissue stiffness thus reducing the need for invasive techniques. This is particularly important since this approach reduces the risk of an infection and patient discomfort.



Figure 1
Figure 1: Figure 1. Charging curve of 100 nF capacitor. It takes approximately 300 ms to charge the capacitor to 1.8 kV. The voltage was measured with a 1 GΩ resistor and the oscilloscope impedance was 10 MΩ. That makes for an attenuation factor of 100 meaning that at this load the voltage across the capacitor is close to 1.8 kV.

Poster: A102, March 5th, 16:00 - 18:00, 1st floor

PLASMA CREATION WITH A FARNSWORTH-HIRSCH FUSOR

T. Lindén1, S. Galambosi2, V. Litichevskyi1, S. Maisala3, P. Metsä3, J. Orava2, H. Saarikko2, O. Solin2

Tomas.Linden@Helsinki.FI
1 Helsinki Institute of Physics
2 Department of Physics, University of Helsinki
3 Computing Center, University of Helsinki


A Farnsworth-Hirsch fusor is a device using only an electric field to ionize, accelerate and confine ions to produce fusion reactions. The Farnsworth-Hirsch fusor is an example of an Inertial Electrostatic Confinement (IEC) fusion device. A fusor provides likely the simplest way to achieve deuterium-deuterium fusion. Fusors have also been used to study the deuterium-$^3$He fusion reaction. Fusors have been used as neutron sources for activation analysis, isotope generation and material research.

Because of its relative simplicity, small size and low cost a fusor can be used as an educational tool teaching students to work with radiation detection, radiation safety, plasma physics, high voltage systems, vacuum technology and electronics. A fusor is being built in a joint project between the Department of Physics at the University of Helsinki and the Helsinki Institute of Physics mainly for educational purposes. In the presentation preliminary results from the fusor project are shown.


Poster: A103, March 5th, 16:00 - 18:00, 1st floor

MULTILAYERED FERRITE-VANADIUM DIOXIDE STRUCTURES FOR MICROWAVE APPLICATIONS

Andrey A. Nikitin1,2, Vitaliy V. Vitko2, Aleksei A. Nikitin1,2, Alexey B. Ustinov1,2, Vitaliy V. Karzin2, Andrey E. Komlev2, Boris A. Kalinikos2, Erkki Lähderanta1

and.a.nikitin@gmail.com
1 Department of Physics, Lappeenranta University of Technology, Finland
2 Department of Physical Electronics and Technology, St. Petersburg Electrotechnical University “LETI”, Russia


Nowadays increased demands to the composite frequency-agile magnetic materials for microwave applications are evident.  A combination of ferrites with other materials such as ferroelectrics or piezoelectrics is promising for microwave devices due to electric and magnetic tunability of the waveguiding properties [1].  A metal-insulator transition (MIT) in the oxides of transition elements, such as vanadium dioxide (VO2) having huge conductivity changes, offers a novel tuning mechanism for the spin-wave devices. This tuning mechanism manifests itself as a modification of the spin-wave dispersion due to controllable variation of the VO2 conductivity.  This mechanism takes origin from the work of Bongianni [2] that describes an influence of a conductive layer on propagation of the surface magnetostatic waves.  In contrast to this work, we develop a theory that describes a dispersion of the surface spin waves (SWs) in the ferrite-dielectric-metal multilayered structures taking into account finite values of the layer permittivities and conductivities.  According to developed theory, we propose a novel tuning mechanism that is achieved due to the controllable variation of the conductivity in the layered structures exhibiting the MIT as functions of temperature or electric field.  The investigated structure is shown in Fig. 1(а).  A ferrite waveguide consists of an yttrium iron garnet (YIG) film (2) on a gadolinium gallium garnet substrate (1).  A dielectric-metal structure is composed of a dielectric layer (3), a VO2 film (4) on a sapphire substrate (5).  For the calculations, we use typical parameters of the YIG-film structures: d1 = 500 µm,  $\epsilon$1 = 14; d2 = 10 µm, $\epsilon$2 = 14, H0 = 1500 Oe, M = 1570 G.  A thickness of the VO2 film is d4 = 500 nm.  In order to obtain the MIT in the VO2 the laser pulse with the intensity of 11 W/mm2 and the pulse duration of 0.2 ms are chosen for calculation. It allowed to heat the VO2 from 333 К up to 345 К that provided a change in the VO2 conductivity from σ4 = 20 Ω-1cm-1 up to σ4 = 2350 Ω-1cm-1.  The dispersion characteristics calculated for these two values of conductivity are shown in Fig 1.b by the blue dashed and red solid lines, respectively. An increase in σ4 provides a change in the dispersion and produces the wavenumber variation shown in Fig. 1.c by the red solid line. Insertion of the intermediate silicon dioxide layer ($\epsilon$3 = 4.6, d3 = 10 µm,) narrows the frequency range of the conductivity influence (see black doted line in Fig 1.b and Fig. 1.c). However, the influence of the σ4 on the wavenumber variation in both structures is equal in the remaining frequency range. Therefore, a thick enough intermediate layer may be used for a thermal isolation of the YIG film from the VO2.  The temperature distributions are found by using appropriate COMSOL models and will be presented at the conference.



References
[1] M. M. Vopson, Crit. Rev. Solid State Mater. Sci. 40, 223 (2015).
[2] W.L. Bongianni J. Appl. Phys., 43, 2541 (1972).



Figure 1
Figure 1:

Figure 1. (a) Sketch of the ferrite-dioxide vanadium layered structure. (b) Dispersion characteristic of the SWs in the investigated . (c) Wavenumber variation.


Poster: A104, March 5th, 16:00 - 18:00, 1st floor

INVESTIGATION OF THE SPIN-WAVE PHASE SHIFT INDUCED BY PUMPING SIGNAL

N.A. Kuznetsov1,2, A.B. Ustinov1,2, E. Lähderanta1

ustinov_rus@yahoo.com
1 Lapeenranta University of Technology, P.O.Box 20, FIN-53851 Lappeenranta, Finland
2 St.Petersburg Electrotechnical University, 197376, St. Petersburg, Russia


Intensive spin wave propagation in magnetic films can be accompanied by the various nonlinear phenomena [1]. One of the recently investigated phenomenon is nonlinear phase shift of intensive spin waves propagating in side-coupled magnetic film wave-guides [2,3].

Purpose of this work is investigation of the phase shift of two nonlinearly coupled spin waves propagating in one magnetic waveguide. Experiments were carried out with experimental prototype of ferrite-film nonlinear phase shifter similar to that described in [4]. The prototype utilized 5.7-μm, 9.6-μm and 13.6-μm-thick, 2-mm-wide, and 40-mm-long yttrium iron garnet (YIG) film waveguides. The films were epitaxially grown on 500-μm-thick gadolinium gallium garnet substrate. The surface spin waves in the YIG film waveguides were excited and detected by microstrip transducers separated by a distance changing from 3.5 to 7.5 mm. The bias magnetic field H=1280 Oe was applied in the YIG film plane. This corresponds to the carrier spin wave frequencies in the range of 5500-6300 MHz.

A low power working signal and a high-power pump signal excited spin waves in the magnetic film at different frequencies. An induced nonlinear phase shift of the working signal was measured. Experimental dependences of the working signal’s differential nonlinear phase shift Δφ from the pump input power Pin were measured for different frequencies of the input signal. The results show, that the nonlinear phase shift of the working signal was increased with increasing frequency of the working signal and also with increasing amplitude of the pump signal. The phase shift was decreased for all frequencies with increasing film thickness. Theoretical model describing the observed phenomena was developed.

[1] B. A. Kalinikos et al., Solid State Physics. Vol. 64. P. 193-235 (2013).
[2] A. V. Sadovnikov et al., Phys. Rev. B 96, 144428 (2017).
[3] Q. Wang et al., Sci.Adv. 4, e1701517 (2018).
[4] A. B. Ustinov et al., J. Appl. Phys. 113, 113904 (2013).


Poster: A105, March 5th, 16:00 - 18:00, 1st floor

OPTIMIZING ROTATION CONTROL IN ACOUSTIC LEVITATION

P. Helander1, T. Puranen1, A. Meriläinen1, G. Maconi1, A. Penttilä1, M. Gritsevich1,2, I. Kassamakov1, A. Salmi1, K. Muinonen1,3, E. Hæggström1

petteri.helander@helsinki.fi
1 Department of Physics, University of Helsinki, P.O. box 64, 00014, Helsinki, Finland
2 Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia
3 Finnish Geospatial Research Institute FGI, Geodeetinrinne 2, 02430 Masala, Finland


Acoustic levitation allows non-contacting manipulation of millimeter-scale samples, which enables non-destructive, disturbance-free measurements. In the recent years, large phased array transducers have made better control over the emitted acoustic field possible [1]. For example, dynamic fields allow the transport of the sample and spatially asymmetric fields have been used to demonstrate limited orientation control [2]. Our recent results [3], however, show full 3-axis rotation control in levitation. This was achieved with a custom control algorithm capable of creating an acoustic field with an asymmetric shape. Even though we were able to create a desired field shape, the problem of finding the optimal field shape remains.

Here we present simulation results describing the 3-axis orientation control capability as a function of field shape. We show that the most reliable angular trapping is achieved with definite 3D asymmetry. We compare the case of the ideal field against the field realizable with a representative phased array levitator. The force and torque on the levitated sample were obtained by solving the complete scattering problem using the pressure acoustics module of COMSOL Multiphysics® (v. 5.3).

1. Marzo, Asier, et al. "Holographic acoustic elements for manipulation of levitated objects." Nature communications 6 (2015): 8661.
2. Cox, L., et al. "Acoustic lock: Position and orientation trapping of non-spherical sub-wavelength particles in mid-air using a single-axis acoustic levitator." Applied Physics Letters 113.5 (2018): 054101.
3. Kassamakov, Ivan, et al. "Light scattering by ultrasonically-controlled small particles: system design, calibration, and measurement results." Photonic Instrumentation Engineering V. Vol. 10539. International Society for Optics and Photonics, 2018.


Poster: A106, March 5th, 16:00 - 18:00, 1st floor

PRESSURE MEASUREMENT OF A MEMBRANE-ENCLOSED CAVITY BY MULTIPOINT SHOCKWAVE EXCITATION

P. Ihalainen1, H. Malinen1, D. Veira Canle1, A. Salmi1, A. Kontiola, E. Hæggström1

peetu.ihalainen@helsinki.fi
1 Electronics Research Laboratory, Dept. of Physics, PL 64, 00014 Univ. of Helsinki, Finland


Surface waves can be excited on membranes by non-linear acoustic waves. In this study, a shock wave impinging on a membrane excites a surface wave whose propagation speed is proportional to the tension on the membrane. Here, a membrane-enclosed cavity was pressurized to different pressures using a manometer. We built a custom made setup capable of exiting two shock fronts in rapid succession, propagating through two guiding structures. We generated two shock waves, which impinged on a membrane 4 mm apart from each other. A laser Doppler vibrometer (LDV) measured the propagating waves amplitude at the zenith of the membrane. Since the distance between the excitations points was fixed, we could perform an absolute speed of sound measurement on the membrane wave by calculating the difference of the time-of-flight of the waves. This allows remote characterization of membrane-like samples to determine properties such as the tension, or the elasticity if the membrane tension is known.





Figure 1
Figure 1: Fig 1. Schlieren image of a dual shockwave generation. The shock waves are directed by the tubes located on the left side of the image, here vortexes are generated by the sharp openings of the cylinders. On the right-hand side one can see the propagating shock wave delayed by 5 ms.

Poster: A107, March 5th, 16:00 - 18:00, 1st floor

FREQUENCY DEPENDENCY OF FOCAL DISTANCE OF SCANNING ACOUSTIC MICROSCOPE LENS

O. Tommiska1, A. Meriläinen1, J. Mäkinen1, J. Hyvönen1, A. Salmi1, E. Hæggström1

1 Department of Physics, Division of Materials Physics, University of Helsinki


Scanning acoustic microscope (SAM) permits high resolution, non-destructive imaging of samples. Acoustic microscopy allows studying both the topology and material properties, such as elasticity and porosity, of the samples. In a SAM, the image is formed by focusing a high frequency broadband signal with an acoustic lens. Said acoustic lens is usually fitted to focus a certain central frequency. When using a broadband signal, the focal distance of an acoustic lens might vary across the frequency band (aberration). If the frequency dependence is significant, it can lead to a loss of information during a measurement.

In our previous work we had studied this frequency dependency, by creating a COMSOL Multiphysics® simulation model of our SAM lens [1]. The geometry of the acoustic lens simulation model was based on a scanning white light interferometric 3D microscope image that we had measured. The frequency domain simulation results indicated that the focal distance depends on the frequency.

In this work, we performed a measurement to validate our simulation results. In the measurement an USAF 1951 microscope resolution test chart was imaged at different distances. Measured signals were analyzed in the frequency domain and for each frequency component the distance of highest signal amplitude was determined. The measurement results show a frequency dependency of the focal distance like the one suggested by the simulations.

[1] Tommiska O., et al. (2018). Multiphysics Simulation of a High Frequency Acoustic Microscope Lens. Paper presented at COMSOL Conference 2018 Lausanne.



Figure 1
Figure 1: Picture of sample, and comparison of simulation and measurement results.

Poster: A108, March 5th, 16:00 - 18:00, 1st floor

TWO-LAYER NANOFIBER PATCH FOR ADVANCED CHRONIC WOUND TREATMENT

J. Hunnakko1

joel.hunnakko@helsinki.fi
1 Electronics Research Laboratory, Department of Physics, University of Helsinki, Helsinki


Two-layer nanofiber patch for advanced chronic wound treatment

Joel Hunnakko 1, Ivo Laidmäe 1,2 , Joni Mäkinen 1, Tuomas Puranen 1, Ari Salmi 1, Kai
Kronström , Anton Nolvi 1, Timo Rauhala 1, Karin Kogermann 2, Jyrki Heinämäki 2, Heikki J.
Nieminen 3, Tero Oinonen, Petteri Helander 1, Edward Hæggström 1

1 Electronics Research Laboratory, Department of Physics, University of Helsinki, Helsinki,
Finland, Gustaf Hällströmin katu 2A

2 Institute of Pharmacy, University of Tartu, Tartumaa, Eesti, Nooruse 1, 50411, Tartu linn

3 Medical Ultrasonics Laboratory (MEDUSA); Dept. of Neuroscience and Biomedical Engineering
(NBE), Aalto University School of Science, P.O. Box 12200, FI-00076 AALTO, Finland (Street
address: Room F260, Rakentajanaukio 2 C, Espoo);

Treating chronic wounds with patches requires certain properties from the patch, e.g. moisture
control and resistance to microbes [1]. There are different types of wound patches and
dressings on the market designed for chronic wound treatment -- some wound patches are
based on nanofibers. To produce nanofiber patches, electrospinning is commonly used.
Traditional electrospinning does not offer dynamic control over the produced material
structures, e.g. fiber diameter. We have developed an ultrasound electrospinning device [2]
that allows us to modify the fiber diameter during the production process. Previously we
demonstrated fiber diameter control in different samples. Here, we demonstrate real time
control of fiber diameter and create a two-layered sample by adjusting the fiber diameter of a
polyethylen oxide (PEO) nanofiber. Such internal structure of the wound patch is a way to
create more advanced wound care products. To verify that a suitable structure was generated,
the samples were imaged with scanning electron microscopy.

1 [Wound dressings, 2018, Errol J. Britto, Christopher A. Morrison.]
2[Ultrasound-enhanced electrospinning, 2018, Heikki J. Nieminen, Ivo Laidmäe, Ari Salmi, Timo
Rauhala, Tor Paulin, Jyrki Heinämäki & Edward Hæggström]


Poster: A109, March 5th, 16:00 - 18:00, 1st floor

GUIDING NON-LINEAR MECHANICAL WAVES WITH TUBULAR STRUCTURES

H. Malinen1, D. Veira Canle1, J. Heikkilä1, P. Ihalainen1, A. Salmi1, E. Hæggström1

henri.malinen@helsinki.fi
1 Department of Physics, Division of Materials Physics, University of Helsinki


Shockwaves are perturbations that propagate faster than sound in the medium through which they propagate. Their characteristics include an abrupt change of pressure and usually non-symmetry of the negative and positive pressure swings. Shock waves can be used to actuate linear ultrasonic waves in targets. Currently, shock waves are used in medicine in various treatments e.g. destroying kidney stones in lithotripsy.

We created a system for producing shock waves with an electric discharge and guiding the waves with a tubular structure similar to a Reddy tube$^1$. A combination of a flexible tube and 3D-printed parts was used to assemble the shock wave guiding system. The spark was generated between tungsten electrodes inside a 3D-printed chamber. The chamber had an adapter where the flexible tube was connected. The tubular wave guide can be used to direct shockwaves for actuation into hard-to-reach locations. Schlieren imaging was used to image the shock waves exiting the tube after travelling a distance of 25 cm in the tube and retaining their shock characteristics.

Different geometries at the end of the tube were tested to reduce vortex formation. For this purpose, the flexible tube was coupled into a 3D-printed structure. Schlieren images demonstrated that the tube generated a vortex ring, and the distribution of energy between the shockwave and the vortex ring was modified by altering the curvature of the tube opening.

1 “Manually operated piston-driven shock tube”, K. P. J. Reddy and N. Sharath, 2013



Figure 1
Figure 1: Schlieren image of the shock wave exiting the tube.

Poster: A110, March 5th, 16:00 - 18:00, 1st floor

LARGE AREA HIGH RESOLUTION IMAGING OF GROWTH ZONES IN WOOD

J. Hyvönen1, A. Meriläinen1, H. Help-Rinta-Rahko2, J. Alonso Serra3, A. Salmi1, E. Hæggström1

jere.hyvonen@helsinki.fi
1 Electronics Research Laboratory, Dept. of Physics, PL 64, 00014 Univ. of Helsinki, Finland
2 Helsinki X-ray Laboratory, Dept. of Physics, PL 64, 00014 Univ. of Helsinki, Finland
3 Institute of Biotechnology, University of Helsinki, 00014 Helsinki, Finland


Scanning acoustic microscopy (SAM) permits simultaneous structural and mechanical imaging, which has been utilized in many fields [1]. We developed a custom-made SAM that employs coded signal excitation, which permits a rapid measurement of large areas (20 min for 2mm², 1 µm steps) with high resolution [2].
We show one application of our technique: to understand the development of growth zones in wood. The cross-sections of a one-year-old birch’s branches were imaged (Fig 1 left) with a custom-built SAM. Images taken with 400 MHz transducer at 1 µm steps allows distinguishing single cell walls. Histograms of the acoustic impedance (Fig 1 right) feature higher values on the bottom part of the branch. The result demonstrates the microscopic origin of the well-known macroscopic phenomenon of the formation of tension wood. We foresee applications of our microscope for all areas of biology that would benefit from µm resolution mechanical images of large areas (up to ~mm2).

REF 1: Briggs and Kolosov “Acoustic Microscopy (Second Edition)”, Oxford University Press, 2010.
REF 2: Meriläinen A. I. et al. ”Solid state switch for GHz coded signal ultrasound microscopy”, Electronics Letters. vol 49, no. 3, pp. 169-170 Jan. 2013



Figure 1
Figure 1: Figure 1: Left) Acoustic impedance map of a one-year-old spruce branch cross-section imaged with 300 to 500 MHz, 1µs, linear chirp. Right) Histograms of impedances in the marked red and blue subsections showing difference in the median acoustic impedance.

Poster: A111, March 5th, 16:00 - 18:00, 1st floor

STUDY OF NON-LINEAR MECHANICAL WAVE INTERACTION WITH BIO-MEMBRANE

J. Heikkilä1, E. Lampsijärvi1, A. Salmi1, E. Hæggström1

jesse.t.heikkila@helsinki.fi
1 Electronics Research Laboratory, P.O.B. 64, FIN-00014 University of Helsinki, Finland


The propagation of a non-linear mechanical wave (shockwave) and its interaction with a curved bio-membrane was visualized and measured. The shockwave was guided to the membrane in a plastic tube. The impinging shockwave excites a wave that travels across the membrane.

The measurement setup consists of a straight dual-field-lens (f=200mm) schlieren arrangement, a laser Doppler vibrometer (Polytec OFV-5000 with OFV-534 laser unit) and an oscilloscope (Tektronix TBS 1202B) to measure the output signal of the LDV. The schlieren setup was used together with an USB-camera (IDS UI-3480CP), a custom-made LED pulser, a green LED (LedEngin LZ1-00G102) and a signal generator (Tektronix AFG 3252). The shockwaves were created with a triggerable three-electrode high-voltage spark gap. The generation of shockwaves was optimized to produce nearly identical shocks, which made the use of a stroboscopic detection method possible. This was required to create high-resolution videos of shockwaves, which is unfeasible even with state of the art high speed cameras due to poor resolution at sufficient frame rates. The spark gap was triggered using a thyristor driven by a FET connected to an Atmega328p microcontroller. A current transformer triggered the signal generator feeding the LED pulser with controlled delays.

The speed and amplitude of the shockwave were measured from schlieren images. For speed measurements the LED was pulsed 3 times at 400 kHz for each image to measure speed from the propagation distance between pulses. An averaged line profile was extracted from each image. Median and Savitzky–Golay filters were used for noise reduction. Signals from the LDV were used to measure amplitudes of excited waves on the membrane. We present results of the amplitude of the membrane wave and the speed of the incident and reflected shockwaves as a function of incident angle of the shockwave onto the membrane.



Figure 1
Figure 1: A labeled Schlieren image and a graph of the membrane wave amplitude versus incident angle

Poster: A121, March 5th, 16:00 - 18:00, 1st floor

IONOSPHERE AND ARCTIC RESEARCH WITH SUOMI 100 SPACE WEATHER CUBESAT

Esa Kallio1, Ari-Matti Harri2, Anita Aikio3, Arno Alho1, Markku Alho1, Mathias Fontell1, Riku Järvinen1,2, Kirsti Kauristie2, Antti Kestilä2, Olli Knuuttila1, Petri Koskimaa2, Johannes Norberg2, Jouni Rynö2, Esa Turunen3, Heikki Vanhamäki3

esa.kallio@aalto.fi
1 Aalto University, School of Electrical Engineering, Department of Electronics and Nanoengineering, Espoo, Finland
2 Finnish Meteorological Institute, Helsinki, Finland
3 University of Oulu, Finland


The Suomi 100 nanosatellite was launched on Dec. 3, 2018 (http://www.suomi100satelliitti.fi/). The 1 Unit (10 cm x 10 cm x 10 cm) polar orbit satellite performs geospace, ionosphere and arctic region research with a white light camera and a radio wave spectrometer instrument which operates in the 1-10 MHz frequency range.

The Suomi 100 satellite is a manifestation of a modern technology that provides new possibilities to study Earth’s ionosphere by using small spacecraft and 3D computer simulations. Suomi 100 satellite type of nanosatellite, so called CubeSat, provides a cost effective possibility to provide in-situ measurements in the ionosphere.

Moreover, combined CubeSat observations with ground-based observations give a new view on auroras and associated electromagnetic phenomena. Especially joint CubeSat – ground based observation campaigns enable the possibility of studying the 3D structure of the ionosphere.

At the same time, increasing computation capacity has made it possible to perform simulations where properties of the ionosphere, such as propagation of the electromagnetic waves in the medium frequency, MF (0.3-3 MHz), and in the high frequency, HF (3-30 MHz), ranges is based on a 3D ionosphere model and on first-principles modelling.

Electromagnetic waves at those frequencies are strongly affected by ionospheric electrons and, consequently, those frequencies can be used for studying the plasma. On the other hand, even if the ionosphere originally enables long-range telecommunication at MF and HF frequencies, the frequent occurrence of spatio-temporal variations in the ionosphere disturbs communication channels, especially at high latitudes. Therefore, study of the MF and HF waves in the ionosphere has both a strong science and technology interests.

We present computational simulation and measuring principles and techniques to investigate the arctic ionosphere by a polar orbiting CubeSat which radio instrument measures HF and MF waves. We introduce 3D simulations, which have been developed to study the propagation of the radio waves, both ground generated man-made radio waves and space formed space weather related waves, through the 3D arctic ionosphere with a 3D ray tracing simulation and a local scale full kinetic electromagnetic simulation. We also introduce the Suomi 100 CubeSat mission and its initial observations.



Figure 1
Figure 1: A photograph taken by Suomi 100 satellite (http://www.suomi100satelliitti.fi/media/avaruuskuvia/Ulko-Hebridit-20181217.png). See more Suomi 100 satellite’s photographs at http://mittaukset.suomi100satelliitti.fi/kuvat/. (© Aalto University)

Poster: A122, March 5th, 16:00 - 18:00, 1st floor

POROSITY AND NON-LOCALITY OF INJECTION AT THE EARTH'S BOW SHOCK: GLOBAL SIMULATION RESULTS

M. Battarbee1, U. Ganse1, Y. Pfau-Kempf1, L. Turc1, T. Brito1, M. Grandin1, T. Koskela1,2, M. Palmroth1,3

markus.battarbee@helsinki.fi
1 Department of Physics, University of Helsinki
2 Department of Physics and Astronomy, University of Turku
3 Finnish Meteorological Institute


We study the interaction of solar wind protons with the Earth's quasi-parallel bow shock through a combination of hybrid-Vlasov and high-statistics test-particle simulations. We employ the Vlasiator high-fidelity global hybrid model to include effects due to bow shock curvature, tenuous upstream populations, and foreshock waves. We investigate the position of the bow shock by using multiple parametrizations, and propose a porosity measure to describe shock front non-locality. Our results support the notion of upstream structures causing patchwork reconstruction of the shock front in a non-uniform manner.

We show that in the context of the global bow shock, reformation and shock non-locality cannot be directly linked with temporal pulses of particle injection. Instead, we present data supporting the notion of acceleration non-locality, with energization taking place throughout a larger shock transition zone. We quantify the size of this zone as porosity. Non-localized acceleration is found for shocks exhibiting both large and small porosity values.

We additionally show that the density of suprathermal particles upstream of the shock may not be a useful metric for the probability of injection at the shock, as large-scale foreshock dynamics have a greater effect on energetic particle accumulation at a given position in space. Our results have significant implications for statistical and spacecraft studies of the shock injection problem.



Figure 1
Figure 1: Proton number density overlaid with bow shock positions according to criteria for plasma density (black), magnetic field magnitude (red), solar wind core heating (orange), magnetosonic Mach number (pale blue), and magnetic field to bow-normal angle (purple). Panel (a) is for Simulation 1 ($B_\mathrm{sw}=5\,\mathrm{nT}$), panel (b) for Simulation 2 ($B_\mathrm{sw}=10\,\mathrm{nT}$), both at $t=500\,\mathrm{s}$.

Poster: A123, March 5th, 16:00 - 18:00, 1st floor

SELF-ORGANIZING STRUCTURES IN THE FORESHOCK OF EARTH'S MAGNETOSPHERE

M. Ganse1, L. Turc1, Y. Pfau-Kempf1, M. Battarbee1, T. Brito1, M. Dubart1, M. Grandin1, M. Palmroth1,2

urs.ganse@helsinki.fi
1 Department of Physics, University of Helsinki
2 Finnish Meteorological Institute


A steady stream of particles ejected from the Sun, known as the solar wind, continuously hits Earth's magnetic field with supersonic velocities, thus creating a bow shock of approximately parabolic shape.

Upstream of this bow shock, in cases where the interplanetary magnetic field gives it a quasi-parallel geometry, shock-reflected protons stream back into the incoming solar wind plasma. This two-stream plasma situation excites wave instabilities, leading to the creation of well-known 30-second plasma waves. The resulting waves do not form in neat parallel wavefronts, but tend to present a complex spatial obliquity structure, which has been notoriously difficult to image and understand using satellite measurements alone.

We investigate the processes affecting global-scale foreshock structure formation by employing the global hybrid-Vlasov simulation system Vlasiator, in which full and noise-free information about proton distribution functions is available at every point of the simulation, and their kinetic behaviour and interaction with electromagnetic field is solved.

Our results indicate multiple processes are affecting and co-interacting in global-scale structure formation in the foreshock, including beam instabilities, wave-particle trapping and scattering. We find that oblique wave structures are the norm rather than the exception, and that details of the resulting structure sizes vary strongly with solar wind parameters.



Figure 1
Figure 1: Plasma density in the foreshock region of Earth's magnetosphere's bow shock. Backstreaming particles are exciting wave instabilities with varying wavelengths and obliquities.

Poster: A124, March 5th, 16:00 - 18:00, 1st floor

THE ROLE OF NOISE IN THE CHOICE OF THE FLOW WAVE NUMBER AFTER FIRST INSTABILITY

Maria Gritsevich1, Olga Krivonosova2, Dmitry Zhilenko2

olga@imec.msu.ru
1 University of Helsinki
2 Moscow State University


In the present study we consider how the wave number selection in spherical Couette flow, in the transition to azimuthal waves after the first instability, occurs in the presence of noise. The outer sphere was held stationary while the inner sphere rotational speed was increased linearly from a subcritical flow to a supercritical one. In a supercritical flow, one of two possible flow states, each with different azimuthal wave numbers, can appear depending upon the initial and final Reynolds numbers and the acceleration value. Noise perturbations were added by introducing small disturbances into the rotational speed signal. With an increasing noise amplitude, a change in the dominant wave number from m to m±1 was found to occur at the same initial and final Reynolds numbers and acceleration values. The flow velocity measurements were conducted by using laser Doppler anemometry. Using these results, the role of noise, as well as the behaviour of the amplitudes of the competing modes in their stages of damping and growth, were determined.
This work was supported by Russian foundation for basic research, projects nos. 19-05-00028 and 18-08-00074.


Poster: A125, March 5th, 16:00 - 18:00, 1st floor

SURVEY OF ELECTRON VELOCITY DISTRIBUTIONS IN THE EARTH’S MAGNETOSPHERE SIMULATED WITH VLASIATOR

Thiago Brito1, Urs Ganse1, Yann Pfau-Kempf1, Markus Battarbee1, Lucile Turc1, Maxime Grandin1, Maxime Dubart1, Tuomas Koskela1, Minna Palmroth1

thiago.brito@helsinki.fi
1 University of Helsinki


Vlasiator is a global magnetospheric hybrid-model that solves the Vlasov equation for ions, while treating the electrons as a charge-neutralizing fluid. Solving the full kinetic motion of electrons in magnetospheric spatial and temporal scales is way beyond the capability of current HPC capabilities. However, the combination of subgrid-scale methods and implementations such as gyrokinetics or multifluid electron representation can be carried out in order to resolve small scale phenomena and to more efficiently introduce electron effects and thus self-consistently simulate the whole magnetospheric environment. Aiming towards this goal, we present here the results of the first global 2D simulation of the Earth magnetosphere solving the Vlasov equation for the electron population instead of merely treating it as a fluid. Due to the extreme computational cost of performing this simulation, it can only be done for a duration of hundreds or maybe a few thousand electron gyroperiods, which means that we can simulate less than a second of real time. However, this is already enough time so that an equilibrium is reached and it is possible to investigate the evolution and structure of the electron distribution functions in some regions of the magnetosphere.


Poster: A126, March 5th, 16:00 - 18:00, 1st floor

LATITUDINAL DIFFERENCES OF GEOMAGNETIC FLUCTUATION COVERAGE

P. Peitso1, E.I. Tanskanen1

pyry.peitso@aalto.fi
1 Aalto University, School of Electrical Engineering, Department of Electronics and Nanoengineering


Several space weather phenomena are closely linked to rapid geomagnetic fluctuations. The detailed studies of the latitudinal distribution, as well as quantifying the amount of these high-frequency fluctuations is useful for high-latitude space weather forecasting. We utilize the newly developed Fractional Derivative Rate (FDR) method to illustrate the daily coverage of these fluctuations using a wide range of different latitudes. The FDR method calculates a daily percentage value where a given amount of geomagnetic fluctuations is met or exceeded. This allows the efficient usage of high time-resolution magnetometer data in situations where time series of several years are to be studied. In addition to the standard 0.20 nT/s threshold, corresponding to the onset of a typical substorm, several thresholds from 0.10 nT/s to 0.60 nT/s are tested to further quantify different types of fluctuations. In addition, the UT variation of these rapid magnetic field changes is studied and results presented.


Poster: A127, March 5th, 16:00 - 18:00, 1st floor

SELF-SIMILARITY OF MAGNETIC FIELD STRUCTURE IN CORONAL MASS EJECTIONS

S. W. Good1, E. K. J. Kilpua1, A. T. LaMoury2, R. J. Forsyth2, J. P. Eastwood2, C. Möstl3

simon.good@helsinki.fi
1 University of Helsinki, Finland
2 Imperial College London, UK
3 Austrian Academy of Sciences, Austria


Coronal mass ejections (CMEs) are large eruptions of magnetic field and plasma from the Sun that propagate into the solar system. When CMEs hit the Earth, they can cause a range of damaging effects to ground and orbital infrastructure; a current endeavour of the space physics community is to accurately forecast this ‘space weather’. Understanding how the magnetic field structures of CMEs change as they travel from the Sun is critical to accurate space weather prediction. By analysing CME observations made by spacecraft at Mercury, Venus and the orbital distance of the Earth, we show that the magnetic field structures of CMEs often evolve self-similarly with propagation distance from the Sun. This finding supports the case for using observations from upstream monitors on the Sun-Earth line to produce routine space weather forecasts.


Poster: A128, March 5th, 16:00 - 18:00, 1st floor

IN SITU OBSERVATIONS OF A MAGNETOSHEATH HIGH-SPEED JET TRIGGERING MAGNETOPAUSE RECONNECTION

H. Hietala1,2, T. D. Phan3, V. Angelopoulos2, M. Oieroset3, M. O. Archer4,5, T. Karlsson6, F. Plaschke7,8

heli.hietala@utu.fi
1 Dept. of Physics and Astronomy, University of Turku, Finland
2 Dept. of Earth, Planetary, and Space Sciences, University of California, Los Angeles, USA
3 Space Science Laboratory, University of California, Berkeley, USA
4 School of Physics and Astronomy, Queen Mary University of London, London, UK
5 The Blackett Laboratory, Imperial College London, UK
6 Space and Plasma Physics, School of Electrical Engineering, KTH Royal Institute of Technology, Stockholm, Sweden
7 Space Research Institute, Austrian Academy of Sciences, Graz, Austria
8 Institute of Physics, University of Graz, Graz, Austria


Magnetosheath high-speed jets (HSJs)$-$dynamic pressure enhancements typically of $\sim$1 Earth radius in size$-$impact the dayside magnetopause several times per hour. Here we present the first in situ measurements suggesting that such an impact triggered magnetopause reconnection. We use observations from the five THEMIS spacecraft in a string-of-pearls configuration on August 7, 2007. First the magnetopause moved inwards past THB (the outermost probe) to a location between THE and THA (the innermost probe). In the magnetosheath THB observed a HSJ with a large velocity towards the magnetopause ($V_N\,\sim\,-300\,$km/s). Then the magnetopause moved back out. Before the HSJ, there was no evidence for reconnection. After the HSJ, there were clear reconnection outflows ($V_L\,\sim\,-260\,$km/s). We infer that the HSJ impact compressed the thick ($60-70\,d_{\mathrm{i}}$), high shear ($140-160^{\circ}$) magnetopause until it was thin enough to reconnect. HSJs could therefore act as a driver for bursty dayside reconnection.


H. Hietala et al., In situ observations of a magnetosheath high-speed jet triggering magnetopause reconnection, Geophys. Res. Lett., 45, (2018) 1732–1740, doi:10.1002/2017GL076525


Poster: A129, March 5th, 16:00 - 18:00, 1st floor

DAYSIDE MAGNETOPAUSE RECONNECTION IN A QUASI-3D GLOBAL HYBRID-VLASOV SIMULATION

Y. Pfau-Kempf1, M. Battarbee1, T. Brito1, M. Dubart1, U. Ganse1, M. Grandin1, T. Koskela1, L. Turc1, S. Hoilijoki2, M. Palmroth1,3

yann.kempf@helsinki.fi
1 Department of Physics, University of Helsinki, Helsinki, Finland
2 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, United States of America
3 Space and Earth Observation Centre, Finnish Meteorological Institute, Helsinki, Finland


The leading global magnetospheric hybrid-Vlasov model Vlasiator is expected to soon deliver the world's first fully three-dimensional noiseless and realistic simulations of near-Earth space. Promising results have already been obtained using two-dimensional simulations of the magnetosphere. Furthermore, detailed investigations have shown that even at coarse resolutions kinetic phenomena are well described by Vlasiator, going beyond classic fluid models like magnetohydrodynamics.

Here, we will present simulations performed in a quasi-three-dimensional setup. The noon-midnight meridional plane is expanded in the normal direction to form a slab and the dipolar magnetic field assumes a cylindrical geometry. This allows to study the behaviour of dayside magnetopause reconnection including the formation of flux transfer events. Even in this setup with a limited extent in the third dimension the magnetic field topology becomes non-trivial, in line with recent spacecraft observations.


Poster: A131, March 5th, 16:00 - 18:00, 1st floor

AALTO VIRTUAL PLANETARIUM

Markku Alho1, Esa Kallio1, Riku Järvinen1,2

markku.alho@aalto.fi
1 Aalto University
2 Finnish Meteorological Institute


The novel and accessible consumer-grade virtual and augmented reality technologies enable immersive experiences in three-dimensional space. Taking advantage of these technologies, we have developed outreach and education software for virtual environments to engage with both the public and students on space physics topics.

The immersive technologies are especially suited for exploration of three-dimensional structures, such as plasma flows and magnetic field in space physics. The Aalto Virtual Planetarium demonstrates space research activities that involve Aalto University, including: visualizations of plasma physics simulations of the Sun and inner Solar System bodies (presently: Mercury, Venus, Earth and the comet 67P/Churyumov-Gerasimenko); space probes and missions (Rosetta, BepiColombo, Suomi100); heliospheric structure; and the Solar System to scale. The user can move and scale freely in the Solar System, from towering over the ecliptic down to visiting the Philae lander in 1:1 scale on the surface of 67P. Pre-set tour points with virtual posters provide descriptions of objects and phenomena in their natural context.

The Virtual Planetarium viewing sessions will be given during the conference, with showtimes specified later.

See http://space.aalto.fi/software/ for further details.


Poster: A132, March 5th, 16:00 - 18:00, 1st floor

PROMOTING PHYSICS AMONG CHILDREN AND YOUTH IN HELSINKI DETECTOR LABORATORY

V. Litichevskyi1, E. Brücken1, E. Tuominen1, J. Ott1, T. Naaranoja1, L. Martikainen1, S. Kirschenmann1, P. Luukka1, J. Aaltonen1, S. Bharthuar1, F. Garcia1, J. Heino1, I. Kassamakov1, P. Koponen1, T. Lampen1, T. Linden1, R. Turpeinen1, K. Osterberg1, A. Karadzhinova-Ferrer2, M. Kalliokoski2

vladyslav.litichevskyi@helsinki.fi
1 P.O.Box 64 (Gustaf Hällströmin katu 2), FI-00014 University of Helsinki, Finland
2 Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia


Helsinki Detector Laboratory is a national infrastructure specialized in the instrumentation of particle and nuclear physics. It is a joint laboratory between Helsinki Institute of Physics (HIP) and the Department of Physics of the University of Helsinki. The Laboratory provides premises, equipment, know-how and technical support for research projects developing radiation detector technologies. The Laboratory team has extensive expertise in the modelling, design, construction and testing of semiconductor and gas-filled radiation detectors. In addition, the personnel and scientists working in the Laboratory are active in promoting the enthusiasm for physics among children and youth.

Providing education in the instrumentation of physics is of outmost importance in Detector Laboratory. In the framework of the Masters Program for Particle Physics and Astrophysical Sciences (PARAS), the laboratory offers a study module of instrumentation consisting of four different courses of detector technologies including hands-on laboratory exercises. In addition, the laboratory regularly organizes one-week research training courses in detector technology for particle physics for Nordic post-graduate students. The laboratory team also supervises doctoral and master students in their thesis works, especially in the framework of the Doctoral Program in Particle Physics and Universe Sciences (PAPU).
In the laboratory, special effort is devoted to societal interaction among young people. Every year hundreds of school children and youth visit the laboratory for demonstrations of physics and instrumentation. As small as eight-year old children have enjoyed doing educative hands-on works in the laboratory - or done laboratory works at their own school together with the visiting laboratory team. In addition, every year the laboratory team takes part in several common outreach events, such as CERN Master Classes, EU Researchers’ Night, Think Corner exhibitions, and Bring-Your-Child-to-Work -days. Furthermore, every year several secondary school TET-trainees and day labour (Taksvärkki-trainees) do their practice in the laboratory.


Poster: A141, March 5th, 16:00 - 18:00, 1st floor

PRODUCTION OF ITER-RELEVANT DEPOSITED LAYERS FOR FUEL-RETENTION INVESTIGATIONS

A. Hakola1, K. Heinola2,3, K. Mizohata3, J. Likonen1, C. Lungu4, C. Porosnicu4, E. Alves5, R. Mateus5, I. Bogdanovic Radovic6, Z. Siketic6, V. Nemanic7, C. Pardanaud8

antti.hakola@vtt.fi
1 VTT, Finland
2 Atomic and Molecular Data Unit, International Atomic Energy Agency, Vienna, Austria
3 Department of Physics, University of Helsinki, Helsinki, Finland
4 National Institute for Laser, Plasma and Radiation Physics, Magurele, Bucharest, Romania
5 Instituto Superior Técnico, Universidade de Lisboa, Bobadela, Portugal
6 Rudjer Boskovic Institute, Zagreb, Croatia
7 Jozef Stefan Institute, Ljubljana, Slovenia
8 Aix-Marseille Université, Laboratoire PIIM, Marseille, France


The next step on the road to produce energy by nuclear fusion is the construction and operation of the ITER reactor in the late 2020’s. Crucial issues in the design of ITER are to ensure that the lifetime of the reactor wall is sufficiently long upon exposure to high-power plasma discharges and that the accumulation of radioactive tritium from the plasma fuel into wall structures remains low. These requirements have led one to select beryllium (Be) and tungsten (W) as materials for the different plasma-facing components of ITER. To investigate plasma operations as well as interactions between the plasma and the wall in an ITER-relevant environment, the JET test reactor has been equipped with an ITER-Like Wall (ILW) back in 2009. Experience with JET ILW has been promising but some interesting observations require further investigations. One of them are thick deposited layers, rich in Be and various impurities like carbon (C) and oxygen (O), that have been formed on surfaces at the bottom of the vessel. The formation mechanisms and properties of such deposits is being investigated within a large European research project under the EUROfusion Consortium.

The project focuses on producing and characterizing different Be-containing deposited layers that would be proxies to those observed on JET-ILW or predicted for ITER. For safety reasons, deuterium (D) is used as a representative element of plasma gas instead of tritium. The prepared samples consist of Be, Be-O, and Be-O-C films, whose composition, thickness, and surface structure have been varied during the production phase. In addition, the temperature of the layers have been scanned between the room temperature and 600°C to cover the conditions that may occur during the operation of both JET-ILW and ITER. The properties of the deposits have been determined using a variety of surface-analysis tools in the participating laboratories.

The analyses show that the thicker the sample, the larger will be the amount of D retained in the growing film. The largest D levels have been observed for Be-O-C-D samples, and the relative atomic fractions can climb as high as 40-50 at.%. Furthermore, the retained fraction of D in the co-deposits increases with the decreasing O content which, for its part, can be controlled by adding more C in the film. Defects and various traps as well as strong C-D and O-D bonds are largely responsible for the measured D inventory. Finally, the higher the surface temperature, the smaller is the amount of D retained. Simultaneously, the amount of C, N, and O impurities increases. Compared to the JET-ILW samples, the as-deposited Be-O-C-D layers show many similarities especially when it comes to their C and O contents and the characteristics of fuel release. The differences can be largely attributed to high surface temperatures and temperature excursions that the wall components have been subjected to during their exposure.


Poster: A142, March 5th, 16:00 - 18:00, 1st floor

5D PARTICLE DISTRIBUTIONS IN NON-TRIVIAL FUSION-PLASMA GEOMETRIES

U. Lauranto1, J. Kontula1, J. Varje1

unna.lauranto@aalto.fi
1 Aalto University


Magnetic confinement fusion devices confine hot plasma by means of a magnetic field. According to a famous theorem in algebraic topology, the so-called hairy-ball theorem, the only geometry in which a vector field is at all points surface-aligned is a torus. Since a magnetic field is a vector field, fusion devices based on magnetic confinement are all topologically toruses. However, especially in the case of stellarators, these surfaces can be far from a circular torus and, therefore, the enclosed volumes are not necessarily straightforward to evaluate.

ASCOT is a simulation tool for minority species in magnetically confined plasmas, most often used to study the behavior of energetic particles, such as fusion-born 3.5 MeV alpha particles. Distributions of different plasma species are needed in toroidal coordinates, which is the natural coordinate system for toroidal magnetic confinement devices. Furthermore, macroscopic quantities, such as pressure and density, are frequently needed as a function of the so-called flux surface coordinate only.

The particle distributions in toroidal coordinates are implemented to ASCOT5, which is a new version of the code compatible with modern super-computer architectures. The particle distributions are phase space distributions, which contain time, spatial coordinates in the toroidal coordinate system and velocity coordinates. The velocity coordinates are either velocities parallel and perpendicular to the magnetic field, or velocity coordinates in the cylindrical coordinate system. For calculating the particle density, the sizes of the volume elements are needed. With complex plasma geometries, the volume elements cannot be calculated analytically. Therefore, the calculation for volume element sizes by means of the Monte Carlo method is implemented. The particle distributions of 10 000 neutral-beam markers in the presence of a Maxwellian background plasma in the JET-tokamak are analyzed. As expected, although a majority of energetic ions from the neutral beam are ionized near the edge, the actual density of the ions rapidly decreases towards the edge due to the strongly growing volume elements.


Poster: A143, March 5th, 16:00 - 18:00, 1st floor

IMPROVING THE STATISTICS OF SYNTHETIC FAST-ION LOSS DETECTOR SIMULATIONS IN ASCOT

J. Kilpeläinen1, A. Snicker1

1 Aalto university, Department of Applied Physics


In order to generate electricity with a thermonuclear fusion power plant, the fusing plasma needs to be brought to burning condition. The fusion burn is a state where the reactants of the deuterium-tritium fusion, fusion-born 3.5 MeV alpha particles, will heat the plasma via Coulomb collisions. Hence, the plasma will be kept warm without external heating. In order to keep the plasma hot enough for fusion to occur, those alpha particles need to be confined to deposit their energy in the plasma rather than to leak the energy to the plasma facing components of the machine. Hence, understanding the transport of non-thermal fast ions species in a fusion plasma is of paramount and fundamental importance.

Fast-ion losses in toroidal fusion devices can be simulated with ASCOT particle following code. Synthetic signal of fast-ion loss detectors can also be simulated but since approximately 0.05 % of all simulated markers hit the detector, obtaining statistically relevant results is computationally challenging. Therefore, the number of markers hitting the detector must be increased. Markers that will definitely not hit the detector should be discarded before running the simulation.

For this purpose, a MATLAB routine is written to analyze and filter the markers. Multiple filtering methods are introduced. The best filtering method enables the number of detector hits to improve by approximately an order of magnitude.



Poster: A144, March 5th, 16:00 - 18:00, 1st floor

THE CHEMICAL PATH OF ERODED SOIL FIELD SOIL FROM FIELD TO BOTTOM SEDIMENT WITH K-EDGE X-RAY ABSORPTION NEAR EDGE SPECTROSCOPY.

Antti-Jussi Kallio1,2,3

antti-jussi.kallio@helsinki.fi
1 Simo Huotari
2 Petri Ekholm
3 Jouni Lehtoranta


Eutrophication is a global challenge, where chemical processes in bottom sediments play a key role. Yet, eutrophication science lacks understanding of the role of terrestrial matter on the processes that mobilize or immobilize phosphorus, a central algal nutrient, in sediments. During burial the sediment microbial processes change the redox-conditions which affect the chemistry of settled soil. Here, methods were developed to follow the evolution of the chemical state of soil during burial with iron K-edge X-ray absorption near edge spectroscopy. [1] with a novel home laboratory-based spectrometer. The chemical state of the sediment, and especially the chemistry of iron is linked to the release and binding of phosphorus in the bottom sediments [2]. The chemical path of field soil was simulated with chemical incubations, where soil was mixed with sea water and carbon and/or sulfate was added to match various conditions faces in sea bottom. In order to measure the spectra of soil-water mixtures, a sample preparation method was developed, where the sample is gellified in both aerobic and anaerobic conditions, and placed inside an airtight sample environment. The measured spectra were compared with spectra obtained from reference iron compounds and the iron species were quantified. The sulfate and addition of organic carbon enhanced the formation of iron sulfides, causing iron bound phosphorus to be released to water. As expected the release of phosphorus was higher in anaerobic conditions than in aerobic conditions. The addition of organic carbon and sulfate enhanced the formation of iron sulfides, causing iron bound phosphorus to be released to the aquatic system. The results shed light on iron chemistry in anoxic sediments, which can be used in management of eutrophication

[1] A.-P. Honkanen, S. Ollikkala, T. Ahopelto, A.-J. Kallio, M. Blomberg, S. Huotari, Johann-type laboratory-scale X-ray absorption spectrometer with versatile detection modes, arXiv:1812.01075, December 2018

[2] P. Ekholm, J. Lehtoranta, Does control of soil erosion inhibit aquatic eutrophication?, Journal of Environmental Management 93:140-6, January 2012



Figure 1
Figure 1: Hypothesis of the path of the phosphorus in the aquatic system (upper panels). Measurement setup (lower left panel) and measured spectra (lower right panel) [1] [2].

Poster: A151, March 5th, 16:00 - 18:00, 1st floor

THE PETRA PHONE PROPOSAL: DETECTING HIGH-ENERGY NEUTRINOS IN BEDROCK

T. Enqvist1, J. Joutsenvaara2, P. Kuusiniemi1, K. Loo1,3, M. Slupecki1, W.H. Trzaska1

timo.t.enqvist@gmail.com
1 Department of Physics, University of Jyväskylä
2 Kerttu Saalasti Institute, University of Oulu
3 Institute of Physics and Excellence Cluster PRISMA, Johannes Gutenberg-Universität Mainz


Due to an extremely small flux and low interaction cross section of ultra-high energy neutrinos (energy above EeV) the target size and mass should be as large as possible. Such large target sizes can be obtained with water and ice and these two materials are currently the only ones used or planned to be used in future detectors equipped with acoustic sensors. An acoustic signal, a characteristic bipolar pressure pulse, is generated by a particle cascade following the interaction of an ultra-high energy neutrino.

We have proposed [1] that instead of water or ice bedrock could be utilized as a target material for the detection of ultra-high energy neutrinos. With the density of rock 3-times larger and the speed of sound 4-times larger compared to water, the signal amplitude of the generated bipolar pressure pulse in rock should be larger by an order of magnitude. This is also confirmed in our preliminary simulations. In addition, due to higher density of rock and longer attenuation length in rock compared to water, the event rate is much higher.

The Pyhäsalmi mine in central Finland has a unique infrastructure, rock conditions and broad borehole network for testing and developing the idea of acoustic detection of ultra-high energy neutrinos. To our knowledge the utilization of bedrock for that purpose has never been considered before. For an in-depth review of the field see, for example, Ref. [2].

A borehole network with acoustic sensors would allow to construct a large-volume detector array with quite moderate costs. We present the physics motivation, simulations comparing water and rock, and ideas for testing and prototyping the detection of acoustic signals in bedrock in the Pyhäsalmi mine.


[1] W.H. Trzaska et al., Proc. ARENA 2018, to be published in EPJ Web of Conferences

[2] R. Lahmann, Nucl. Part. Phys. Proc. 273-275 (2016) 406


Poster: A152, March 5th, 16:00 - 18:00, 1st floor

MEASUREMENT OF THE TOP-QUARK MASS USING THE BEST-ASSISTED R DISTRIBUTION IN CMS

Minsuk Kim1

minsuk.kim@helsinki.fi
1 Helsinki Institute of Physics


A measurement of the top-quark mass is presented using a template method based on the Bi-Event Subtraction Technique (BEST). Top-quark-pair candidate events with one muon and at least four jets in the final state are selected in proton-proton collision data recorded by the CMS experiment in Run 1. The dataset corresponds to an integrated luminosity of 19.7 fb$^{-1}$. The top-quark mass is extracted from the distribution of the ratio $R$ of three-jet and two-jet invariant-mass combinations, corresponding to the top-quark and W boson candidate masses, respectively. The BEST method is used to determine the shape of the background distribution. By taking the ratio, a significant reduction of the dominant systematic uncertainty associated to the jet energy scale is achieved. The top-quark mass is measured to be $m_t = 172.61 \pm 0.57~(\rm stat.) \pm 0.90~(\rm syst.)$ GeV, in agreement with previous measurements.



Figure 1
Figure 1: Summary of the CMS mass measurements in Run 1, including the result derived from BEST backgrounds.

Poster: A153, March 5th, 16:00 - 18:00, 1st floor

DIJET INVARIANT MASS STUDIES WITH ALICE AT THE LHC

O. Saarimäki1

oskari.a.m.saarimaki@jyu.fi
1 University of Jyväskylä


Relativistic heavy ion collisions are used to study extremely hot and dense quark-gluon plasma (QGP). QGP is born in the collisions for a very short time, so the methods for studying it rely on the particles which are born in the heavy ion collision itself. One such method is to observe the energy loss of high energy quarks and gluons which propagate through the thermalized QGP medium. High energy partons, as quarks and gluons are together called, hadronize and what is left to measure is a well collimated shower of hadrons called a jet. Jets have indeed been measured to lose energy, and this effect is generally called jet quenching. Jet quenching has been used to quantify the transport coefficient of the medium which describes the energy loss.

Because of the momentum conservation, a high energy parton has a back-to-back pair. These two jets together form a dijet system. There is a possibility that a single parton is born in the outer region of the collision zone and thus would not traverse the medium enough for energy loss to happen, but a back-to-back two parton system has a higher chance that at least one of the partons has traversed the medium [1], and this is why dijets are a interesting subject to study in heavy ion collisions. I have chosen to inspect dijet invariant mass spectrum as previous studies from RHIC [2] and LHC [3,4] indicate that it can be sensitive to modifications caused by the QGP medium.

The final goal of my work is to observe the centrality dependency of a dijet invariant mass spectrum in lead-lead collisions. I have the first results from proton-lead collisions which is a natural step to see if cold nuclear matter modifies the spectrum. The results are corrected of detector inefficiencies using unfolding algorithms. In addition I have also compared different Monte Carlo event generators including Pythia, JEWEL and Jetscape.


[1] Thorsten Renk and K. Eskola. ”Prospects of medium tomography using backto-back hadron correlations”, arXiv: hep-ph/0610059 [hep-ph]
[2] J. Adams et al. ”Evidence from d + Au measurements for final state suppression of high p(T) hadrons in Au+Au collisions at RHIC”, arXiv: nucl-ex/0306024 [nucl-ex]
[3] Georges Aad et al. ”Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at $\sqrt{s_{NN}} = 2.76$ TeV with the ATLAS Detector at the LHC”, arXiv: 1011.6182 [hep-ex]
[4] Serguei Chatrchyan et al. ”Jet momentum dependence of jet quenching in PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV”, arXiv: 1202.5022 [nucl-ex]


Poster: A154, March 5th, 16:00 - 18:00, 1st floor

ALICE UPGRADE FOR LHC RUN 3 AND RUN 4

Maciej Slupecki1

maciej.slupecki@cern.ch
1 University of Jyvaskyla, Department of Physics


The LHC has entered a two-year upgrade period of Long Shutdown 2 (LS2) in December 2018. It will be followed by Run 3 and 4, during which ALICE will conduct high-precision measurements of rare probes over a broad range of transverse momenta focusing on low signal-to-background probes at low $p_\mathrm{T}$. To achieve this goal a sustained Pb$-$Pb readout rate of up to 50 kHz must be maintained while operating either continuously or with a minimum bias trigger. Altogether ALICE will gain two orders of magnitude in the statistics over the combined data collected during Run 1 and Run 2. To cope with the challenges to register and analyse such a huge amount of data, ALICE is implementing new hardware and software solutions. In particular, three new detectors are currently installed: the Inner Tracking System (ITS), the Muon Forward Tracker (MFT) and the Fast Interaction Trigger (FIT) detector. Their arrangement is presented in the figure below.

The base building block of the new trackers is an ALIPIDE (ALICE Pixel Detector) chip, a custom designed Monolithic Active Pixel Sensor (MAPS) incorporating the requirements imposed by the physics program, including high-granularity, low material budget and radiation hardness. The new sensor will improve vertexing and tracking, especially at low $p_\mathrm{T}$. The use of ALIPIDE by the Muon Forward Tracker will add vertexing capabilities to the Muon Spectrometer covering a broad range of transverse momenta and allowing ALICE to measure beauty down to $p_\mathrm{T} \sim 0$ from displaced J/$\psi$ vertices and to have an improved precision for the $\psi$(2S) measurement. It will also add high-granularity data to the forward multiplicity information acquired by FIT. In addition to providing inputs for the new Central Trigger Processor, FIT will serve as the main luminometer, collision time, multiplicity, centrality, and reaction plane detector for the ALICE experiment.


Poster: A155, March 5th, 16:00 - 18:00, 1st floor

BRIDGING LOW AND HIGH VELOCITY STOPPING FORCE REGIMES: A MODIFICATION OF THE BETHE-BLOCH MODEL

Sascha Lüdeke1, Arto Javanainen1,2

sascha.a.f.ludeke@jyu.fi
1 Department of Physics, University of Jyvaskyla, FI-40014 Jyvaskyla, Finland
2 Electrical Engineering and Computer Science Department, Vanderbilt University, Nashville, TN 37235 USA


The stopping force of low and high velocity heavy ions is well investigated and described by different models. Bohr [Lindhard and Sørensen, 1996] and Bethe-Bloch [Ferrariis and Arista, 1984] presented models that cover the intermediate and high velocity regimes, while the Firsov [Teplova et al., 1962] the LSS [Lindhard and Scharff, 1961] models describe the low velocity regime.

Despite various models, presently no unifying theory or model exists covering the entire particle velocity range. This work proposes a semi-empirical modification to the Bethe-Bloch model able to describe the entire velocity range utilizing two parameters.

Based on the relations shown in Equations (1) and (2), an addition to the stopping number formula was made for the Bethe-Bloch model.


$(1)\qquad ln(1+x) \approx x, \text{for }x\ll1\\\\(2) \qquad ln(1+x) \approx ln(x), \text{for } x \gg 1$

This resulted in the following form for the stopping number for the Bethe-Bloch model as displayed in Equation (3), where $p_0$ and $p_1$ indicate fitting parameters.


$(3)\qquad L_{Bloch-mod} = p_0\ln\left(1 + p_1\cdot C\cdot\frac{m_e v^3}{Z_1 I v_0}\frac{1}{\sqrt{1+\left(\frac{Cv}{2Z_1 v_0}\right)^2}}\right)\\$

When using Eq. (1) and (2) the stopping number converges towards Bethe-Bloch model for high velocities. At low velocities, the modified expression converges into a form very similar as described by the Firsov and LSS models. This indicates the ability of this approach to cover the low and high velocity regimes of the ion stopping force calculations. The results estimated by this model for the stopping force of oxygen ions in aluminum are shown in Fig. 1.

Figure 1 (left side) compares stopping force values calculated from different models and SRIM simulations to measured data taken from literature for oxygen ions in aluminum targets. This graph underlines the improved ability of the modified model to cover the entire velocity range over the classical models as well as over SRIM simulations.

Figure 1 (right side) compares the relative deviation of the modified model from the measured data to the same deviation for the SRIM simulation. It shows that the performance of the proposed model is comparable to the performance of SRIM in the intermediate and high velocity regime and outperforms it in the low energy regime.

The final goal of this work will be to determine $p_0$ and $p_1$ values based experimental data of various ion target combinations and investigate the possibility of modelling functions that will correlate the fitting parameters to the atomic numbers of target and projectile.


References:
- Lindhard, J. and Sørensen, A. H. (1996). On the relativistic theory of stopping of heavy ions, Phys. Rev. A 53, 2443–2456.

- de Ferrariis, L. and Arista, N. R. (1984). Classical and quantum-mechanical treatments of the energy loss of charged particles in dilute plasmas, Phys. Rev. A29, 2145–2159.

- Teplova, Y. A., Nikolaev, V. S., Dimitriev, I. S. and Fateeva, L. N. (1962). Slowing down of multicharged ions in solids and gases, Zh. Eksp. Teor. Fiz. 42, 44–60, [English translation: Sov. Phys. JETP 15, 31-41 (1962)].

- Lindhard, J. and Scharff, M. (1961). Energy dissipation by ions in the keV region, Phys. Rev. 124, 128–130.



Figure 1
Figure 1: Left: Stopping force values for various models for oxygen ions in an aluminum target. Additionally experimental data taken from literature [https://www-nds.iaea.org/stopping/] are presented. Right: Relative deviation of the proposed model and SRIM simulations [http://www.srim.org/] from the experimental stopping force values.

Poster: A156, March 5th, 16:00 - 18:00, 1st floor

SEARCH FOR PROTON HALOS IN THE EXCITED STATES OF 12N

J. Louko1, N. Burtebaev2, A.N. Danilov3, A.S. Demyanova3, S. V. Khlebnikov4, Yu. G. Sobolev5, V.I. Starastsin3, W. Trzaska1, G.P. Tyurin4

jussi.i.louko@student.jyu.fi
1 Department of Physics, University of Jyväskylä
2 Institute of Nuclear Physics, National Nuclear Center of Republic of Kazakhstan
3 NRC Kurchatov Institute
4 V. G. Khlopin Radium Institute
5 Flerov Laboratory for Nuclear Research, JINR


We propose to use the $^{3}$He($^{12}$C, T)$^{12}$N reaction to populate and identify the so-called halo states among the low-lying excited states of the $^{12}$N. These exotic nuclear states are expected to have an enlarged size and a diffused surface region surrounding a core with normal nuclear density. The $^{12}$N nucleus was chosen as it is part of the A=12 triplet, including $^{12}$B, $^{12}$C, and $^{12}$N. From the known properties of the isobar analog and mirror states it is deduced that the states known to have excess size in $^{12}$C, should have the corresponding halo counterparts in $^{12}$N and $^{12}$B.

To verify the feasibility of the proposed reaction and of our experimental approach, we have conducted a four-day test at the Jyväskylä K-130 cyclotron in December 2018. The intensity of the 40 MeV $^{3}$He beam was 15-30 pnA. The beam was focused into thin carbon targets with various thicknesses (0.23, 0.5 and 0.9 mg/cm$^{2}$). Six sets of dE-E telescopes were mounted on the rotating table inside of the Large Scattering Chamber (LSC). During the allocated time we were able to measure energy spectra of tritons around thirty points between 10 and 80 degrees in the center of mass system. The collected data were used to extract the differential cross-sections of the ground state (1$^{+}$) and of the 0.961 MeV (2$^{+}$), 1.190 MeV (2$^{-}$), 1.800 MeV (1$^{-}$) and 2.439 MeV (0$^{+}$) excited states in $^{12}$N nucleus and plot them as a function of the scattering angle. The outcome will be compared to the model predictions based on the distorted wave born approximation (DWBA) yielding the information on the size of the excited states. Using this method, we have found, for instance, two neutron halo states in $^{12}$B [1].

In the poster details of the experimental approach [2] will be presented together with the preliminary results.

[1] T.L.Belayeva et al., Phys. Rev. C 98, 034602 (2018)
[2] W.H.Trzaska et al., Nucl. Inst. and Meth. A 903 (2018) 241



Figure 1
Figure 1: (a) Particle identification with the dE-E telescope. There is a clear separation between protons, deuterons and tritons. (b) Triton spectrum at 35\degree from the reaction $^{3}$He($^{12}$C, T)$^{12}$N at E($^{3}$He)=40 MeV. The dominant peaks correspond to the ground and the first excited state in $^{12}$N.

Poster: A157, March 5th, 16:00 - 18:00, 1st floor

IN-BEAM STUDIES OF 186PB EMPLOYING THE SAGE SPECTROMETER

J. Ojala1

joonas.k.ojala@jyu.fi
1 University of Jyväskylä


We will present result of in-beam spectroscopic study of $^{186}$Pb nucleus with the SAGE spectrometer. The SAGE spectrometer combines $\gamma$-ray and conversion electron spectroscopy [1]. Simultaneous observation of $\gamma$-rays and conversion electrons is important when internal conversion is enhanced.$\\$

Nucleus can possess microscopically different shapes which can typically be spherical, prolate or oblate. In $^{186}$Pb, there is experimental evidence for shape coexistence. Alpha-decay studies have shown unique triplet of 0$^{+}$ states, each of which can be associated with spherical, prolate and oblate shape [2]. Existence of different deformed minima has been confirmed in in-beam $\gamma$-ray spectroscopic studies, where rotational bands have been observed [3,4]. The combined conversion electron and $\gamma$-spectropic measurement provides more information for the highly converted transitions. For example, 0$^{+}$$\rightarrow$ $ $0$^+$ transitions proceeds mainly through internal conversion. Such E0 transitions are typically present in nuclei featuring shape coexistence. $\\$

We have conducted simultaneous conversion electron and $\gamma$-ray studies for $^{186}$Pb using the SAGE+RITU+GREAT+TDR [5,6,7] instrumentation employing recoil-decay tagging method for $^{106}$Pd$(^{83}$Kr,3n$)^{186}$Pb and $^{154}$Gd$(^{36}$Ar,3n$)^{186}$Pb reactions. Experiments were performed in 2013 and 2016 at Accelerator Laboratory of University of Jyväskylä.$\\$

In this presentation, we will show evidence for the 2$^{+}$$\rightarrow$ $ $2$^+$ transition which mainly proceed via internal conversion. Our results can be used e.g. to confirm the existence of different shapes and to estimate the deformation difference between the lowest 2+ states [2].
$\\$
$\textbf{References}$
$\\$
[1] Pakarinen, J., et al. Eur. Phys. J. A, 50(3):53, 2014.$\\$

[2] Andreyev, A. N., et al. Nature, 405:430, 2000.$\\$

[3] Heese, J., et al. Phys. Lett. B, 302(4):390 – 395, 1993.$\\$

[4] Pakarinen, J., et al. Phys. Rev. C, 72:011304, 2005.$\\$

[5] Leino, M., et al. Nucl Instrum Methods Phys Res B, 99(1):653 – 656, 1995.$\\$

[6] Page, R., et al. Nucl Instrum Methods Phys Res B, 204:634 – 637, 2003.$\\$

[7] Lazarus, I., et al. IEEE Trans. Nucl. Sci., 48(3):567–569, 2001.$\\$

[8] Kibédi, T. and Spear, R. At. Data Nucl. Data Tables, 89(1):77 – 100, 2005.$\\$


Poster: A158, March 5th, 16:00 - 18:00, 1st floor

SPACETIME-FREE QUANTUM THEORY AND EFFECTIVE SPACETIME STRUCTURE

M. Raasakka1

matti.raasakka@aalto.fi
1 Department of Electronics and Nanoengineering, Aalto University


Quantum theory and gravity, taken together, seem to imply that spacetime cannot be described by a classical continuum geometry at very small distances. Instead of substituting something in place of classical geometry, as is often done in approaches to quantum gravity, I examine the idea that spacetime geometry is an effective description of certain statistical properties of the quantum state of a system. The approach is motivated by operational considerations and the intimate relationship between quantum entanglement and spacetime geometry, which has been uncovered in recent years (see e.g. [1,2,3]).

In order to study such a proposal, I first provide a mathematical formulation for quantum theory in the absence of a predefined background spacetime geometry. I then discuss several ways in which one can infer aspects of spacetime structure for the system from the quantum state.

It is necessary, of course, to recover the familiar physics in some suitable regimes of the theory. The emergence of quantum field theory and gravity impose somewhat contradictory requirements on the theory, which sheds some light on the difficulty of formulating a quantum theory of gravity. Namely, for quantum field theoretic models, the local algebras of the theory must be hyperfinite type $III_1$ factors according to the classification of von Neumann algebras [4]. However, in order for gravitational effects to arise, the local entropy density must be finite [2], which is not the case for type $III_1$ algebras. This observation leads us to consider locally finite-dimensional models and their infinite-dimensional limits.

The presentation is partly based on the papers [5,6].


References:
[1] J. Eisert, M. Cramer, M.B. Plenio , "Area laws for the entanglement entropy - a review", Rev. Mod. Phys. 82:277 (2010), arXiv:0808.3773 [quant-ph].
[2] T. Jacobson, "Entanglement Equilibrium and the Einstein Equation", Phys. Rev. Lett. 116:201101 (2016), arXiv:1505.04753 [gr-qc].
[3] E. Witten, "Notes on Some Entanglement Properties of Quantum Field Theory", Rev. Mod. Phys. 90:45003 (2018), arXiv:1803.04993 [hep-th].
[4] J. Yngvason, "The Role of Type III Factors in Quantum Field Theory", Rept. Math. Phys. 55:135 (2005), arXiv:math-ph/0411058.
[5] M. Raasakka, "Spacetime-Free Approach to Quantum Theory and Effective Spacetime Structure", SIGMA 13:006 (2017), arXiv:1605.03942 [gr-qc].
[6] M. Raasakka, "Local Lorentz covariance in finite-dimensional Local Quantum Physics", Phys. Rev. D 96:086023 (2017), arXiv:1705.06711 [gr-qc].


Poster: A159, March 5th, 16:00 - 18:00, 1st floor

STUDIES OF ELASTIC, INELASTIC AND TOTAL PROTON-PROTON CROSS SECTIONS WITH TOTEM AT THE LHC

F. Oljemark1, on behalf of the TOTEM collaboration

fredrik.oljemark@helsinki.fi
1 University of Helsinki and Helsinki Institute of Physics


The TOTEM [1] experiment at the Large Hadron Collider is devoted to a deeper understanding of the proton structure by precise measurement of elastic, inelastic and total cross-sections and comprehensive studies of diffraction. Forward-going charged particles are detected at pseudorapidities of $3.1 \leq |\eta| \leq 4.7$ and $5.3 \leq |\eta| \leq 6.5$ by the T1 and T2 telescopes, respectively, and the leading protons by silicon detectors in Roman Pots.

TOTEM has studied in detail both elastic and inelastic proton-proton (pp) interactions at 2.76, 7, 8 and 13 TeV [2]. Here we present an analysis of the dip in the elastic cross section at 2.76 [3] and 13 TeV [4], and another analysis [6] very precisely constraining the $\rho$-parameter at 13 TeV ($\rho$ is the ratio of real to imaginary part of the nuclear pp interaction amplitude at $t = 0$), and measuring the total cross section in a complementary way.

At small momentum transfer $t$, the elastic cross section is exponential, but around $|t| \approx 0.5$ GeV$^2$ (energy-dependent) the cross section reaches a dip, followed by a maximum with a cross-section ratio max-to-dip around 1.8. We show that the dip position moves to smaller t-values as the energy increases, while the max-to-dip ratio is approximately constant [5].

At 13 TeV, we have data from two different beam configurations: $\beta^*=2500$ m down to the Coulomb-Nuclear interference region, and $\beta^*=90$ m [2] ($ 0.04 \leq |t|\leq 4$ GeV$^2 $, covering the exponential fall, the dip, and beyond). The very low $t$-reach of the former allows extracting an absolute normalization using the Coulomb amplitude, giving a completely independent measurement of the total cross section. Combining this with the previously published TOTEM measurement [2] gives an average of $\sigma_{tot} \approx 110.5 \pm 2.4$ mb.


[1] G. Anelli et al. (TOTEM Collaboration), JINST, 3 (2008) S08007. DOI:10.1088/1748-0221/3/08/S08007
[2] G. Antchev et al. (TOTEM Collaboration), First measurement of elastic, inelastic and total cross-section at $\sqrt s$ = 13 TeV by TOTEM and overview of cross-section data at LHC energies, CERN-EP-2017-321 (accepted for publication in Eur. Phys. J. C)
[3] G. Antchev et al. (TOTEM Collaboration), Elastic differential cross-section $d\sigma/dt$ at $\sqrt s$ =2.76 TeV and implications on the existence of a colourless 3-gluon bound state, CERN-EP-2018-341 (to be submitted to Eur. Phys. J. C)
[4] G. Antchev et al. (TOTEM Collaboration), Elastic differential cross-section measurement at $\sqrt s$ =13 TeV by TOTEM, CERN-EP-2018-338 (to be submitted to Eur. Phys. J. C)
[5] See the presentation by Kenneth Österberg, also at the Physics Days 2019, for comparisons with proton-antiproton interactions at the TeVatron, and theoretical interpretations.
[6] G. Antchev et al. (TOTEM Collaboration), First determination of the $\rho$ parameter at $\sqrt s$ = 13 TeV - probing the existence of a colourless three-gluon bound state, CERN-EP-2017-335 (submitted to Eur. Phys. J. C).




Figure 1
Figure 1: Elastic differential cross section at 13 TeV as a function of the momentum transfer $|t|$. The data shows a clear dip at $0.47 \pm 0.004$ (stat) $\pm 0.01$ (syst) GeV$^2$, followed by a second maximum. The cross-section ratio of the second maximum to the dip is given in the figure.

Poster: A160, March 5th, 16:00 - 18:00, 1st floor

CENTRAL EXCLUSIVE PROCESSES IN THE LHC ERA

L. Forthomme1

laurent.forthomme@cern.ch
1 Department of Physics and Helsinki Institute of Physics, University of Helsinki


With the recent experimental evidence [1] for the production of a pair of electrons or muons through the fusion of two photons, involving the detection of outgoing primary particles, the Large Hadron Collider (LHC) at CERN has moved yet one step forward in the understanding of photon-induced reactions at unprecedented scales.

As one component of the broader class of central exclusive processes, characterised by a clean final state even in busiest environments, these processes (as pictured in Figure 1) allow to probe the inner structure of matter (for instance through its large sensitivity to protons structure functions in a wide kinematics range), and the complexity of interactions at the electroweak scale.

In this poster, we will illustrate the phenomenological characterisation (theoretical modellings, computational toolbox), and experimental study of such processes as performed within the CMS programme at HIP. This will allow to introduce the $k_{\rm T}$ factorisation procedure [2] allowing to divide analytically the central exclusive process theoretically contribution from the global proton-proton picture, while accounting for the transverse momentum of incoming partons, and the Precision Proton Spectrometer [3] constructed and operated since 2016 at the CMS experiment at LHC.

[1] CMS-TOTEM Collaborations, A. M. Sirunyan et al., JHEP 07, 153 (2018), arXiv:1803.04496.
[2] G. Gil da Silveira, L. Forthomme, et al., JHEP 02 (2015) 159, arXiv:1409.1541.
[3] CMS-TOTEM Collaborations, M. Albrow, M. Arneodo, et al., CMS-TOTEM Precision Proton Spectrometer, Tech. Rep. CERN-LHCC-2014-021. TOTEM-TDR-003. CMS-TDR-13, CERN, Geneva, Sep, 2014.



Figure 1
Figure 1: A generic two-photon process produced in proton-proton collisions. Each photon emission from the initial proton may lead to an intact final state proton, or an excited system dissociating in a diffractive state.

Poster: A171, March 5th, 16:00 - 18:00, 1st floor

INTERACTION OF CHARGED POLYMERS IN AQUEOUS SOLUTION IN THE PRESENCE OF EXCESS IONS

Zahra Mohammadyarloo1, Sousa Javan Nikkhah1, Sahin Buyukdagli2, Maria Sammalkorpi1, Tapio Ala-Nissilä3,4

zahra.mohammadyarloo@aalto.fi
1 Department of Chemistry and Materials Science, Aalto University School of Chemical Engineering, P.O. Box 16100, FI-00076 Aalto, Finland
2 Department of Physics, Bilkent University, Ankara 06800, Turkey
3 Interdisciplinary Centre for Mathematical Modelling and Department of Mathematical Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
4 Department of Applied Physics and QTF Center of Excellence, Aalto University School of Science, P.O. Box 11000, FI-00076 Aalto, Finland


Charged polymers, polyelectrolytes (PEs) are immensely relevant in both biology and technology. For example nucleic acids, DNA, RNA, cytoskeletal filaments F-actin, microtubules, and most proteins are PEs. Synthetic polyelectrolytes form the basis of polyelectrolyte multilayers and complexes which are highly versatile, tunable materials with applications e.g. in drug delivery, battery materials, responsive coatings, and water purification. Electrostatic interactions between the PEs and the presence of ions and ion aggregates play an important role both for biological function and technological applications of the PEs. Here, we focus on resolving the effect of monovalent and multivalent salt solutions on simple PE systems via molecular modelling methods. We compare an all-atom molecular dynamics simulations predicted ion distribution around polyelectrolytes described chemically specific detail and the resulting interactions between PEs with mean field Poisson-Boltzmann predictions. We connect the findings with synthetic PE layer-by-layer assemblies mechanical characteristics and DNA interactions in salt solutions.


Poster: A172, March 5th, 16:00 - 18:00, 1st floor

FUNCTIONAL CONFORMATIONAL DYNAMICS OF THE CHOLESTEROL CARRIER NPC1

Giray Enkavi1, Xavier Prasanna1, Ilpo Vattulainen1

giray.enkavi@helsinki.fi
1 Department of Physics, University of Helsinki


Cholesterol is an essential lipid that modulates for the structure and permeability of cellular membranes. Besides, cholesterol is the precursor for many bioactive molecules, such as steroid hormones, bile acids, oxysterols, and vitamin D. Constituting 20-25 mol% of plasma membrane lipids, cholesterol is ubiquitously present in all vertebrate cells. Due to its structural and functional roles, a network of cellular signaling and transport systems tightly regulates cholesterol trafficking. Intracellular cholesterol trafficking involves various soluble and membrane proteins, which act as sensors or transporters in lipid metabolism, vesicle trafficking, and signal transduction.
NPC1 and NPC2 work together to facilitate endosomal and lysosomal cholesterol trafficking. The two proteins have been implicated in many severe disease conditions. They get their name from the Niemann-Pick C disease, a rare fatal genetic lysosomal storage disorder. Mutations of NPC1 or NPC2 result in accumulation of lipids, such as unesterified cholesterol, glycosphingolipids, sphingomyelin and sphingosine, progressively causing neuronal degeneration and early death. NPC2 is a small soluble protein that shuttles cholesterol between the internal membranes of the lysosomes and the late endosomes. Moreover, NPC1 takes the cholesterol fro mt eh internal memrbanes to NPC1, a transmembrane protein residing in the limiting membrane. NPC1, in turn, is responsible for cholesterol efflux through the limiting membrane.
In our previous work, we investigated NPC2-membrane binding revealing the roles of various lipid components in a lipid-mediated regulation mechanism for NPC2-meadiated cholesterol transport. Recently, cryo-EM and several crystal structures of NPC1 were released. With this information available, we extend our investigations to cholesterol transport mechanism by NPC1. NPC1, unlike NPC2, is complex transmembrane protein with several domains. The structures only capture a single conformational state of the protein; thus, the transport mechanism for this complex machine remains unknown at the molecular level. We employed biased and unbiased molecular dynamics simulations, as well as, molecular modeling approaches to characterize the conformational dynamics of NPC1 in its transport cycle.


Poster: A173, March 5th, 16:00 - 18:00, 1st floor

STUDYING LANGUAGE LATERALIZATION OF THE BRAIN WITH FUNCTIONAL MRI

K.I. Nousiainen1,2, L.J. Kuusela1,2

katri.nousiainen@hus.fi
1 Medical Imaging Center, Helsinki University Hospital, Helsinki, Finland
2 Department of Physics, University of Helsinki, Helsinki, Finland


Language is a lateralized brain function; in the majority of individuals, language-processing areas are located in the left hemisphere of the brain. Determining the dominant hemisphere of language processing is necessary for example in the pre-surgical planning of epilepsy patients. Functional magnetic resonance imaging (fMRI) studies the brain functions non-invasively [1]. From the fMRI data, a laterality index that describes the degree of language lateralization can be calculated [2].

In this work, we present a calculation method for the laterality index that uses nine local regions of interests obtained from Montreal Neurological Institute’s structural atlas [3]. We applied this method to the fMRI data of five epileptic subjects. The data was acquired with a 3 Tesla MRI scanner, while subjects performed three language-related tasks, and analyzed with an FSL-software [4]. The laterality indices were calculated with an in-house written code.

Figure 1 shows an example of the results from the FSL-analysis (Fig. 1a) and the laterality index calculation (Fig. 1b). The presented method determines the language lateralization correctly for three subjects out of five. For two subjects, who suffer from poor quality of the fMRI data, it yields contradicted results. Consequently, the results of the laterality index calculation can provide additional information to the epilepsy patients’ clinical data, when the acquisition and preprocessing of the fMRI data are successful.


[1] Chen, J.E. & Glover, G.H. (2015). Neuropsychology review, 25(3), 289-313.
[2] Seghier, M.L. (2008). Magnetic resonance imaging, 26(5), 594-601.
[3] Collins, D.L. et al. (1995). Human brain mapping, 3(3), 190-208.
[4] Jenkinson, M. et al. (2012). Neuroimage, 62(2), 782-790.



Figure 1
Figure 1: a) the active areas during a language task in red-yellow and the regions of interest in grey, b) a table of the regions of interest and corresponding laterality indices

Poster: A174, March 5th, 16:00 - 18:00, 1st floor

MRI-ONLY PLANNING FOR EXTERNAL BEAM RADIATION THERAPY OF PELVIS: DOSIMETRIC EVALUATIONS WITH ARTIFICIAL TREATMENT PLANS

T. Pavel1, J. Keyriläinen2, S. Suilamo2, Y. Häme, M. Pesola

tomas.pavel@philips.com
1 Philips Healthcare
2 Turku University Hospital


MRI-only radiation therapy (RT) is a novel treatment workflow utilising magnetic resonance imaging (MRI) as a sole modality, and thus avoiding spatial inaccuracies caused by the CT (computed tomography) and MRI image registration. In the MRI-only workflow, MRI images are used for contouring of sensitive organs and treatment targets as well as for generation of synthetic CT providing electron density information for dose calculations. When it comes to the evaluation of dosimetric accuracy of the synthetic CT, clinical CT-based RT plans have to be available as an essential input for dose comparisons. However, the collection of a sufficient amount and distribution of clinical data becomes problematic to impossible, mainly, when one needs to evaluate the safe use of synthetic CT for the treatment of multiple cancer types with varying tumour location.


We present the automated dose evaluation method with artificial RT plans enabling extensive dose evaluation studies with significantly reduced demands on the availability of clinical data sets. Described method was used to evaluate commercial synthetic CT solution, namely Philips MRCAT (Magnetic Resonance for Calculating ATtenuation, Philips Oy, Vantaa, Finland), with an intended application for external beam photon therapy for male and female pelvic cancer patients. All patient data used in this study were provided by the Turku University Hospital (Turku, Finland). Reference CT-based artificial RT plans were automatically generated in the Pinnacle3 (version 16.0.2, Philips Medical Systems Inc., Fitchburg, WI, USA) treatment planning system. Overall, 631 reference RT plans were optimised using 64 patient datasets for irradiation of semi-randomly positioned spherical PTV (planning target volume) structures. These RT plans were recalculated on MRCAT images. Reference and recalculated dose distributions were compared using dose comparison methods including mean PTV dose difference, gamma analysis, and dose volume histograms.


The outcome of this work is the evidence of possibility to increase the confidence about the safe use of the synthetic CT by the use of dose evaluations with a large number of artificial RT plans alongside the conventional approach using only clinical RT plans, while not increasing demands on clinical data availability.


Poster: A175, March 5th, 16:00 - 18:00, 1st floor

LATERAL DIFFUSION ALONG CURVED LIPID BILAYERS

P. Kaurola1,2, M. Javanainen3,1,2, I. Vattulainen1,2,4

petri.kaurola@helsinki.fi
1 Department of Physics, University of Helsinki, FI-00014 Helsinki, Finland
2 Laboratory of Physics, Tampere University of Technology, FI-33101 Tampere, Finland
3 Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, CZ-166 10 Prague 6, Czech Republic
4 MEMPHYS – Center for Biomembrane Physics, Department of Physics, University of Southern Denmark, 5230 Odense, Denmark


Cell membranes are complex heterogenous fluids where lipids and proteins, the main constituents of biological membranes, are under constant motion caused by thermal fluctuations. The lateral diffusion along lipid bilayers is considered to be a major factor in promoting lipid-protein interactions which are an essential part of proper protein function. Due to the importance of these interactions, lateral diffusion, and membrane dynamics in general, have been of scientific interest for decades; however, many of the specifics of the lateral diffusion in membranes remain unclear.

Commonly, attempts to describe lipid and protein dynamics in membranes have been based on using simplified planar membrane models with only one or a few different lipid types and with a low concentration of proteins. Though these studies, both experimental and computational, have provided us insight into the basic nature of diffusion in membranes, such approaches strip away many of the properties of biological membranes; we are well aware that the biological membranes consist of numerous different lipid types, they are packed with proteins, and at least to some extent, they are almost exclusively curved. Fortunately, the advancements in experimental methods and the increase in computing power are starting to allow us to introduce these complexities into diffusion studies in order to help us understand diffusion in real biomembranes.

In this work, we are the first to study how both membrane curvature and protein crowding affect lipid and transmembrane protein diffusion by using molecular dynamics simulations. By studying systems with varying membrane tube radius and protein to lipid ratio, we are able to see the explicit differences in diffusion caused by curvature and protein crowding. From our results, we are able to see that nanoscale curvature has a hindering effect on diffusion, which is comparable in magnitude to crowding corresponding to biological concentrations. Additionally, we see that unlike crowding, curvature doesn’t strongly contribute to anomalous diffusion of proteins.



Figure 1
Figure 1: One of the nine simulated membranes tubes with inner radius approximately 15 nm and with 1:75 protein to lipid ratio.

Poster: A176, March 5th, 16:00 - 18:00, 1st floor

CONTRAST AGENT IMAGING AND TISSUE DENSITY ESTIMATION WITH DUAL-ENERGY COMPUTED TOMOGRAPHY

Tuomas Vuokko1, Liisa Porra1,2

tube@saunalahti.fi
1 Helsinki University - Department of Physics
2 HUS


Since its invention in the 1970’s, computed tomography (CT) has revolutionized radiology by enabling faster and more precise imaging of the human body. Traumatic imaging as well as the examination of longer-term changes, such as cancer, are improving fast as technology evolves. Dual-energy CT has already been in clinical use for quite some time, and has brought in new, still less well-known tools for us to use. The aim of this study is to examine contrast-agent enhanced imaging and tissue density calculations in dual-energy CT images. In addition, potential benefits of dual-energy CT compared to traditional computed tomography imaging are also discussed.

A second-generation dual-energy CT device Siemens SOMATOM Definition Flash and its accompanying Syngovia software were used to implement this study. Iodine solution mixed with water was used as a contrast agent, in proportion to the typical real-life contrast concentrations encountered within patients. The test tubes, containing varied concentrations of iodine, were placed on a CIRS 062M density phantom, which is used to calibrate CT devices and then scanned with dual energy protocols. The data obtained from the images were tabulated using the Syngovia software and the ImageJ program.

Measuring the concentration of the contrast agent within the diagnostic range of 0-300 HU, the obtained data was close to the expected, calculated values (within a mean ± 6% with the torso phantom, ± 4% with the head phantom). In the virtual native images reconstructed by Syngovia, i.e. the images where the effect of the contrasting agent is removed, the densities were close to the expected value of water (0 HU). With higher concentrations of iodine, however, the algorithms failed, losing accuracy rapidly beyond 200 HU. The estimated effective atomic number and tissue density were also within reasonable range of their expected values.

This study shows that estimation of iodine concentrations works sufficiently with low iodine concentrations, and the virtual native algorithm removes contrast agent in the images well. However, with higher iodine concentrations, especially the virtual native algorithm fails.

There are already several dual-energy CT applications, and the area is developing fast. As computing power increases, complex algorithms will be faster to execute and more precise. The visual tools and different reconstruction algorithms will improve diagnostics, and the additional computational information provided by dual-energy tomography is undeniable.


Poster: A181, March 5th, 16:00 - 18:00, 1st floor

REAL-TIME RESPONSE FROM IMAGINARY-TIME SIMULATION: HACKING QUANTUM STATISTICS WITH PATH-INTEGRAL MONTE CARLO

J. Tiihonen1, I. Kylänpää1, T.T. Rantala1

1 Computational Physics Laboratory, Tampere University, P.O. Box 692, FI-33014 Tampere, Finland


Variants of Quantum Monte Carlo (QMC) excel at describing complex quantum statistical problems, such as exact many-body correlations. Unfortunately, most QMC methods operate in imaginary time, which makes estimation of real-time dynamic properties a notorious challenge [1]. Theoretically, the estimation is possible by performing analytic continuation on generalized susceptibilities, or the quantum correlation functions. However, the practical implementation is an ill-posed inversion problem, which defies even the most sophisticated methods. Here we focus on a particular numerical approach: Maximum Entropy [1].

For demonstration, we consider the standard electric field response of small atoms and molecules: static and dynamic multipole polarizability. The polarizability is a fundamental property involved in a plethora of physical models, including infrared activity, optical dispersion, and x-ray scattering. Indeed, the full spectrum of electric field response is often decoupled in the adiabatic approximation. However, we have managed to go beyond the Born–Oppenheimer approximation by using multiscale path-integral Monte Carlo simulation: In a recent work, we present the combined electronic, rovibrational and nonadiabatic effects on the total dynamic polarizability [2]. The results include estimates of the exact multipole spectra, dynamic polarizabilities, and – for the general enjoyment – accurate van der Waals coefficients from first principles.


[1] M. Jarrell & J.E. Gubernatis, Bayesian inference and the analytic continuation of imaginary-time quantum {Monte Carlo} data, Physics Reports 269, 133 (1996)

[2] J. Tiihonen and I. Kylänpää and T.T. Rantala, Computation of Dynamic Polarizabilities and van der Waals Coefficients from Path-Integral Monte Carlo, Journal of Chemical Theory and Computation 14, 5750 (2018)


Poster: A182, March 5th, 16:00 - 18:00, 1st floor

MEASURING PHYSICS PUBLICATIONS: WHICH WAY FORWARD?

Eva Isaksson1, Riku Hakulinen1

Eva.Isaksson@helsinki.fi
1 Helsinki University Library, University of Helsinki


Use of metrics to evaluate research publications has been on constant increase in the recent years. Several new databases and approaches have been introduced, and the existing ones are constantly being enhanced with new metadata and new functionalities. What does this mean from the point of view of physics publications?

At Helsinki University Library, we use several metrics databases: Web of Science (with InCites) from Clarivate, Scopus (with SciVal) from Elsevier, Google Scholar, Microsoct Academic, Altmetric.com, and most recently Dimensions from Digital Science. The choice of database depends on the needs of each discipline. To address the above question, we have put four of these databases to test, using physics publication data from University of Helsinki.

We look at coverage, timeliness, impact and metadata. We ask whether altmetrics is really meaningful from the point of view of physics? Does physics benefit from recent developments in metrics, such as Emerging Sources Citation Index for Web of Science?

There is a growing trend to use linked sources to enhance the existing metadata, e.g. the Web of Science and Scopus partnerships with ImpactStory to add Open Access metadata. Dimensions, the newest player in the field of metrics databases, claims to bring together metadata from several sources. Which disciplines are driving this development – and is physics among them


Poster: A183, March 5th, 16:00 - 18:00, 1st floor

ATMOSPHERIC MOLECULAR CLUSTERS PARTICIPATING IN NEW PARTICLE FORMATION

Jakub Kubecka1, Vitus Besel1

jakub.kubecka@helsinki.fi
1 INAR, University of Helsinki, Finland


Atmospheric air pollutants are responsible for plenty of human diseases and even for up to 7 million premature deaths per year (WHO, 2014). Not just pollutants but also other aerosol particles play an important role in atmospheric chemistry and processes such as light scattering, absorption of radiation, cloud nucleation or ice crystallization. Approximately half of all particles in the Earth’s atmosphere are formed from gaseous precursors, while the remaining aerosols are directly emitted as particles. Unfortunately, the processes of formation are still not well understood. In our research, we focus on small acid-based clusters relevant to the atmosphere which seem to be driving growth of particles.

For an accurate theoretical analysis of molecular clusters, the proper configurational sampling of clusters and analysis of the whole potential surface is required. Unfortunately, since the number of possible cluster conformations growths exponentially with the size of the molecular cluster, proper searching of the PES becomes computationally very expensive. Thus, we have to use certain strategies in order to reach proper configurational sampling of the desired quality and to find the global minimum. And this is the point, where modern techniques such as genetic algorithms, machine learning etc. can be taken into action. In this work, we present how we are dealing with the configurational sampling of atmospheric molecular clusters and point out the main obstacles of the final procedure.

Proper configuration sampling of molecular cluster is important because molecular cluster kinetics and population dynamics variables such as evaporation rates are exponentially dependent on quantum properties (free energy). And consequently, non-proper configurational sampling might cause orders of magnitude differences in evaporation rates. We present a general way for a theoretical configurational sampling of equilibrated molecular clusters containing mixtures of different molecules such as sulphuric acid (H$_2$SO$_4$), ammonia (NH$_3$), dimethyl-amine (NH(CH$_3$)$_2$) or guanidine (CH$_5$N$_3$). By calculating their quantum properties, we are able to calculate cluster populations in the atmosphere (using Atmospheric Clusters Dynamic Code (ACDC)), and thus see, how different molecules participate in new particle formation.

ACKNOWLEDGEMENTS: This work is supported by the European Research Council project 692891-DAMOCLES, Academy of Finland and ATMATH project. We would like to thank also CSC–Finnish IT Centre for access to computer clusters and computational resources.


Poster: A184, March 5th, 16:00 - 18:00, 1st floor

MODELLING OF UNCERTAINTIES IN ULTRASOUND SENSOR LOCATIONS IN PHOTOACOUSTIC TOMOGRAPHY

Teemu Sahlström1, Aki Pulkkinen1, Jenni Tick1, Tanja Tarvainen1,2

teemu.sahlstrom@uef.fi
1 Department of Applied Physics, University of Eastern Finland
2 Department of Computer Science, University College London


Photoacoustic tomography (PAT) is an emerging biomedical imaging modality which combines optical contrast of soft tissues with high spatial ultrasound resolution [1]. Some of the most prominent applications include imaging of soft tissues, detection of skin and breast cancer, and small animal imaging in biomedical applications. In PAT, imaged target is illuminated by a short pulse of visible or near-infrared light. The resulting ultrasound waves, generated by the photoacoustic effect, are measured on the boundary of the target using ultrasound sensors. In the inverse problem of PAT, the initial pressure distribution is reconstructed from these time-varying ultrasound measurements. The image reconstruction problem in PAT is an ill-posed inverse problem, and thus even small errors in modelling or measurements can result to large errors in the reconstructed image. These modelling errors can arise from several different sources such as simplifications of the numerical approximation of the forward model or incorrect assumptions of the model parameters.

In this work, the inverse problem of PAT is approached in the framework of Bayesian inverse problems [2,3]. We study modelling errors due to uncertainties in ultrasound sensor locations using Bayesian approximation error approach [2]. Ultrasound propagation is modelled using $k$-space method [4]. The approach was evaluated with simulations. It was noticed that errors in modelling of sensor locations resulted in significant reduction of the quality and accuracy of the reconstructed images. The effects of these modelling errors were, however, well compensated by using the approximation error approach, and the images were greatly improved. Thus, based on the results, the approximation error approach could be beneficial in situations where knowledge of accurate ultrasound sensor positions is not available.

[1] P. Beard, ”Biomedical photoacoustic imaging”, Interface Focus 5(1):602-631, 2011.
[2] J. Kaipio and E. Somersalo, “Statistical and computational inverse problems”, Springer
New York, 2010.
[3] J. Tick, A. Pulkkinen and T. Tarvainen, ”Image reconstruction with uncertainty quantification in photoacoustic tomography”, The Journal of the Acoustical Society of America 139(4):1951-1961, 2016.
[4] B. Treeby and B. Cox, ”$k$-Wave: MATLAB toolbox for the simulation and reconstruction
of photoacoustic wave fields”, Journal of Biomedical Optics 15(2):1-12, 2010.


Second Floor

Time: March 5th, 16:00 - 18:00

Poster: A201, March 5th, 16:00 - 18:00, 2nd floor

CONTROLLING ELECTRON AND PHONON HEAT FLUXES IN SUSPENDED SEMICONDUCTOR-SUPERCONDUCTOR JUNCTIONS

E. Mykkänen1, J. S. Lehtinen1, L. Grönberg1, A. Shchepetov1, A. V. Timofeev1, D. Gunnarsson1, A. Kemppinen1, A. J. Manninen1, M. Prunnila1

1 VTT Technical Research Centre of Finland Ltd


For scalable solid-state quantum technologies, there appears to be no alternative to the temperature operation below 1 K, and it is evident that the required low-temperature infrastructure has been an obstacle for development of quantum devices. Although maintenance-free dry dilution refrigerators are now available, using large refrigerators limits the application only to large facilities such as future quantum data centres.

To solve this problem, we have developed a thermionic solid state mK - cooler platform for quantum devices [1]. The platform is a mm-scale silicon sub-chip (Fig. 1) that is suspended by micron scale semiconductor-superconductor (Sm-S) tunnel junctions [2]. The junctions function as both: thermal isolation and electrical coolers. The interfacial thermal boundary resistance, due to the lattice mismatch between the junctions and the sub-chip, provides phonon isolation. The superconducting energy gap enables cooling of the platform by quasi-particle filtering [3] and it provides highly efficient barrier for electron mediated heat transport. So far we have demonstrated refrigeration of the platform by 40 % from bath temperature of 170 mK.

We have simulated that the platform can be improved by sophisticated material and phonon engineering and multistage cascade structure to enable cooling even from 1.5 K to about 100 mK. This would be a highly cost effective method, both economically and in energy, to reach low temperatures with potential to bring quantum technologies to tabletop pulse tube refrigerators. Furthermore, the platform is a useful test bench for nanoscale heat transport.

This project was financially supported by H2020 programme FET-open project EFINED (project number 766853).

References:
[1] E. Mykkänen et al. Efficient thermionic operation and phonon isolation by a semiconductor-superconductor junction, https://arxiv.org/abs/1809.02994
[2] J. Muhonen, M. Meschke, J. Pekola, Rep. Progr. Phys. 75, 046501 (2012).
[3] D. Gunnarsson el al., Sci. Rep. 5, 17398 (2015).



Figure 1
Figure 1: An example of our cooler platform. The diameter of the sub-chips varied between c. 300 $\mu$m and 1 mm.

Poster: A202, March 5th, 16:00 - 18:00, 2nd floor

EXTENDING THE APPLICATIONS OF MACHINE LEARNING INTERATOMIC POTENTIALS: RADIATION DAMAGE ANALYSIS IN SILICON

A. Hamedani1, J. Byggmästar1, K. Nordlund1, G. Alahyarizadeh2, R. Ghaderi2, F. Djurabekova1, G. Sosso3

ali.hamedani@helsinki.fi
1 Department of Physics, University of Helsinki, P.O. Box 43, FI-00014, Helsinki, Finland
2 Engineering Department, Shahid Beheshti University, G.C, P.O. Box 1983969411, Tehran, Iran
3 Department of Chemistry and Centre for Scientific Computing, University of Warwick, Coventry, UK


Machine learning interatomic potentials build a bridge between the accuracy of first principles calculations and fastness of the empirical interatomic potentials in atomistic-level simulations; making it possible to simulate systems containing thousands of atoms with DFT accuracy. In this approach, the topology of the potential energy surface is non-parametrically learned from reference electronic structure data which covers considerable configurational diversity of the system. Resultant potential has the DFT accuracy of reference data, however, is several orders of magnitude faster than DFT. Gaussian Approximation Potential (GAP) was used to simulate radiation damage in Si for the very first time. This potential accurately reproduces density functional theory reference results for a wide range of observable properties, including crystal, liquid, and amorphous bulk phases, as well as point, line, and plane defects. To modify the potential to be implemented in radiation damage simulations, the repulsive potential was added so that DFT calculated threshold displacement energies in <111>, <110> and <110> directions were accurately captured. Moreover, the behavior of modified potential in various shorter interatomic distances along above-mentioned directions was meticulously checked against DFT simulations. This potential is a starting point to conduct radiation damage simulations with DFT accuracy in large scale which would undoubtedly enhance our insights about radiation-induced defects in Si.


Poster: A203, March 5th, 16:00 - 18:00, 2nd floor

HYBRID CLUSTER-EXPANSION AND DENSITY FUNCTIONAL THEORY APPROACH AND ITS APPLICATION FOR OPTICAL ABSORPTION IN TiO2

O. Vänskä1,2, M. P. Ljungberg2,3, P. Springer2, D. Sánchez-Portal4,3, M. Kira2, S. W. Koch2, I. Tittonen1

osmo.vanska@aalto.fi
1 Department of Electronics and Nanoengineering, Aalto University, Finland
2 Department of Physics and Material Sciences Center, Philipps-Universität Marburg, Germany
3 Donostia International Physics Center, Spain
4 Centro de Física de Materiales CFM-MPC, Centro Mixto CSIC-UPV/EHU, Spain


Semi-empirical methods, like the $\mathbf{k}\cdot\mathbf{p}$ perturbation theory, can yield parameters for the band structure and interaction matrix elements that are necessary for various approaches describing many-body dynamics of semiconductors. However, there are several “nontrivial” systems where many of the needed parameters for this kind of modeling are not well characterized. These cases include, e.g., hybrid organic-inorganic materials, many types of interfaces, and van der Waals layered structures. Thus, it is important to search for new approaches to overcome this restriction of semi-empirical schemes.

In our work [1], we put together density functional theory (DFT) and cluster-expansion (CE) method [2]. By using DFT, we evaluate the properties of electronic states and approximate the interaction matrix elements need for CE with a minimal amount of experimental input. Afterwards, we model the many-body dynamics efficiently via CE. As a result, we obtain the hybrid CE and DFT scheme that does not require many empirical parameters for a modelling of many-body phenomena in nontrivial systems.

We utilize the hybrid approach for rutile TiO$_2$, which might seem to be a typical semiconductor material, but actually possesses many characteristics of a nontrivial system. For example, the effective masses in TiO$_2$ are not well known and it has a direct but dipole-forbidden band gap. To demonstrate the plausibility of the hybrid approach, we model the near-bandgap optical absorption in TiO$_2$. We found strong evidence that the experimentally detected [3] excitonic signature below the band gap originates from a dipole-forbidden but quadrupole-allowed 1s exciton.




[1] O. Vänskä, M. P. Ljungberg, P. Springer, D. Sánchez-Portal, M. Kira, S. W. Koch, $\textit{J. Opt. Soc. Am. B}$ $\textbf{33}$, C123, (2016).
[2] M. Kira and S. W. Koch, $\textit{Semiconductor Quantum Optics}$ (Cambridge University Press, Cambridge, 2012).
[3] J. Pascual, J. Camassel, and H. Mathieu, $\textit{Phys. Rev. Lett.}$ $\textbf{39}$, 1490 (1977).


Poster: A204, March 5th, 16:00 - 18:00, 2nd floor

VACANCY DEFECTS IN Si AND Ge-DOPED HVPE-GaN INVESTIGATED BY POSITRON ANNIHILATION SPECTROSCOPY

I. Prozheev1, F. Tuomisto1, M. Iwinska2, M. Bockowski2

igor.prozheev@aalto.fi
1 Department of Applied Physics, Aalto University, Finland
2 Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw, Poland


Silicon and germanium impurities can be used for n-type doping of GaN. In-grown gallium vacancy defects (V$_{\mathrm{Ga}}$) are known to act as compensating acceptors in n-type GaN and are often complexed with an impurity donor atom [1]. Such defects can be identified with the use of positron annihilation spectroscopy. It is a characterization method suitable to identify native vacancy defects in III-nitrides. In semiconductors thermalized positrons can be trapped at neutral and negative vacancy defects as well as negatively charged non-open volume defects. Changes in the positron-electron annihilation radiation evidence the event of positron trapping at these defects [2].
We have applied positron annihilation spectroscopy to study in-grown vacancy defects in GaN crystals grown by hydride vapor phase epitaxy (HVPE) and doped with Si or Ge [3,4]. All Si and Ge-doped samples appear to contain V$_{\mathrm{Ga}}$-related defects in negative charge state as evident from the increase of $\tau_{ave}$ towards low temperatures at comparable concentrations of $5\times10^{15}- 7\times10^{15}$ cm$^{-3}$. However, the Si donors appear to be highly compensated while the Ge donors are not compensated efficiently. Efficient passivation of Si dopants and absence of evidence of other negatively charged defects suggest complex mechanism of electrical compensation to be further investigated.

[1] F. Tuomisto et al., Journal of Crystal Growth 350, 93 (2012).
[2] F. Tuomisto and I. Makkonen, Reviews of Modern Physics 85, 1583 (2013).
[3] M. Iwinska et al., Journal of Crystal Growth 456, 91 (2016).
[4] M. Iwinska et al., Journal of Crystal Growth 480, 102 (2017).



Figure 1
Figure 1: Average positron lifetime as a function of temperature for a) Si-doped GaN and b) Ge-doped GaN.

Poster: A205, March 5th, 16:00 - 18:00, 2nd floor

VORTICES IN SUPERFLUID 3HE-B UNDER MAGNETIC FIELD

M. Nakahara1,2, K. Kasamatsu3, R. Mizuno4, T. Ohmi3

nakahara@shu.edu.cn
1 Deparment of Mathematics, Shanghai University, China
2 Research Institute for Science and Technology, Kindai University, Japan
3 Department of Physics, Kindai University, Japan
4 Department of Physics, Osaka University, Japan


Superfluid $^3$He-B possesses three locally stable vortices known as a normal-core vortex (the $o$-vortex), an A-phase-core vortex (the $v$-vortex), and a double-core vortex (the $d$-vortex). In this work, we study the effects of a magnetic field parallel or perpendicular to the vortex axis on these structures by solving the two-dimensional Ginzburg-Landau equation for two different sets of strong coupling corrections. The energies of the $v$- and the $d$-vortices have nontrivial dependence on the magnetic field. As a longitudinal magnetic field increases, the $v$-vortex is energetically unstable even for high pressures and the $d$-vortex becomes energetically most stable for all possible range of pressure. For a transverse magnetic field the energy of the $v$-vortex becomes lower than that of the $d$-vortex in the high pressure side. In addition, the orientation of the double cores in the $d$-vortex prefers to be parallel to the magnetic field at low pressures, while the $d$-vortex with the double cores perpendicular to the magnetic field is allowed to continuously deform into the $v$-vortex by increasing the pressure. The figure shows the phase diagram of the vortices under (a, b) a parallel magnetic field and (c, d) a perpendicular magnetic field. Theoretical strong coupling corrections by Sauls and Serene (Phys. Rev. B {\bf 32}, 4782 (1985) are employed in (a) and (c), while experimental fitting of strong coupling corrections by Choi {\it et al.} (Phys. Rev. B {\bf 75}, 174503 (2007); {\bf 87}, 019904(E) (2013)) are used in (b) and (d). The symbols $v,d,d_x, d_y$ without parentheses denote the stable vortex type while those in $(\quad)$ denote the metastable vortex type. The $d_x$- and the $d_y$-vortices are the $d$-vortex whose double core orientation is parallel and perpendicular to the transverse magnetic field, respectively.

Details of this work is available in arXiv:1901.02638.



Figure 1
Figure 1: Phase diagrams of the vortices under (a, b) a parallel magnetic field and (c, d) a perpendicular magnetic field to the vortex axis. Set I is employed in (a) and (c), while Set II is used in (b) and (d). The symbols $v,d,d_x, d_y$ without parentheses denote the stable vortex type while those in $(\quad)$ denote the metastable vortex type. The $d_x$- and the $d_y$-vortices are the $d$-vortex whose double core orientation is parallel and perpendicular to the transverse magnetic field, respectively.

Poster: A206, March 5th, 16:00 - 18:00, 2nd floor

DYNAMICAL INSTABILITIES AND DECAY DYNAMICS OF ISOLATED MONOPOLES

T. Ollikainen1, P. Kuopanportti2, A. Gammal3, M. Möttönen1

tuomas.ollikainen@aalto.fi
1 QCD Labs, QTF Centre of Excellence, Department of Applied Physics, Aalto University, PO Box 13500, FI-00076 Aalto, Finland
2 Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland
3 Instituto de Física, Universidade de São Paulo, 05508-090 São Paulo, Brazil


Monopoles in spinor Bose-Einstein condensates (BECs) are examples of the many topological structures that can appear in this cold-atom system [1,2]. In previous research, isolated monopoles, created in the polar magnetic phase of the spin-1 BEC, were observed to decay into Dirac monopoles in the presence of a quadrupole magnetic field [3,4]. In the absence of the magnetic field, the isolated monopoles are predicted to decay into polar-core spin vortices [5].

Here, we theoretically study the dynamical instabilites of the isolated monopoles in spin-1 BECs. We numerically solve the Bogoliubov equations in order to find the quasiparticle excitations. We find two types of complex-frequency modes which correspond to dynamical instabilities. One of these modes leads to the fast decay of the polar magnetic phase into the ferromagnetic phase. The other instability mode leads to the deformation of the monopole defect into a half-quantum vortex ring, often referred to as the Alice ring [6]. In addition, we study the evolution of these emergent Alice rings using the three-dimensional spin-1 Gross--Pitaevskii equation, and show that their initial orientation and subsequent dynamics depend on the angular momentum of the excitation mode.

[1] M. W. Ray, E. Ruokokoski, S. Kandel, M. Möttönen, and D. S. Hall, Nature 505, 657 (2014).
[2] M. W. Ray, E. Ruokokoski, K. Tiurev, M. Möttönen, and D. S. Hall, Science 348, 6234 (2015).
[3] K. Tiurev, E. Ruokokoski, H. Mäkelä, D. S. Hall, and M. Möttönen, Phys. Rev. A 93, 033638 (2016).
[4] T. Ollikainen, K. Tiurev, A. Blinova, W. Lee, D. S. Hall, and M. Möttönen, Phys. Rev. X 7, 021023 (2017).
[5] K. Tiurev, P. Kuopanportti, A. Gunyhó, M. Ueda, and M. Möttönen, Phys. Rev. A 94, 053616 (2016).
[6] J. Ruostekoski and J. R. Anglin, Phys. Rev. Lett. 91, 190402 (2003).


Poster: A207, March 5th, 16:00 - 18:00, 2nd floor

POLARIZED EMISSION SPECTRA OF PLASMON-MOLECULE SYSTEM

K. S. U. Kansanen1, A. Asikainen1,2, G. Groenhof3, J. J. Toppari1, T. T. Heikkilä1

kalle.s.u.kansanen@student.jyu.fi
1 University of Jyväskylä, Department of Physics, Nanoscience center
2 Aalto University, Department of Computer Science
3 University of Jyväskylä, Department of Chemistry, Nanoscience center


It has been recently observed that a system consisting of strongly coupled surface plasmon polaritons (SPP) and fluorescent molecules can emit s-polarized light [S. Baieva et al., ACS Photonics, 2017, 4(1), 28-37], even though the pure SPP produces only p-polarized light. To explain the s-polarized emission, we include the vibrations of the molecules in the microscopic Hamiltonian. This leads to an additional channel of decoherence in the SPP-molecule system. Using the input-output equations, we construct a modification of the $P(E)$ theory, used in the context of dynamical Coulomb blockade, to describe the vibrations. Both the s- and p-polarized spectra can be obtained from the $P(E)$ theory and can be characterized with only a few parameters.


Poster: A208, March 5th, 16:00 - 18:00, 2nd floor

EFFECTS OF CASCADES OVERLAPPING WITH DEFECT CLUSTERS IN FUSION-RELEVANT MATERIALS

J. Byggmästar1, F. Granberg1, A. E. Sand1, A. Pirttikoski1, K. Nordlund1

jesper.byggmastar@helsinki.fi
1 Department of Physics, University of Helsinki, Finland


The wall materials in fission and fusion reactors are subjected to continuous irradiation. High-energy neutrons initiate collision cascades within the crystal structure of the wall material, as their kinetic energy is transferred to atoms in their path. Collision cascades take place on femto- and picosecond time scales, and are therefore out of reach for direct experimental observation. On the other hand, atomistic simulations such as molecular dynamics provide the perfect tool for understanding and observing the damage created by collision cascades in materials.

Previous simulation studies have mainly focused on cascades in defect-free single crystalline materials. However, in irradiation experiments and during the lifetime of nuclear reactors, the material is irradiated up to high doses. At these doses, cascades become increasingly likely to overlap with previously formed defects in the crystal structure. Here, we report extensive simulations of collision cascades overlapping with self-interstitial clusters in the fusion-relevant materials iron and tungsten. We discuss the effects of overlapping cascades, including a reduced production of new point defects, and cascade-induced changes in the morphology of the pre-existing interstitial clusters. Our results provide crucial input for larger-scale simulation methods that are needed for a multi-scale model of radiation damage across length and time scales.


Poster: A209, March 5th, 16:00 - 18:00, 2nd floor

SUPERCONDUCTIVITY NEAR A MAGNETIC DOMAIN WALL

Faluke Aikebaier1, Pauli Virtanen2,1, Tero Heikkilä1

faluke.aikebaier@jyu.fi
1 Department of Physics and Nanoscience Center, University of Jyväskylä
2 NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore


We study the equilibrium properties of a ferromagnetic insulator/superconductor structure near a magnetic domain wall. We show how the domain wall size is affected by the superconductivity in such structures. Moreover, we calculate several physical quantities altered due to the magnetic domain wall, such as the spin current density and local density of states, as well as the resulting tunneling conductance into a structure with a magnetic domain wall.


Poster: A210, March 5th, 16:00 - 18:00, 2nd floor

RIXS SPECTRUM OF d-d EXCITATIONS IN STRONGLY CORRELATED COMPOUND Sr3NiIrO6 CAPTURED WITH SINGLE QUASIPARTICLE MODEL HAMILTONIAN

L. Keller1,2, S. Huotari1, K. Gilmore2,3

levi.keller@helsinki.fi
1 University of Helsinki
2 European Synchrotron Radiation Facility
3 Brookhaven National Laboratory


Fifth period transition metal compounds feature a rich variety of phenomena attributed to the interplay of electron correlation and spin-orbit coupling. One such compound is the 5d-oxide $\rm Sr_3NiIrO_6$, which features strong exchange-based magnetic anisotropy in linear chains, and weaker symmetry frustrated interchain magnetic interactions, leading to a record-breaking coercive magnetic field and an unusually large magnonic gap [1,2,3]. Resonant Inelastic X-ray Scattering (RIXS) is a powerful technique that can directly probe the low-lying excitations that are implicated in these phenomena. Recent measurements of the RIXS spectrum at the L$_3$ edge of Ir4+ elucidate the sub-eV quasiparticle excitation spectrum governing this behavior[2]. Our single-quasiparticle model Hamiltonian calculation accurately reproduces the contribution of crystal-field excitations to this portion of spectrum. The ground state thus identified is in agreement with results of neutron scattering experiments and quantum chemistry calculations [2][4]. Inclusion of a local spin-flip term as suggested by [4] fails to improve the agreement, suggesting that an extended model is necessary to capture the contribution of dispersive modes to the RIXS spectrum.

[1] Birol et al. (2018) https://arxiv.org/pdf/1805.03733.pdf
[2] LeFrancois E. et al. (2016) https://doi.org/10.1103/PhysRevB.93.224401
[3] Toth et. al. (2016) https://doi.org/10.1103/PhysRevB.93.174422
[4] LeFrancois E. et al. (2014) https://doi.org/10.1103/PhysRevB.90.014408


Poster: A211, March 5th, 16:00 - 18:00, 2nd floor

ON THE ATOMISTIC BEHAVIOR OF METAL SURFACES UNDER HIGH ELECTRIC FIELDS

Ekaterina Baibuz1, Andreas Kyritsakis1, Ville Jansson1, Flyura Djurabekova1

ekaterina.baibuz@gmail.com
1 Helsinki Institute of Physics and Department of Physics, University of Helsinki


Atomic diffusion on metal surfaces under the electric field gradient has been under the attention of scientists since late 1960s, when the first Field Ion Microscope experiments showed that the adatoms on a surface tend to diffuse towards higher fields [1, 2]. However, there’s been no fundamental understanding of such dynamics up until now. Tsong and Kellog attempted to develop a theory for the directional atomic diffusion due to the field gradient in [3]. Although, this theory was sufficient enough to explain their experimental observations, it lacked a rigorous basis, thus, putting the scientific community in doubt. The interest to the directional diffusion under field increased further with the development of atom probe microscopy and accelerator technology.

This work [4] brings light on the dynamic behavior of metal surfaces under high electric fields on the atomic scale. Combining classical electrodynamics and density functional theory (DFT) calculations, we propose a general and rigorous theoretical framework where we show that the behavior of a surface atom in the presence of an electric field can be described by the polarization characteristics of the permanent and field-induced charges in its vicinity.

We use DFT calculations for the case of a W adatom on a W{110} surface to confirm the predictions of our theory and quantify its system-specific parameters. We compare our quantitative predictions for the diffusion of W-on-W{110} under field with the experimental measurements by Tsong and Kellog, and offer an explanation of the physical phenomena behind the field-driven atomic self-diffusion on metal surfaces.

This work is a crucial step towards developing atomistic computational models of surface diffusion under electric field for long-term simulations.

[1] Ehrlich, G., & Hudda, F. G. (1966). Atomic view of surface Self‐Diffusion: Tungsten on tungsten. The Journal of Chemical Physics, 44(3), 1039-1049.
[2] Tsong, T. T. (1972). Direct observation of interactions between individual atoms on tungsten surfaces. Physical Review B, 6(2), 417.
[3] Tsong, T. T., & Kellogg, G. (1975). Direct observation of the directional walk of single adatoms and the adatom polarizability. Physical Review B, 12(4), 1343.
[4] Kyritsakis, A., Baibuz, E., Jansson, V., & Djurabekova, F. (2018). On the atomistic behavior of metal surfaces under high electric fields. arXiv preprint arXiv:1808.07782.



Figure 1
Figure 1: Charge redistribution induced by (a) the presence of an adatom (no external eld), (b) a positive 3 GV/m applied field (anode) on a system with adatom (DFT simulations)

Poster: A212, March 5th, 16:00 - 18:00, 2nd floor

DROPLET ADHESION MEASUREMENTS IN THE WETTING CHARACTERIZATION OF TOPOGRAPHICALLY COMPLEX, REPELLENT SURFACES

M. J. Hokkanen1, V. Liimatainen2, M. Vuckovac1, V. Jokinen3, V. Sariola4, Q. Zhou2, R.H.A. Ras1,3

matti.hokkanen@aalto.fi
1 Aalto University, School of Science
2 Aalto University, School of Electrical Engineering
3 Aalto University, School of Chemical Engineering
4 Tampere University, Faculty of Biomedical Sciences and Engineering


Wetting characterization of materials is at present done almost exclusively via optical contact angle measurements of macroscopic sessile droplets. Yet, it is well-recognized that contact angle measurements suffer from severe difficulties particularly on highly non-wetting and non-flat surfaces that are of great practical interest. Droplet adhesion measurements have been proposed as an alternative to contact angles as they may accommodate for greater sensitivity while bypassing some of the problems associated with the conventional approach.

We have developed Scanning Droplet Adhesion Microscopy (SDAM) for wetting characterization of highly repellent and topographically challenging surfaces with unpreceded accuracy. In this approach, liquid droplet suspended from a sensitive force sensors enables point-by-point probing of the droplet-surface interaction down to ~10 nN force range. Automated measurement platform is used to systematic wetting mapping of a variety of surfaces that exhibit extreme liquid repellency with complex surface topography, e.g. superhydrophobic coatings, biological samples and microfabricated pillar surfaces with high lateral precision.

In this talk, I will overview the experimental methodology utilized in SDAM, and review the key results from our recent works. Particular attention will be given on the relationship between the droplet adhesion forces and the advancing and receding contact angles, and the benefits adhesion force measurements are able to offer.



Figure 1
Figure 1: TOP: Cartoon of the SDAM measurement cycle showing the droplet suspended from the force sensor modified with a hydrophilic disc. BOTTOM: the associated force curve. Snap-in and pull-off forces signifying the droplet-surface adhesion are indicated.

Poster: A213, March 5th, 16:00 - 18:00, 2nd floor

IRRADIATION STABILITY OF NANOCRYSTALLINE EQUIATOMIC MULTICOMPONENT ALLOYS

E. Levo1, F. Granberg1, K. Nordlund1, F. Djurabekova2,1

emil.levo@helsinki.fi
1 University of Helsinki
2 Helsinki Institute of Physics


The development of future energy production concepts has increased the demand on novel materials that are suitable for extreme operational environments. High temperatures, strong magnetic fields, corrosive environments and prolonged irradiation are some of the realities these novel materials will experience. One promising group of such novel materials consists of alloys built up of at least five elements at near-equimolar concentrations resulting in a high configurational entropy. Due to this characteristic they are called high entropy alloys (HEA) and exhibit many promising properties when it comes to their utilization in energy production. A subgroup of the HEA-family consists of alloys built up of elements at equiatomic concentrations, so called equiatomic multicomponent alloys (EAMC-alloys). These EAMC-alloys can be built up of less than five elements, and they also exhibit very promising properties with energy production in mind. Research in the HEA-family with its subgroups has become an increasingly important field in the last couple decades. HEAs have been realised to be potential candidates as components for future nuclear reactors that involve extreme radiation exposure. The irradiation response in HEAs has been studied both experimentally and computationally, with results showing good radiation tolerance. [1,2]

Polycrystalline materials have often been found superior to their single crystalline counterparts. The reduction in grain size to the nanoscale improves mechanical abilities drastically, which is very attractive for materials in many applications. Combining this nanocrystalline structure with the aforementioned multicomponental high entropy alloy design, we study by the means of molecular dynamics several nanocrystalline Ni-based EAMC-alloys, and elemental Ni under prolonged irradiation for their structural stability. The results show that the alloys encompass a higher structural stability than elemental Ni, as they withstand higher doses of irradiation before their nanocrystallinity collapses. [3]

[1] F. Granberg et al. “Mechanism of radiation damage reduction in equiatomic multicomponent single phase alloys”, Phys. Rev. Lett. 116, 13 (2016), 135504
[2] E. Levo et al. “Radiation damage buildup and dislocation evolution in Ni and equiatomic multicomponent Ni-based alloys”, J. Nucl. Mater. 490 (2017) 323-332
[3] E. Levo et al. “Radiation stability of nanocrystalline single phase multicomponent alloys”, J. Mat. Res. (Accepted for publication)


Poster: A214, March 5th, 16:00 - 18:00, 2nd floor

QUANTUM MONTE CARLO SIMULATION OF POSITRONS IN SOLIDS

K. Simula1, I. Makkonen1, N. Drummond2

kristoffer.simula@aalto.fi
1 Aalto University
2 Lancaster University


Positron annihilation spectroscopy is a non-destructive method used to extract information from atomic matter. It is particularly useful in characterizing open-volume defects, which often determine the electronic, mechanical and thermal properties of crystals. Experimental use of positron annihilation spectroscopy requires a strong theoretical background for drawing conclusions from the measurements and making a link between the atomic structures of the defects detected and the indirect information in the measured spectra [1].

The most widely used method for making theoretical simulations of positron annihilation is density functional theory (DFT). While it is proven to be highly practical, there is demand for more accurate methods. This is why we have developed a Quantum Monte Carlo (QMC) [2] method for simulating positrons in solids. QMC does not require the approximation of multiple functionals, and it is able to sample directly two-body quantities, as well as expectation values in the momentum space. The trial wave functions are in a localized blip basis. With QMC, the electron-positron wavefunction is first optimized with variational Monte Carlo method (VMC), after which it is passed forward for the extremely accurate diffusion Monte Carlo simulation (DMC).

At the moment, we have obtained accurate predictions of the lifetimes of positrons in diamond phase-carbon and -silicon. We are also studying correlation between electrons and a positron in the crystals as a function of location in real space by sampling enhancement factors, and examining how correlations play a role in annihilation events. In the future we may also simulate the Doppler broadening spectrum of annihilating electron-positron pair.

[1] Tuomisto, Filip, and Ilja Makkonen. "Defect identification in semiconductors with positron annihilation: experiment and theory."
Reviews of Modern Physics 85 (2013)
[2] Foulkes, W. M. C., et al. "Quantum Monte Carlo simulations of solids."
Reviews of Modern Physics 73 (2001)



Figure 1
Figure 1: Spherically averaged pair correlation functions of positron-electron pairs in diamond-phase silicon and carbon, calculated with QMC. Calculations were done by binnig values of the configurations of the systems, sampled from the VMC-optimized wave functions with the Metropolis algorithm.

Poster: A221, March 5th, 16:00 - 18:00, 2nd floor

ELECTRON CASCADES IN GRAPHENE AFTER SWIFT HEAVY ION IMPACTS

H. Vazquez1, A. Kononov2, N. Medvedev3, A. Schleife2, F. Djurabekova1

henrique.vazquezmuinos@helsinki.fi
1 Helsinki Institute of Physics and Physics Department, University of Helsinki
2 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
3 Institute of Physics, Czech Academy of Sciences, Na Slovance 2, Prague 8, 18221, Czech Republic


SHI irradiation of graphene could be used to engineer sensors or nanofilters of unparalleled performance [1,2]. Furthermore, recent work suggested that Swift Heavy Ions (SHI) can produce pore-like defects in suspended graphene [3]. SHIs produce strong electron excitation in the material and generate an electron cascade; this process is especially important in two-dimensional materials, where the excited electrons can be emitted into the vacuum.

In this work we study the initial electron cascade after the ion impact in graphene and the role of the electronic dynamics in the formation of defects. We use two methods Monte Carlo (MC) and Time Dependent Density Functional Theory (TDDFT) to simulate the initial electronic dynamics, and combine them with Two-temperature Molecular Dynamics model (TTMD) to investigate the mechanism of defect formation at atomic level.

We report a strong electron emission in graphene after the SHI impact. Both simulation techniques MC and TDDFT show almost identical secondary electron emission spectra; additionally, TDDFT exhibits a strong electron capture by the projectile at low velocities. Both approaches show large transient positive charge around the ion impact point in the graphene layer; additionally, TDDFT predicts a fast charge equilibration within the layer. TTMD atomistic simulations show a strong reduction of the nanopore size with increasing electron emission. These results show that the early electron dynamics after the SHI impact can play a decisive role in the mechanism of defect formation in 2D materials.



References:

[1] Han, Tae Hee, et al. "Steam etched porous graphene oxide network for chemical sensing." Journal of the American Chemical Society 133.39 (2011): 15264-15267.
[2] Jiang, De-en, Valentino R. Cooper, and Sheng Dai. "Porous graphene as the ultimate membrane for gas separation." Nano letters 9.12 (2009): 4019-4024.
[3] Vázquez, H., et al. "Creating nanoporous graphene with swift heavy ions." Carbon 114 (2017): 511-518.
[4] Gruber, Elisabeth, et al. "Ultrafast electronic response of graphene to a strong and localized electric field." Nature communications 7 (2016): 13948.


Poster: A222, March 5th, 16:00 - 18:00, 2nd floor

ANALYSING THE NANOSCALE MOISTURE BEHAVIOR OF WOOD WITH SMALL-ANGLE SCATTERING

P.A. Penttilä1,2, N. Carl2, R. Schweins2, M. Altgen1, M. Östergberg1, L. Rautkari1

paavo.penttila@aalto.fi
1 Department of Bioproducts and Biosystems, Aalto University, Espoo, Finland
2 Institut Laue-Langevin, Grenoble, France


Wood is an extremely abundant and important raw material, that has been traditionally utilized as a construction material and in paper-making. More recently, wood-based biomass has been used as a source of cellulose nanofibrils and other nanosized components for advanced materials, food, cosmetics and medical applications. Water and moisture content of wood play crucial roles in many of its applications, affecting for instance the mechanical properties and chemical reactivity. However, relatively little is known on the interactions between water and the elementary constituents of the plant cell wall. Of particular interest are the interactions of water and the cellulose microfibrils (CMFs), which are semi-crystalline aggregates of cellulose molecules embedded in a matrix of hemicelluloses and lignin.

The nanoscale moisture behavior of wood can be efficiently studied with small-angle scattering of x-rays (SAXS) and neutrons (SANS). A newly proposed model based on hexagonally packed cylinders was used to observe moisture-induced changes in the cross-sectional size and packing of the CMFs. The model works for both SAXS and SANS data, yielding reasonable values for both the CMF diameter (2.0-2.5 nm) and the interfibrillar distance (4 nm in wet state, 3 nm in dry state). As illustrated by the example SAXS data of Figure 1, it could also be used to follow the time development of the CMF diameter and packing distance in wood samples during drying in room air.

Further synchroton-SAXS experiments under controlled humidity conditions were carried out with the aid of a specifically-designed humidity chamber. The structural changes were correlated with moisture content measured with dynamic vapor sorption (DVS). Both the DVS and small-angle scattering data demonstrated the hysteresis of wood's moisture behavior and highlighted its relation to the nanoscale structure.



Figure 1
Figure 1: Equatorial SAXS data from pine wood measured during drying in air, with the changes in microfibril diameter ($2R$) and packing distance ($a$) shown in the inset.

Poster: A223, March 5th, 16:00 - 18:00, 2nd floor

GEOMETRY INDEPENDENT THERMAL CONDUCTANCE MEASUREMENT SETUP FOR 2D MEMBRANES

T. Loippo1, T.A. Puurtinen1, I.J. Maasilta1

teolloip@student.jyu.fi
1 University of Jyväskylä


With thermal conductivity measurements, it is important to get an accurate value for the heater temperature. The simpler geometry used before in our experiments [1] cannot provide the heater temperature reading directly, but a geometry-based view factor needs to be taken into account. Thus, one would like to design an experiment, where the view factor should be close to one, making the temperature analysis trivial. This would mean that the thermometer measures the heater temperature directly.
Here, we demonstrate such a new geometry independent measurement setup for measuring thermal conductance of 2D membranes (see figure 1). The new setup has been designed, fabricated and tested. Testing was performed on a suspended 300 nm thick SiN membrane. Initial results are in line with previous measurements performed with a simpler geometry [1]. In addition, simulations of phonon radiation power explain the measured temperature dependence in the experiment. However, a correction had to be added to take noise power into account. In other words, there is some additional heating power in the measured data.

References
[1] I.J. Maasilta, T.A. Puurtinen, Y. Tian, Y. et al., J Low Temp. Phys. 184, 211 (2016).



Figure 1
Figure 1: SINIS thermometer located inside the SNS heater element.

Poster: A224, March 5th, 16:00 - 18:00, 2nd floor

INTERNAL STRUCTURE AND DYNAMICS OF LIPID NANODISCS

Piotr Stepien1, Bozena Milanovic2, Chetan Poojari3,4, Wojciech Galan2, Agnieszka Polit5, Ilpo Vattulainen3,4,6, Anna Wisniewska-Becker1, Tomasz Rog3,4

chetan.poojari@helsinki.fi
1 Department of Biophysics, Jagiellonian University, Krakow, Poland
2 Department of Computational Biophysics and Bioinformatics, Jagiellonian University, Krakow, Poland
3 Department of Physics, University of Helsinki, Helsinki, Finland
4 Computational Physics Laboratory, Tampere University, PO Box 692, FI-33014 Tampere, Finland
5 Department of Physical Biochemistry, Jagiellonian University, Krakow, Poland
6 MEMPHYS – Center for Biomembrane Physics


Lipid nanodiscs are nanosized patches of lipid bilayers that are wrapped by scaffold proteins or polymers, which stabilize the nanomembrane structure by protecting the hydrocarbon chains at the edges of the nanodisc from unfavourable interactions with water. These nanostructures have numerous applications in nanotechnology, pharmaceutics, and membrane protein studies. Here, based on electron paramagnetic spectroscopy experiments and atomistic molecular dynamics simulations, we explore how the structure and the dynamics of lipids constituting nanodiscs depends on their position in these assemblies. The results reveal that there are three different fractions of lipids: lipids in the center of the nanodisc, lipids in direct contact with the scaffold proteins, and intermediate lipids at the interfacial region of these two distinct fractions. We found that the central lipids are highly ordered and characterized by slow diffusion, and the bilayer in this part of the disc is the thickest. The properties of the central lipids have similarities to lipids often found in highly ordered gel or liquid-ordered membrane phases. The lipids in direct contact with the scaffold protein were observed to be less ordered and to diffuse relatively rapidly, and they also maintained the liquid state even below the main phase transition temperature. Intermediate lipids were also found to be characterized by low order and quite rapid diffusion. The transition enthalpies between the central lipid region and the other two lipid fractions were observed to be largely similar to those observed for lipids going through the main phase transition. Concluding, the studies revealed both the structure and the dynamics of the lipid nanodiscs to be exceptionally heterogeneous. The results suggest that this nanoscale compartmentalization will affect the properties of membrane proteins hosted by nanodiscs.


Poster: A231, March 5th, 16:00 - 18:00, 2nd floor

PHOTONIC NANOJET STRUCTURE FOR 3D SUPER-RESOLUTION

M. Järvinen1, T. Vainikka1, T. Ylitalo1, A. Nolvi1, P. Raatikainen1, T. Arstila1, J. Kantonen2, K. Ahlers1, I. Kassamakov1, E. Hæggström1

miikka.jarvinen@helsinki.fi
1 Electronics Research Lab., Dept. of Physics, University of Helsinki, Helsinki, Finland
2 Dept. of Pathology, Haartman Institute, University of Helsinki, Helsinki, Finland


Super-resolution (SR) imaging in optical microscopy has been a great interest in the field of optics and photonics, particularly SR assisted by sub-diffraction limit focused beam [1], [2]. At the moment most of the works are concerning two dimensional SR [2]. We designed a sub-diffraction limit beam generating structure, which allows us to perform three-dimensional (3D) SR. We tested our design by imaging a polymer and a biosample. Our results show that we are able to achieve less than 100 nm lateral resolution and better than 10 nm axial resolution.

[1] Z. Chen, A. Taflove, and V. Backman, “Photonic nanojet enhancement of backscat-tering of light by nanoparticles: a potential novel visible-light ultramicroscopy tech-nique,” Optics express, vol. 12, no. 7, pp. 1214–1220, 2004.
[2] A. Heifetz, S.-C. Kong, A. V. Sahakian, A. Taflove, and V. Backman, “Photonic nanojets,” Journal of computational and theoretical nanoscience, vol. 6, no. 9, pp. 1979–1992, 2009.



Figure 1
Figure 1: 3D SR image of a polymer sample taken with the PNJ-generating structure.

Poster: A232, March 5th, 16:00 - 18:00, 2nd floor

ENHANCEMENT OF FLUORESCENCE WITH NANOSTRUCTURES

S. Maurya1, M. Nyman1, Ville Kivijärvi1, Andriy Shevchenko1, Matti Kaivola1

somendu.maurya@aalto.fi
1 Department of Applied Physics, Aalto University, P.O.Box 13500, FI00076 Aalto, Finland


Fluorescence enhancement in organic dyes and quantum dots is an active research area with wide-range applications, e.g., in fluorescence imaging, medical diagnostics, coherent and incoherent light sources, and single-molecule detectors and sensors [1,2]. In this work, we propose a simple way to increase brightness of fluorescent thin films by simultaneously enhancing their optical pumping and far-field intensity with the help of periodic metal-dielectric nanostructures.

In particular, we consider fluorescent films of dye-doped polymers, e.g., thin layers of polymethyl methacrylate (PMMA) doped with infrared dye IR-780. The fluorescence is enhanced by placing the film on a layer of silver and creating a dielectric stripe pattern on its surface. The pattern enhances optical pumping by converting the incident light into waveguide modes via diffraction. The structures are optimised numerically, using the COMSOL Multiphysics software. We achieved the pumping efficiency of more than 90% at a pumping wavelength of 633 nm for a film with 290 nm thickness. The pumping enhancement is considerable, as only 7% of pumping light can be absorbed in an unpatterned film with the same thickness. In addition, the designed structure yields highly directional emission of fluorescent light at an angle of 21° with respect to the surface normal. The far-field intensity enhancement factor that we have predicted is 465, and it can be further increased by refining the design.


[1] Nyman, M.; Shevchenko, A.; Shavrin, I.; Ando, Y.; Lindfors, K.; Kaivola, M. "Enhancement of far-field intensity of fluorescent films", submitted.
[2] Nyman, M.; Kivijärvi, V.; Shevchenko, A.; Kaivola, M. “Generation of light in spatially dispersive materials”. Phys. Rev. A 2017, 95, 043802.



Figure 1
Figure 1: Fig. 1. Fluorescence enhancement by nanopatterning: (a) Schematic of the proposed structure. The grat-ing on the top of the fluorescent film (dye-doped PMMA) is made of glass with the thickness-to-period ratio of 0.45. The film is separated from the reflecting silver film by a 10 nm layer of glass. The structure is pumped at normal incidence, and fluorescence is collected at an angle of 21°. (b) The intensity distribution of the pump light in the structure showing the intensity enhancement.

Poster: A233, March 5th, 16:00 - 18:00, 2nd floor

HIGH-POWER SHORT-WAVELENGTH INFRARED SUPERCONTINUUM GENERATION IN A MULTIMODE FLUORIDE FIBER

Zahra Eslami1, Piotr Ryczkowski1, Caroline Amiot1, Lauri Salmela1, Goery Genty1

Zahra.eslami@tuni.fi
1 Photonics Laboratory, Physics Unit, Tampere University, 33014 Tampere, Finland


Supercontinuum (SC) is a broadband light source generated by nonlinear processes in optical fibers. Depending on the type of fibers and pump source, the spectral range of a SC source can span from the visible to infrared and with up to Watt-level average power [1]. Recently, the generation of broadband SC sources operating in the mid-infrared (MIR) has attracted significant interest due to a wide range of potential applications in spectroscopy [2], microscopy [3], molecular fingerprinting [4], environmental monitoring and LIDAR [5].
Fibers made of non-silica soft glasses such as fluoride, tellurite and chalcogenide are good candidates for SC generation in the MIR due to their high intrinsic nonlinearity and wide transparency window in this wavelength range [4]. To date, various SC, in terms of bandwidth and power, have been demonstrated in soft glass fibers. However, they have been principally generated in single-mode fibers, which generally cannot sustain high level of average power due to their small core size and lower damage threshold. This generally imposes a limit on the maximum output power which can be a limitation for practical applications, especially in long-distance remote sensing for which high power is key. In order to overcome this problem, multimode fibers with large core size and higher damage threshold are a promising alternative.
Here, we demonstrate for the first time the generation of an octave-spanning SC by injecting 1 MHz, 350 fs pulses from an optical parametric amplifier in a meter-long multimode step-index InF3 fiber with 100 µm core diameter. We performed a systematic study of the SC spectrum as a function of the pump wavelength and the largest SC spectrum spanning from 1100 nm to 2500 nm with 600 mW output power was generated when injecting the pulses at 1960 nm, in the anomalous dispersion regime of the fundamental mode as shown in Fig. 1a. The output beam profile of the SC was characterized in different wavelengths bands (see Fig. 1b), illustrating the highly multimode nature of the SC generation process. Numerical simulation results (not shown here) shows that higher-order soliton dynamics and dispersive wave generation in multiple higher-order modes are key contributions to reaching octave-spanning bandwidth in a fiber with large core size. Our results open up a promising route towards ultra-high power broadband sources in the MIR for applications where a single-mode spatial intensity distribution is not essential such as e.g. in remote sensing.

References
[1] J. M. Dudley, G. Genty, and S. Coen, "Supercontinuum generation in photonic crystal fiber," Rev. Mod. Phys. 78, 1135–1184 (2006).
[2] A. B. Seddon, B. Napier, I. Lindsay, S. Lamrini, P. M. Moselund, N. Stone, and O. Bang, "Mid-infrared Spectroscopy/Bioimaging: Moving toward MIR optical biopsy," Laser Focus World 52, 50–53 (2016).
[3] S. Dupont, C. Petersen, J. Thøgersen, C. Agger, O. Bang, and S. R. Keiding, "IR microscopy utilizing intense supercontinuum light source," Opt. Express 20, 4887 (2012).
[4] C. R. Petersen, U. Møller, I. Kubat, B. Zhou, S. Dupont, J. Ramsay, T. Benson, S. Sujecki, N. Abdel-Moneim, Z. Tang, D. Furniss, A. Seddon, and O. Bang, "Mid-infrared supercontinuum covering the 1.4-13.3 μm molecular fingerprint region using ultra-high NA chalcogenide step-index fibre," Nat. Photonics 8, 830–834 (2014).
[5] S. Lambert-Girard, M. Allard, M. Piché, and F. Babin, "Differential optical absorption spectroscopy lidar for mid-infrared gaseous measurements," Appl. Opt. 54, 1647 (2015).




Figure 1
Figure 1: Fig. 1. (a) SC spectrum generated in 1-m (blue) and 2-m (black) of InF3 multimode fiber with 100 µm core for a pump wavelength at 1960 nm. (b) Corresponding beam profiles at the output of the 2-m long fiber at different wavelengths as indicated (with bandwidth of 10 nm in each case). Numbers on the profile photos represent the factor by which the SC signal was amplified.


Poster: A234, March 5th, 16:00 - 18:00, 2nd floor

SCANNING-FREE FIELD-INTERFEROMETRIC GHOST IMAGING

E. Ilina1, M. Nyman1, M. Kaivola1, T. Setälä2, A. Shevchenko1

elena.ilina@aalto.fi
1 Department of Applied Physics, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland
2 Institute of Photonics, University of Eastern Finland, P. O. Box 111, FI-80101 Joensuu, Finland


Classical ghost imaging systems make use of spatially incoherent illumination and intensity correlations in the two detection arms of the device. The object to be imaged is placed in the arm with no spatial resolution and the field in the other arm is scanned with a pinhole detector [1]. Typically, the illumination is made spatially incoherent, using a rotating optical diffuser. One of the most intriguing properties of the technique is the possibility to obtain sharp images in the presence of aberrations [2]. In this work, we propose a ghost-imaging system based on observing ordinary optical-field interference instead of measuring intensity correlations. The system utilizes an ordinary camera, which removes the need in a pinhole scanner. In addition, we use a LED as an originally spatially incoherent light source, so that a rotating diffuser is not needed. A large longitudinal coherence length of the source makes the system insensitive to even very strong aberrations. As an example, we retrieve the detailed ghost image of an object screened from the detector by a diffuser (see Fig. 1) [3]. We believe that the proposed approach has a high potential for applications in optical imaging, especially in microscopy for biology and medicine, as well as for other applications based on optical interferometry.


[1] B. I. Erkmen and J. H. Shapiro, Phys. Rev. A 77, 043809 (2008).
[2] T. Shirai, H. Kellock, T. Setälä and A. T. Friberg, J. Opt. Soc. Am. A 29, 1288 (2012).
[3] E. Ilina, M. Nyman, I. Švagždytė, N. Chekurov, M. Kaivola, T. Setälä, A. Shevchenko, "Aberrations-insensitive field-interferometric ghost imaging microscopy," submitted.



Figure 1
Figure 1:

Optical images of Aalto University logo: a and b – the intensity images obtained, respectively, before and after destroying the image with an optical diffuser; c – the retrieved ghost image of the sample.


Poster: A235, March 5th, 16:00 - 18:00, 2nd floor

SPECKLE-FREE UNPOLARIZED LIGHT SOURCE FOR LEVITATING SAMPLE SCATTEROMETER

G. Maconi1, P. Helander1, A. Penttilä1, M. Gritsevich1, T. Puranen1, A. Salmi1, I. Kassamakov1, K. Muinonen1,2, E. Hæggström1

1 Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland
2 Finnish Geospatial Research Institute, Geodeetinrinne 2, 02430 Masala, Finland


We present a light source for our scatterometer, which eliminates interference artifacts, such as speckle. The previous laser source is replaced with a flat broadband light source and the desired wavelength is chosen by means of a laser line filter. Our instrument can measure light scattered in 4π, from millimeter sized samples that are acoustically levitated and orientation-locked. Both the incoming and measured light is filtered by motorized linear polarizers, allowing us to look at any polarization.
The light sources were compared by measuring scattering from a 3 mm ball lens made from NBK-7 glass, as well as a diffuse white sample, consisting of agglomerated 500 nm SiO$_2$ spheres. For the ball lens, the measurement results were compared to theoretical Mie scattering simulations.
The measurements with the new light source show increased self-consistency, while being comparable to previous measurements made using the laser-based source. The broadband light source allows us to choose the polarization of the beam using a calcite polarizer, giving us a polarization ratio of 3000:1 (compared to 300:1 for the film polarizers used with the laser-based source), and guarantees equal beam intensity for any polarization. However, for small samples (< 0.5 mm) and for low reflectivity samples, the laser-based light source is still necessary in order to produce sufficient scattered intensity.


Poster: A241, March 5th, 16:00 - 18:00, 2nd floor

SUPERCONDUCTING TRANSMON QUTRIT: APPLICATIONS TO QUBIT CONTROL AND INITIALIZATION

A. P. Babu1, J. Tuorila1, T. Ala-Nissila1

aravind.babu@aalto.fi
1 Multiscale Statistical and Quantum Physics(MSP) group, QTF Center of Excellence, Department of Applied Physics, Aalto University


The superconducting Josephson junction qubits are considered as a promising candidate for scalable quantum computations [1]. Nonlinearity in the Josephson junction creates an anharmonic potential in the qubit Hamiltonian, due to which the higher energy levels become closer to ground state [2]. High-fidelity and error-free computing requires a detailed understating of the dynamics of these higher energy states. Here, we analyze the dynamics of a superconducting transmon qutrit coupled to a bosonic bath using the Redfield master equation. Qutrit is realized with a three-level quantum system and is the quantum information analogue of the classical trit [3]. The results revels that the bath-induced decay from the second excited state occur faster than from the first (Fig. 1). We investigate the possibility of utilizing this fast decaying state to improve the ground-state initialization protocols of superconducting qubits [4]. We also study the influence of the second excited state in quantum gate operations.

References
[1] J. Koch, T. M. Yu, J. Gambetta, A. A. Houck, D. I. Schuster, J. Majer, A. Blais, M. H. Devoret, S. M. Girvin, and R. J. Schoelkopf, Phys. Rev. A 76, 042319 (2007).
[2] M. J. Peterer, S. J. Bader, X. Jin, F. Yan, A. Kamal, T. J. Gudmundsen, P. J. Leek, T. P. Orlando,W. D. Oliver, and S. Gustavsson, Phys. Rev. Lett. 114, 010501 (2015).
[3] P. B. R. Nisbet-Jones, J. Dilley, A. Holleczek, O. Barter, and A. Kuhn, New Journal of Physics 15, 053007 (2013).
[4] J. Tuorila, M. Partanen, T. Ala-Nissila, and M. Möttönen, npj Quantum Information 3, 27 (2017).



Figure 1
Figure 1: Decay of the first (dashed line) and second (solid line) excited states calculated with the Redfield master equation.

Poster: A242, March 5th, 16:00 - 18:00, 2nd floor

TWO-STEP EVAPORATION TO FABRICATE SIS JUNCTIONS IN SUPERCONDUCTING QUBITS

Tianyi Li1, Joni Ikonen1, Jan Goetz1, Mikko Möttönen1

tianyi.li@aalto.fi
1 Aalto University


Josephson junctions are the key elements of all kinds of superconducting qubits, but also the most uncontrollable elements. The reproducibility and accuracy of fabricating Josephson junctions remain one of the technical challenges ahead for building logical qubits. Traditionally, Al/AlOx/Al tunnel junctions are fabricated by shadow evaporation enabled by double-layer resist, which is not suitable for wafer-size fabrication and sensitive to e-beam lithography parameters. Here we present the progress we have made in fabricating Al/AlOx/Al tunnel junctions by two-step evaporation: the two overlapping electrodes are defined and evaporated in two separate e-beam lithography and evaporation steps respectively. Before evaporating the second electrode, the native oxide layer on the first electrodes needs to be removed by in-situ Ar ion milling, and a thinner oxide layer is formed in a controlled atmosphere. In such a two-step evaporation process, the edges of the electrodes are much smoother than those fabricated using shadow evaporation. We have tried a series of oxidation parameter and found that the resistance of the junctions shows a reasonable dependence on the oxidation parameters. We also found that the resistance of the junctions is very sensitive to the Ar ion milling strength, so that the uniformity of the junction resistance is limited by the FWHM of the Ar ion milling current. We have tested the Xmons with the junctions fabricated using the two-step evaporation, and found they have relatively long relaxation time T1 and decoherece time T2.



Figure 1
Figure 1: SEM images of Al/AlOx/Al tunnel junctions fabricated by shadow evaporation and two-step evaporation respectively. The edges of the electrodes fabricated by two-step evaporation are much smoother than those fabricated by shadow evaporation.

Poster Session B

See titles in compact form

First Floor

Time: March 6th, 15:30 - 17:30

Poster: B101, March 6th, 15:30 - 17:30, 1st floor

FEM-BASED OPTIMIZATION OF ADVANCED WOUND PATCH PRODUCED BY ULTRASOUND ENHANCED ELECTROSPINNING

J. Mäkinen, P. Helander, J. Hunnakko, T. Puranen, K. Kogermann, I. Laidmäe, A. Salmi, J. Heinämäki, E. Hæggström

joni.mk.makinen@helsinki.fi
1 Department of Physics, Division of Materials Physics, University of Helsinki, P. O. Box 64, 00014, Helsinki, Finland
2 Department of Pharmacy, Faculty of Medicine, University of Tartu, Nooruse 1, 50411 Tartu,


Electrospinning can generate nano- and microfibers for, e.g. medical applications or fabrics. In recent years, electrospinning has been used to manufacture wound patches with additional benefits compared to traditional wound patches [1]. Traditional electrospinning methods have limited control over the fiber properties [2]. Changing the fiber properties during spinning is a way produce nanofiber wound patches with advanced properties. We have developed an ultrasound-based electrospinning method capable of changing the fiber diameter on the fly [2]. By varying the fiber thickness, we can control properties, such as fluid permeation and cell adhesion, inside a nanofiber patch.

To study the effect of fiber thickness on the performance of nanofiber patches, we created a method to generate representative models of the electrospun structures. The models were validated by comparing them to scanning electron microscope images of the produced nanofiber patches. The models were then used to quantify the porosity, pore size distribution, and permeability of the patches. The permeability was determined from a computational fluid dynamics (CFD) simulation. COMSOL Multiphysics®, a finite element method (FEM) based simulation software was used for the simulations.

The results indicate a non-linear relation between the fiber diameter and permeability (figure 1b). This relation has also been studied using more simplified rod based analytical model for a fibrin gel by Carr et al. [3]. They report that the pore diameter $D$ is related to the permeability $k$ by $D$ ~ $\sqrt{k}$. Our results indicate that $D$ ~ $\sqrt{d}$, where d is the fiber diameter, giving $d$ ~ $k$, which differs from our observations (figure 1b). This is most likely explained through the fact that our model considers the bending and alignment of the fibers unlike [3]. These results will be utilized in our future wound patch design as input parameters.

References:
[1] S. Chen et al., Recent advances in electrospun nanofibers for wound healing, Nanomedicine, 12, 1335-1352, 2017
[2] H. J. Nieminen et al., Ultrasound-enhanced electrospinning, Sci. Rep., 8, 4437, 2018
[3] E. Carr et al., Fibrin has larger pores when formed in the presence of erythrocites, Am. J. Physiol. Heart Circ. Physiol., 253, H1069-H1073, 1987



Figure 1
Figure 1: a) CFD, creeping flow simulation of a representative fiber structure to determine permeability. b) Simulated permeability for five different fiber thicknesses. These results indicate that $k$ ~ $d^n$, where $n \approx 0.4$.

Poster: B102, March 6th, 15:30 - 17:30, 1st floor

STRUCTURES, CAVITIES AND THE HIDDEN WORLD: THE POSSIBILITIES OF MUON RADIOGRAPHY

J. Joutsenvaara1, M. Aittola2,3, T. Enqvist4, M. Holma3, P. Jalas1, H-M. Karjalainen1, K. Loo4, P. Kuusiniemi4, J. Puputti1, M. Slupecki4, A. Virkajarvi3

jari.joutsenvaara@oulu.fi
1 Kerttu Saalasti Institute, University of Oulu, Finland
2 University of Jyväskylä, Kokkola University Center Chydenius, Finland
3 Muon Solutions Oy, Finland
4 University of Jyväskylä, Department of Physics, Finland


Muon radiography, muography, muon geo-tomography, muon tomography – a loved child has many names. Muon radiography is an emerging method to transilluminate objects from the scale of centimetres up to kilometres. The transillumination is achieved by measuring how many high-energy cosmic-ray induced muons come through the target in the period of the survey. It is, hence, analogous in principle to x-ray imaging. However, contrary to X-rays the use of muons does not require man-made radioactivity as the muon source is the Universe.

The most famous case example of muon tomography is the finding of a secret chamber inside the Great Pyramid in Giza, Egypt [1]. This study required hundreds of days of recorded data and extensive analysis carried out by scientists around the globe. Muons have also been used in homeland security applications, e.g., to transilluminate containers at borders to prevent nuclear smuggling [2]. In geosciences, muon radiography has been successfully utilised, e.g., in monitoring volcanoes. For example, the accumulation and erupt of magma inside the magma chamber in Etna, Italy, has been and is still monitored to provide more accurate information on changes in melt densities [3]. These are just a few examples of muon radiography and tomography applications.

Muon measurements have been conducted for scientific purposes for many decades. However, only in recent years have scientists been able to take steps towards the use of muon radiography (creating density profiles) or muon tomography (3D to 4D imaging) in industrial applications. New application areas include mining operations (e.g., locating valuable resources) and mineral and groundwater exploration. The development of detectors, data acquisition systems and analysis methods have finally made it possible to have economically feasible muon radiography and tomography systems for the industrial sectors. As of now, there are at least seven companies worldwide that are successfully implementing muography for industrial purposes [4]. One of these companies is Muon Solutions Oy, Finland [5].

In our work, we present a review of muon radiography and tomography applications including possibilities these methods can provide for different industries.

[1] K. Morishima et al., Discovery of a big void in Khufu’s Pyramid by observation of cosmic-ray muons, Nature, 552.7685: 386., 2017
[2] K. Gnanvo et al., Imaging of high-Z material for nuclear contraband detection with a minimal prototype of a muon tomography station based on GEM detectors, NIM A, 652, 16-20, 2011
[3] D. Lo. Presti et al., The MEV project: Design and testing of a new high-resolution telescope for muography of Etna Volcano, NIM A, 904, 195-201, 2018
[4] H. K. Tanaka, & L. Oláh, Overview of muographers, Philosophical Transactions of the Royal Society A, 377(2137), 20180143, 2018
[5] Muon Solutions Oy, http://www.muon-solutions.com, 17 Jan 2019


Poster: B103, March 6th, 15:30 - 17:30, 1st floor

HIGH-POWER TIME-REVERSED ULTRASOUND BASED ON NUMERICAL SIMULATIONS AND REAL EXPERIMENTS

J. Mustonen1, T. Rauhala2, P. Moilanen1, M. Gritsevich1, A. Salmi1, E. Hæggström1

joonas.mustonen@helsinki.fi
1 Electronics Research Laboratory, Dept. of Physics, P.O.B. 64, FIN-00014 University of Helsinki, Finland
2 Altum Technologies Oy


Time-reversed ultrasound (TR) is an acoustic method to focus sound in complex media [1]. Time reversal focusing of acoustic waves to a pre-defined location requires either a target with acoustic impedance contrast or a forward propagated signal to reverse. This is challenging especially in situations where forward propagation cannot be performed and the target media is homogenous. As a solution, we propose carrying out numerical simulations to generate the forward propagated signal.

We show that simulated signals can be used to generate a focus in a real-world system. We compare experimental measurements to predictions made by simulations and show that we can focus waves at an arbitrary target point. The real-world signals are generated by a spark gap (14 kV) in ion-exchanged water in a few different locations and recorded with four transducers mounted symmetrically on a cylindrical Plexiglas water tank. The signals are digitally processed before time-reversing. The processed signals are re-transmitted back to media and the resulting pressure field is measured with a hydrophone. Numerical simulations, mimicking the real experimental setup, are carried out by COMSOL Multiphysics. We discuss the differences and similarities between the simulated and experimental data.

References

[1] M. Fink et al., "Acoustic time-reversal mirrors," Inverse Problems, vol. 17, 2001.


Poster: B104, March 6th, 15:30 - 17:30, 1st floor

QUANTIFYING THE EFFECT OF SAMPLE TILT ON SCANNING ACOUSTIC MICROSCOPY MEASUREMENTS

J. Hyvönen1, A. Meriläinen1, A. Salmi1, E. Hæggström1

jere.hyvonen@helsinki.fi
1 Electronics Research Laboratory, Dept. of Physics, PL 64, 00014 Univ. of Helsinki, Finland


Scanning acoustic microscopy (SAM) permits simultaneous structural and mechanical imaging, which has been utilized in many fields [1]. The contrast in SAM measurements arises from the measurement of the amplitude and time-of-flight from a sample, and subsequently, they are used to calculate the localized mechanical properties. Unfortunately, both properties are affected by possible localized sample tilt. Especially for biological samples, ideal preparation is not always possible, and the surface of a sample may have feature patches with different local inclination angle.

Here, we aim to quantify the angle dependency of the focal amplitude of our custom-built scanning acoustic microscope [2]. The measurements, utilizing a 300 to 500 MHz linear 1µs chirp with gaussian envelope, were performed on a flat silicon surface. Multiple B-scans were performed on the sample in different angles. The obtained data was used to compensate the loss of amplitude in a measurement of a biological sample to obtain more precise acoustic impedance values. The results will be utilized to make our instrument more precise when measuring e.g. biological samples.

REF 1: Briggs and Kolosov “Acoustic Microscopy (Second Edition)”, Oxford University Press, 2010.
REF 2: Meriläinen A. I. et al. ”Solid state switch for GHz coded signal ultrasound microscopy”, Electronics Letters. vol 49, no. 3, pp. 169-170 Jan. 2013


Poster: B105, March 6th, 15:30 - 17:30, 1st floor

BERYLLIUM CHARGE STATE DISTRIBUTION FOR OPTIMISATION OF ACCELERATOR MASS SPECTROMETRY

B. Gonsalves1, K. Mizohata1, J. Räisänen1

basil.gonsalves@helsinki.fi
1 University of Helsinki


Accelerator mass spectrometry (AMS) is an ultrasensitive analytical technique in which atoms are counted directly instead of being induced to emit radiation (e.g. in spectroscopy) or scatter bombarding particles (e.g. ion beam techniques). In this technique atoms are extracted from a sample are ionized, accelerated to high energies, separated according to their momentum, charge and energy and then individually counted after identification as having correct atomic number and mass. AMS is able to measure concentrations of one atom in $10^{15}$ atoms, to an accuracy of about one per cent, using a sample size of the order of one mg, and a measurement time of one hour.

$^{10}$Be (t$_{1/2}$ = 1.5 x 10$^{6}$ ) is of cosmogenic origin and due to its very long half-life, is ideal for Geosciences studies. To expand the 5 MV tandem accelerator of Helsinki AMS for $^{10}$Be analysis, transmission of the ion beam through a accelerator is studied. In this work we measured Be charge state distributions after passage through the gaseous (CO$_{2}$ and Ar) terminal stripper to optimise Be beam transmission. Our aim is to optimise the Be beam so that it can be used for AMS studies by selecting the most abundant charge state. This is important to us from a practical view point as it tells us which state gives the highest intensity for Be Accelerator Mass Spectrometry (AMS) measurements.

We measured $^{9}$Be charge states +1, +2, +3, and +4 yields for energies varying from 0.5 to 2 MeV (see Figure 1). $^{9}$Be beam was produced from injected BeO$^{-}$ beam in the terminal stripper with terminal voltages from 1 to 5 MV and yield was determined from the beam currents measured by Faraday cups. Experimental results were compared against experimental values from literature, semi empirical parametrisation models and ionisation theories. The maximum Be yield for 5 MV tandem accelerator was found to be at 4.17 MV terminal voltage for charge state +2. From the literature it can be seen that the ionisation process of Be in the terminal stripper has not been investigated adequately in depth in the past.

[1] L. Calcagnile, G. Quarta, L. Maruccio, V. Gaballo, H. A. Synal, and A. M. Muller, “$^{10}$Be detection at the new AMS beam line at CEDAD: Performance tests and first results,” Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms, vol. 331, pp. 215–219, 2014.
[2] J. L. Yntema, “Heavy ion stripping in tandem accelerator terminals,” Nucl. Instruments Methods, vol. 122, no. C, pp. 45–52, 1974.




Figure 1
Figure 1: Main results of experiment

Poster: B106, March 6th, 15:30 - 17:30, 1st floor

LASER ULTRASOUND GUIDED WAVE TOMOGRAPHY FOR DAMAGE DETECTION IN HEMISPHERICAL SHELLS

D.Veira Canle1, J.Mäkinen1, A.Salmi1, E.Hæggström1

daniel.veiracanle@helsinki.fi
1 Department of Physics, Division of Materials Physics, University of Helsinki, Finland


Laser ultrasound guided wave tomography is a technique that allows damage evaluation in structures that are hard to reach (surgical implants), located in harmful environments (nuclear reactors) and where the structure itself is moving (propellers). In laser ultrasound the sample is the transducer, thus enabling the study of curved structures which would prove challenging with conventional transducers. However, studying curved structures with ultrasound may prove challenging. Resonances may appear due to possible sample symmetries and multiple propagation paths cause wave superposition and reduce the image contrast.

Beam shaping can be used to alleviate this problem. We show that a line source reduces the ambiguity in the interpretation of measured data in comparison to a point source. We performed COMSOL simulations on a stainless-steel hemisphere 5 cm in diameter and showed that the directivity of the acoustic field (power as a function of angle) matches the experimental results.

Combining the highly directed acoustic field with a tomographic measurement on the steel hemisphere we were able to locate a 9.5 mm size hole and perform an image reconstruction on a 3D model. We foresee that the tomographic technique presented in this study would find applications in damage evaluation curved structures such as fuel tanks and fuselages.



Figure 1
Figure 1: Fig 1. COMSOL simulation showing the interaction between the highly directed acoustic field with a 9.5 mm hole on a steel hemisphere. Lamb waves are excited at the center of the hemisphere by a line source 8 mm long and their amplitude is read at the edge of the sample by an LDV. In blue, the Lamb wave travelling through an intact region and in red the one deflected by the defect. Since the Lamb wave cannot travel through the hole, it circles around it, thus arriving later than the blue signal.

Poster: B107, March 6th, 15:30 - 17:30, 1st floor

LASER ULTRASOUND ON A ROTATING DRONE PROPELLER

V. Veromaa, D. Veira Canle, J. Mäkinen, A. Salmi, D. Hæggström

valtteri.veromaa@helsinki.fi
1 Department of Physics, Division of Materials Physics, University of Helsinki, Finland


Propeller inspection and quality control is mandatory for safe operation of aircraft and drones. Nowadays the United States trains more drone pilots than aircraft pilots [1]. The rapid movement of the blades makes possible damage (e.g. cracks) grow rapidly, and thus it would be important to evaluate the integrity of the spinning blades without stopping operation, e.g. at cargo loading stations.

In this study, we continue our work performed on rotating blades [2]. Previously, we demonstrated notch detection in a blade rotating at 415 rpm. Here, we report on our efforts to study much faster moving targets. Lamb waves were excited on a rotating blade with a Q-switched Nd: YAG laser synchronized to the sample rotation, whereas the wave amplitude was obtained with a laser Doppler vibrometer (LDV).

We located a surface breaking crack on a carbon reinforced propeller rotating at 4000 rpm and verified the experimental results with FEM simulations (Fig.1). We foresee that this technique would find applications for quick inspection of drone propellers thus reducing the operational cost and the downtime of these machines. Possible applications of this technology range from inspection of drones for goods delivery to military drones.

[1] “Drone Milestone: More RPA Jobs Than Any Other Pilot Position | Military.com.” [Online]. Available: https://www.military.com/daily-news/2017/03/08/drone-milestone-more-rpa-jobs-any-other-pilot-position.html. [Accessed: 17-Jan-2019].

[2] D. Veira Canle, A. Salmi, and E. Hæggström, “Non-contact damage detection on a rotating blade by Lamb wave analysis,” NDT E Int., vol. 92, 2017.



Figure 1
Figure 1: FEM simulation of a laser ultrasound measurement of a flat aluminum propeller. Through transmission measurement of a propeller blade. An Nd:YAG laser excited Lamb waves (diagonal lines) at one edge of the blade and propagated towards the LDV (pickup point) located at the center. Here both the symmetric modes (north east corner) and the antisymmetric modes (south west corner) are visible.

Poster: B108, March 6th, 15:30 - 17:30, 1st floor

UNDERSTANDING THE PHYSICS OF ULTRASOUND ENHANCED ELECTROSPINNING (USES)

P. Helander1, J. Mäkinen1, J. Hunnakko1, T. Puranen1, K. Kogerman2, I. Laidmäe2, A. Salmi1, J. Heinämäki2, E. Hæggström1

petteri.helander@helsinki.fi
1 Department of Physics, University of Helsinki, P. O. box 64, 00014, Helsinki, Finland
2 Department of Pharmacy, University of Tartu, Nooruse 1, 50411 Tartu, Estonia


Electrospinning is the leading method for producing nanoscale fibers. In traditional electrospinning a strong electrical field pulls a thin fiber from polymer solution trough a nozzle. Although the phenomenon has been studied extensively, no comprehensive model exists as the process includes the complexity of electrohydrodynamics and rheology. Models capable of even partly describing the process are valuable since the ability to predict the properties of the nanofiber eliminates laborious experimental work. The fiber diameter is known to be affected by several system parameters such as the electric field strength, viscosity of the solution and size of the nozzle. [1]
We have developed an improved spinning method: ultrasound enhanced electrospinning (USES) [2]. In USES, the nozzle is replaced with an ultrasonic fountain, a deformation in the solution surface created by acoustic radiation force. Introduction of the acoustic field allows dynamic control of the spinning process via changing the acoustic parameters.
We present a theoretical study on the effects of the acoustic parameters on the produced fiber diameter. The results were obtained by combining the effects of ultrasound to existing electrospinning models. While the linking of the theories is not flawless, it considers the dominant acoustic effects: changes in the fountain geometry and heating by absorption. Other effects, including changes in ejection velocity and sound propagation into the fiber, were found to be minute. The results confirm that it is possible to change the fiber diameter via altering the total acoustic power. The theoretical predictions are compared against experimental results.

1. Haider, Adnan, Sajjad Haider, and Inn-Kyu Kang. "A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology." Arabian Journal of Chemistry 11.8 (2018): 1165-1188.
2. Nieminen, Heikki J., et al. "Ultrasound-enhanced electrospinning." Scientific reports 8.1 (2018): 4437.


Poster: B109, March 6th, 15:30 - 17:30, 1st floor

NUMERICAL AND LABORATORY MODELLING OF ATMOSPHERE TURBULENCE SPECTRA

Dmitry Zhilenko1, Maria Gritsevich2, Olga Krivonosova1

jilenko@imec.msu.ru
1 Moscow State University
2 University of Helsinki


Turbulent flows of a viscous incompressible fluid in a layer between rotating concentric spheres under the action of the modulation of the velocity of one of the spheres have been studied experimentally and numerically. The possibility of the formation of turbulence with spectra qualitatively similar to spectra obtained in measurements in the upper atmosphere is established: with the slope close to –3 at low frequencies and close to –5/3 at high frequencies and with the negative third order longitudinal velocity structure function. It has been shown that such spectra are formed in the regions of a flow that are strongly synchronized under the action of the modulation of the rotation velocity. This work was supported by Russian foundation for basic research, projects nos. 19-05-00028 and 18-08-00074.


Poster: B110, March 6th, 15:30 - 17:30, 1st floor

NEW TYPE OF CENTRIFUGICAL INSTABILITY UNDER TORSIONAL OSCILLATION OF LIQUID

Olga Krivonosova1, Dmitry Zhilenko1, Maria Gritsevich2

olga@imec.msu.ru
1 Moscow State University
2 University of Helsinki


Flows of a viscous incompressible fluid in a spherical layer that are due to rotational oscillations of its inner boundary with respect to the state of rest are numerically studied. With oscillations amplitude increasing the flow become instability and in addition to circulation in meridional plane of the flow toroidal structures appears near the equator. It is found that three kinds of instability are observed, and each kind is associated with its own variation range of frequency oscillations. For large and small frequencies values are observed well known Gortler vortices in the first case and Taylor vortices in the second case. For both cases rotation direction in two vortices is opposite to the direction of rotation in meridional circulation, and one extremes of vorticity is observed. At intermediate frequencies new spatial structures are revealed, with characteristic scale does not exceed half of the spherical layer thickness. Direction of rotation in these structures coincides with the same parameter for meridional circulation, and number of vorticity extremes alternate during the cycle of oscillation.
This work was supported by Russian foundation for basic research, projects nos. 19-05-00028 and 18-08-00074.


Poster: B111, March 6th, 15:30 - 17:30, 1st floor

MODULAR ACOUSTIC LEVITATOR

T. Puranen1, P. Helander1, A. Meriläinen1, G. Maconi1, A. Penttilä1, M. Gritsevich1,2, I. Kassamakov1, A. Salmi1, K. Muinonen1,3, E. Hæggström1

tuomas.puranen@helsinki.fi
1 Department of Physics, P.O. box 64, FI-00014 University of Helsinki, Finland
2 Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia
3 Finnish Geospatial Research Institute FGI, Geodeetinrinne 2, FI-02430 Masala, Finland


Acoustic levitation is a method for non-contacting sample manipulation. This method poses few requirements on the material properties, in contrast to e.g. magnetic and optical levitation which require magnetic and optical properties. Acoustic levitation relies on the precise control over the acoustic field in which a levitated object is suspended. Systems capable of advanced control, i.e. position and orientation control, employ phased arrays with many transducers [1]. The phase and amplitude of each transducer is individually computed and adjusted. The locations of transducers need to be known with subwavelength accuracy for the levitation to work. Existing designs have the transducers mounted to fixed positions. To create a modular design with no need for a priori information about the transducer positions a method to determine the relative position of the transmitters is necessary.

We demonstrate using the levitating transducers also for ultrasonic time-of-flight positioning of the array modules. This allows us to recompute the phase and amplitude for each transmitter to construct a levitating acoustic field. It is beneficial to use the ultrasonic transducers themselves as this removes the need for additional hardware. This technology enables construction of versatile acoustic levitators which can be reconfigured as needed.

[1] Kassamakov, Ivan, et al. "Light scattering by ultrasonically-controlled small particles: system design, calibration, and measurement results." Photonic Instrumentation Engineering V. Vol. 10539. International Society for Optics and Photonics, 2018.


Poster: B112, March 6th, 15:30 - 17:30, 1st floor

SHORT-RANGE SUPERCONTINUUM BASED LIDAR FOR TEMPERATURE PROFILING

Abba Saleh1,2, Antti Aalto1, Piotr Ryczkowski1, Tommi Mikkonen1, Juha Toivonen1

abba.saleh@tuni.fi
1 Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland.
2 Valmet Technologies Oy, Energy Services, Lentokentankatu 11, Tampere, Finland.


Light Detection and Ranging (LIDAR) is a remote-sensing technique based on collection of back scattered (optical frequency) electromagnetic radiation from a target point. The interaction of this radiation with surrounding gases and aerosol particles pave way for the measurement of various surrounding variables, including temperature, pressure, humidity and trace gases concentration. Although the LIDAR technique has been consistently used to measure various observables, the technique is typically limited to one specie at a time. However, this limitation can be mitigated using supercontinuum (SC) sources, which provides a broad spectrum[1] covering the absorption bands of all the investigated gases. Thus, opening the door for simultaneous detection of multiple species[2].

Herein, a new technique is developed for temperature mapping in thermal devices (TD), like combustion power plants, using a SC-based LIDAR. The device is design to measure temperature profile inside the TD using one opening. The technique is based on differential absorption, induced by the gas absorption cross section dependence on temperature, between three wavelength bands as shown in Fig. 1. (b). Water vapor transmittance, with a typical 10 % concentration and a 10 m interaction length, was modeled using the HITRAN 2012 database. Fig. 1. (a). depicts the schematic of the experimental setup. SC light pulses are guided into a furnace containing water vapor. Part of the incident light is scattered by the 1st scatterer placed just before the furnace, which will serve as a reference to the 2nd scatterer. The remaining part of the beam traverses through the furnace undergoing absorption, and subsequently being scattered by the 2nd scatterer behind the furnace. In an ideal combustion environment, the role of the scatterers is played by the naturally present aerosol particles. Moreover, the back-scattered signal from both scatterers are then detected as a function of time simultaneously for three wavelength bands. Therefore, by comparing the transmittance ratio between certain wavelength bands, temperature profile within the TD can be determined.

Our initial laboratory measurements shows a very close agreement with the simulation for all the channels (as shown in Fig. 1. (c)-(e)). Experimental data at elevated temperatures beyond 600°C were not obtained due to some technical limitations, which are currently being solved. Nonetheless, our preliminary results demonstrates a temperature measurement accuracy of 15°C in the range from 400°C to 600°C and a spatial resolution of 30 cm.

Besides temperature profiling, molecular number density or concentration can be measured simultaneously using the same technique, provided that three or more wavelength bands are used. Furthermore, by varying the direction of the incident beam in a non-parallel plane, a full 3D temperature and concentration measurements can be performed simultaneously using just one opening.

References
[1] J.M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135-1184 (2006).
[2] Méjean, G., et al. "Towards a supercontinuum-based infrared LIDAR." Appl Phys B-Lasers O. 77, 357 – 359 (2003).



Figure 1
Figure 1: Fig. 1. (a) Schematic of the experimental setup. (b) Modeled water vapor transmittance at varying temperatures with 10% water vapor concentration, and an interaction length of 10 m. (c)-(e) Preliminary experimental data for the transmittance ratio between all the channels compared to the simulated transmittance ratios.

Poster: B121, March 6th, 15:30 - 17:30, 1st floor

CAPABILITIES OF SATELLITE LASER RANGING IN OBSERVING NEAR-EARTH OBJECTS

O. Wilkman1, Arttu Raja-Halli1, Jyri Näränen1, Niko Kareinen1

olli.wilkman@nls.fi
1 Finnish Geospatial Research Institute FGI


Satellite laser ranging (SLR) is a geodetic technique in which distances to orbiting satellites, equipped with retro-reflectors, are measured from the flight time of reflected laser pulses. With improved laser and detector technology, modern SLR systems are also capable of observing targets without retro-reflectors, such as space debris. These observations are limited to relatively large objects and low orbits.

We study the capabilities of modern SLR systems to observe natural objects passing near the Earth. They present considerable difficulties compared to debris, as they are generally much darker than man-made objects, and arrive with very little advance warning. We compute probabilities of successful SLR ranging observations as a function of target size and altitude.

We find that in general, the possibilities of successful NEO observation with SLR systems are very limited. The small flux of near-Earth objects close to the Earth means that larger objects (observable at altitudes of thousands of km) are expected to be seen only once per decade or century. Small objects arrive regularly, but are observable only at very low altitudes (a few hundred km).

We consider one case in which SLR observations may be possible: objects which approach the Earth, and are captured by its gravitational field to perform an unstable orbit, and afterwards impact the atmosphere. Such objects could be discovered during their first approach, and adequate ephemeris computed for rapid response at suitably located SLR stations. These objects could be of similar size to the Chelyabinsk impact of 2013.


Poster: B122, March 6th, 15:30 - 17:30, 1st floor

RELATIONSHIP BETWEEN SOLAR UV-BRIGHTNESS AND SOLAR MAGNETIC FLUX FROM SDO OBSERVATIONS

I. Tähtinen1, I. Virtanen1, A. Pevtsov1,2,3, K. Mursula1

ismo.tahtinen@oulu.fi
1 ReSoLVE Centre of Excellence, Astronomy and Space Physics research unit, University of Oulu, POB 3000, FIN-90014, Oulu, Finland
2 National Solar Observatory, Boulder, CO 80303, USA
3 Pulkovo Astronomical Observatory, Russian Academy of Sciences, Saint Petersburg, 196140, Russian Federation


Solar chromospheric Ca II K spectral line has been continuously monitored since the start of the 20th century. Enhanced emission in different chromospheric spectral lines has been associated to magnetic fields since 1950s, when large-scale solar magnetic field observations started. The regions of enhanced Ca II K intensity, called plages, coincide very well with magnetic active regions and the Ca II K network resembles magnetic network. The structure of 160 nm brightness enhancements also resembles the magnetic network, as well as the active regions, similarly as the enhanced Ca II K emission. Therefore, images of the Sun in ultraviolet continuum at 160 nm reveal a strikingly similar structure as the photospheric magnetic field.

Our aim is to reconstruct the solar magnetic field from the historical Ca II K historical observations by quantifying the relationship between the 160 nm ultraviolet brightness and the unsigned magnetic flux density. We use 160 nm observations of Atmospheric Imaging Assembly (AIA) and photospheric line-of-sight magnetograms of Helioseismic Magnetic Imager (HMI) both onboard NASA’s Solar Dynamics Observatory (SDO).

We show that, depending on resolution, roughly 83% to 96% of enhanced UV-pixels (plages) spatially coincide with the active regions of the solar magnetic field. The total magnetic flux density depends on the plage area. Largest plages have mean magnetic flux density from 100G to 200G. Pixelwise relationship between the UV-brightness and unsigned magnetic flux density can be modelled with a power law with an exponent of about 0.6 – 0.7 depending on resolution.



Figure 1
Figure 1: Average intensities/flux densities of plages. Left panel: Unsigned magnetic flux density per pixel as
a function of plage size. Color of data marker shows the average 160 nm intensity per pixel. Right panel:
Average 160 nm intensity per pixel as a function of unsigned magnetic flux density per pixel. Color of data
marker depicts the plage size.

Poster: B123, March 6th, 15:30 - 17:30, 1st floor

WHY DO SOLAR ERUPTIONS ERUPT?

Jens Pomoell1, Daniel Price1, Erkka Lumme1, Emilia Kilpua1

1 University of Helsinki


Accurate modelling of the solar coronal magnetic field is of key importance for advancing our understanding of the processes governing the initiation of coronal mass ejections (CMEs) and their potential for causing severe space weather events. Currently, the most advanced models employed in a data-driven event-based context are time-independent, such as the nonlinear force-free field extrapolation model. However, such modelling is limited in that it cannot address the crucial issue whether the magnetic field is stable or evolves to produce an eruption.

In this work, we present our efforts to develop a data-driven time-dependent simulation to model coronal active regions from their birth to eruption. Employing the simulation tool, we discuss the conditions that lead to the formation of eruptive structures in the corona. In particular, we focus on understanding how the energy and helicity injected from the photosphere distributes in the corona to form structures that are prone to erupt.


Poster: B124, March 6th, 15:30 - 17:30, 1st floor

RESPONSE OF THE OUTER RADIATION BELT AND INNER MAGNETOSPHERE TO DRIVING BY SHEATH REGIONS OF CORONAL MASS EJECTIONS

M. Kalliokoski1, E. Kilpua1, D. Turner2, A. Jaynes3, A. Osmane1, L. Turc1, M. Palmroth1,4

milla.kalliokoski@helsinki.fi
1 Department of Physics, University of Helsinki, Helsinki, Finland
2 Space Sciences Department, The Aerospace Corporation, El Segundo, California, USA
3 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA
4 Finnish Meteorological Institute, Helsinki, Finland


The energetic electron content in the Van Allen radiation belts surrounding the Earth can vary dramatically on timescales from minutes to days, and these strong electron fluxes present a hazard for spacecraft traversing the belts. Electron dynamics in the belt is governed by various competing acceleration, transport and loss processes in which wave-particle interactions play an important role, and the response to solar wind driving is yet largely unpredictable. We investigate here the response of electron flux in the outer radiation belt to driving by sheath regions preceding interplanetary coronal mass ejections and the associated wave activity in the inner magnetosphere. We consider events in the Van Allen Probes era (from 2012 to present) to employ the unprecedented energy and radial distance resolved electron flux observations of the twin spacecraft mission. A statistical study of the events is performed using superposed epoch analysis, where the sheaths are superposed separately from the ejecta and resampled to the same average duration. Our results show that enhancements of the electron flux at source (tens of keV) and seed (hundreds of keV) energies are more common than depleting events, while at MeV energies the electron flux has a slightly decreasing trend on average. The magnetospheric wave activity in the ultra low frequency range, as measured by geostationary GOES satellites, is stronger during the sheaths than in the ejecta, whereas the power of chorus waves stays at about the same level despite on average stronger ring current enhancements during the ejecta.


Poster: B125, March 6th, 15:30 - 17:30, 1st floor

MAGNETIC PROPERTIES OF HIGH-SPEED JETS IN THE EARTH’S MAGNETOSHEATH

L. Vuorinen1, H. Hietala1,2, F. Plaschke3

lakavu@utu.fi
1 Department of Physics and Astronomy, University of Turku, Turku, Finland
2 Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, USA
3 Space Research Institute, Austrian Academy of Sciences, Graz, Austria


The solar wind is a stream of charged particles flowing from the Sun. This plasma carries out the Sun’s magnetic field forming the interplanetary magnetic field (IMF). As the supersonic solar wind moves towards the Earth, it encounters an obstacle -- the Earth's magnetic field. Therefore, the solar wind forms a bow shock in front of the Earth's magnetosphere and is slowed down, compressed and deflected. The deflected plasma flows around the Earth between the bow shock and the boundary of the Earth's magnetosphere (magnetopause), in an area called the magnetosheath.

Magnetosheath jets are regions of plasma that move faster towards the Earth than the surrounding magnetosheath plasma. Due to their high dynamic pressure, they can cause indentations when colliding into the magnetopause. These impacts can trigger processes such as magnetopause surface waves and magnetic reconnection, and have been shown to be associated with auroral brightenings.

We statistically study these jets in the subsolar magnetosheath using measurements from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and OMNI solar wind data from 2008--2011. First, we study the occurrence of the jets to better understand where and how they are formed. Secondly, we study the magnetic properties of the jets with our goal to understand whether the magnetic field in them is such that jets enhance or suppress magnetic reconnection at the magnetopause.

We show how the IMF controls where the jets occur: jets occur predominantly downstream of the quasi-parallel bow shock, that is where the IMF is almost parallel to the shock normal. In this region particles are able to reflect from the shock and interact with the inflowing solar wind, causing the quasi-parallel shock to be extended into a turbulent foreshock region. The occurrence pattern of the jets suggests that foreshock processes affect jet formation and implies that the effects of jets are felt more downstream of the quasi-parallel shock.

As the formation of jets is strongly connected to the interplanetary magnetic field, we are interested in how the jets carry these magnetic properties while propagating in the magnetosheath, possibly making it to the magnetopause. Because the Earth's magnetic field is northward in the inner magnetosphere near the subsolar magnetopause, magnetic reconnection is enhanced when the magnetosheath magnetic field is oppositely directed southward. Our preliminary results suggest that during northward IMF it is more likely for a jet to have southward magnetic field compared to rest of the magnetosheath plasma, and the other way around during southward IMF.


Poster: B126, March 6th, 15:30 - 17:30, 1st floor

FORESAIL-1: ENERGETIC PARTICLE AND DE-ORBITING EXPERIMENTS WITH A CUBESAT

R. Vainio1, for the FORESAIL-1 Team1,2,3,4,5

rami.vainio@utu.fi
1 Department of Physics and Astronomy, University of Turku, Finland
2 Department of Physics, University of Helsinki, Finland
3 Department of Future Technologies, University of Turku, Finland
4 School of Electrical Engineering, Aalto University, Espoo, Finland
5 Finnish Meteorological Institute, Helsinki, Finland


The Finnish Centre of Excellence for Research of Sustainable Space (FORESAIL) is an eight-year project (2018-2025) led by the University of Helsinki, Finland, set to investigate the Earth's radiation environment and develop technological solutions that on one hand help the survival of CubeSats in space radiation and on the other demonstrate the revolutionary Coulomb drag mechanism enabling electric solar wind sailing as well as de-orbiting satellites reliably after their use. The centre will launch three CubeSat missions to Low Earth Orbit (LEO), Geostationary Transfer Orbit and beyond, respectively. Here we describe the first mission, FORESAIL-1, set for launch in the turn of 2019-2020.

The FORESAIL-1 mission will be a 3-unit CubeSat launched to polar LEO. The spacecraft bus is based on a modular avionics stack, developed for higher reliability. The stack hosts an on-board computer, based on two cold-redundant ARM R4 based micro-controller units, an UHF radio communication system, an attitude determination and control system based on magnetorquers, and an electrical power system. The mass of the spacecraft is 4.0 kg.

FORESAIL-1 will carry a Particle Telescope (PATE), which has the primary objective of accurately measuring the 80-800 keV electrons precipitating in the atmosphere. For this, it needs an angular resolution good enough to separate the particles that are in the bounce loss cone from those that are not. This will be achieved by using two telescopes, one pointing along the spin axis of the satellite while the other scans the directions perpendicular to it at a rate of 4 rpm. The secondary science target is to observe energetic hydrogen (ions and atoms) at energies 0.3-10 MeV.

FORESAIL-1 will test an innovative application of space weather physics, namely de-orbiting of the satellite by means of Coulomb drag with the ionospheric plasma. FORESAIL-1 deploys a long, thin and negatively charged plasma brake tether, which disturbs the plasma ram flow to create braking thrust. The tether is so thin that it does not form a threat to other satellites. The baseline plan is to use the plasma brake in an early phase of the mission for going from synchronous to a somewhat lower drifting orbit so that the local time sampling characteristics of PATE are improved. The experiment demonstrates the capability of a plasma Coulomb drag device to modify the orbit and to de-orbit a satellite.

We will present the structure and goals of the FORESAIL-1 mission and give a status report on the development activities.



Figure 1
Figure 1: The FORESAIL-1 satellite is a 3-unit CubeSat to be launched in a polar Low Earth Orbit to measure precipitating electrons and protons, as well as solar energetic hydrogen ions and atoms. The mission will also demonstrate a novel de-orbiting method utilizing the plasma Coulomb drag.

Poster: B127, March 6th, 15:30 - 17:30, 1st floor

MODELLING PARTICLE PRECIPITATION WITH A GLOBAL KINETIC MAGNETOSPHERIC MODEL: VLASIATOR

M. Grandin1, M. Battarbee1, T. Brito1, M. Dubart1, U. Ganse1, Y. Pfau-Kempf1, L. Turc1, M. Palmroth1,2

maxime.grandin@helsinki.fi
1 Department of Physics, University of Helsinki, Helsinki, Finland
2 Finnish Meteorological Institute, Helsinki, Finland


Particle precipitation plays a key role in the coupling of the terrestrial magnetosphere and ionosphere. Protons and electrons precipitating from the magnetosphere modify the upper atmospheric chemistry, drive field-aligned currents, and produce aurora. However, quantitative observations of precipitating fluxes are limited. Indeed, ground-based instruments can only provide indirect measurements of precipitation, whereas particle telescopes onboard spacecraft merely enable local observations and inherently coarse time resolution above a given location. On the other hand, global magnetospheric simulations can provide estimations of particle precipitation over larger spatial scales and with higher time resolution. We present the first results of auroral (~1–30 keV energy) proton precipitation estimation using the Vlasiator global hybrid-Vlasov model. The simulation run used in this study is in the noon-midnight meridional plane and is driven by steady solar wind with southward interplanetary magnetic field. At selected locations in the simulated nightside magnetosphere, we first calculate the bounce loss cone angle value, which determines which particles precipitate and which ones are trapped in the inner magnetosphere. Then, using the velocity distribution function representation of the proton population at those selected points, we study the population inside the loss cone. This enables the estimation of differential precipitating flux, i.e., precipitating flux as a function of particle energy, as would be measured by a particle detector onboard a low-Earth-orbiting spacecraft. We can then infer the mean precipitating energy as well as the total precipitating energy flux, which can be compared to those obtained from observations or empirical models.


Poster: B128, March 6th, 15:30 - 17:30, 1st floor

INVESTIGATION OF ION-SCALE WAVES WITHIN A GLOBAL HYBRID-VLASOV SIMULATION

M. Dubart1, U. Ganse1, A. Osmane1, D. Verscharen2,3, K. Klein4,5, M. Grandin1, T. Brito1, M. Battarbee1, Y. Pfau-Kempf1, L. Turc1, M. Palmroth1

maxime.dubart@helsinki.fi
1 University of Helsinki, Helsinki, Finland
2 Mullard Space Science Laboratory, University College London, Dorkin, UK,
3 Space Science Center, Univeristy of New Hampshire, Durham, USA
4 Lunar and Planetary Laboratory, University of Arizona, Tucson, USA,
5 Department of Physics, University of New Hampshire, Durham USA


Waves are ubiquitous in near-Earth space and play an important role in the coupling between its different regions. From the formation of turbulent structures to their impact upon particle precipitation within the ionosphere, the understanding of these waves is a key point to the understanding of space weather events. Here we use the novel Vlasiator global hybrid-Vlasov simulation to investigate the various types of ion-scale waves present in the magnetosheath and the foreshock. We identify the wave modes and we compare them to existing plasma and dispersion solvers results, such as the Arbitrary Linear Plasma Solver, in order to validate the simulations. We discuss their relation with the proton velocity distribution functions, as the hybrid-Vlasov approach provides us with noise-free distribution functions throughout the simulation domain. These results will be used to later investigate the behaviour and impact of the present waves on processes such as particle precipitation.


Poster: B131, March 6th, 15:30 - 17:30, 1st floor

LECTURES NEED DEVELOPMENT: REQUIREMENTS FOR A HIGH-QUALITY PHYSICS EDUCATION FROM A TEACHER’S PERSPECTIVE

A. Tuomela1, L. Timonen1, S. Harmoinen2, S.-M. Huttula1

anu.tuomela@oulu.fi
1 Nano- and molecular systems research unit, University of Oulu, Oulu, Finland.
2 Faculty of Education, University of Oulu, Oulu, Finland.


Teacher-centred lecturing is one of the most common teaching methods at universities worldwide, even though it is definitely not the most effective way to enhance it [1-2]. It is widely studied that individuals learn through experiences. Theories behind experiential learning emphasize that a teacher should not be the authority telling the truth in front of students, but rather a senior helping them to learn [3-4]. A recent model of experiential learning, introduced by Jarvis [4], is a comprehensive model of learning but there are yet hardly any studies about implementing the model from theory to practice.

Large enrolment courses bring challenges for education design. Our practical aim is to redesign a large enrolment physics course through design-based research (DBR) [5]. This study is the first partial study of the DBR process and, throughout the study, requirements for high-quality education from a teacher’s perspective was examined. The teacher’s perspective is important to consider, since the teacher is the main stakeholder of teaching and, in our study, the DBR process. Additionally, there have been numerous studies investigating learning from students’ perspective but not so many focused on teachers’, as professionals, experience of the requirements on high-quality teaching. Analysis method of this qualitative study was inductive content analysis from data including video interviews of the lecturer. Through analysis, requirements that were found based on the data were reflected to existing theories of experiential learning.

Our results show various requirements for high-quality teaching. Most of them are seen difficult to implement in teacher-centred lecturing. DBR certainly is needed also from the theoretical perspective. Additionally, our study revealed requirements considering not only students and learning but also teachers. Inconsideration of these teachers’ requirements could be one reason for teacher-centred lecturing, since traditional lecturing can be regarded as ‘a survival kit’ [6] for the lecturer: lectures can be carefully scripted beforehand. The future of the DBR considers the results of this partial study and implements the Jarvis’ experiential learning model from theory to practice.

[1] O. P. Ajaja. EJSE. 17(1) (2013).
[2] J. K. Knight et al. CBE. 4(4), 298-310 (2005).
[3] D.A. Kolb. Experiential learning: Experience as the source of learning and development. Prentice-Hall. (1984).
[4] P. Jarvis et al. The theory and practice of learning: Psychology Press. (2003).
[5] B. Barab et al. JLS. 13(1) (2004).
[6] P. Jarvis (ed.). The theory and practice of teaching. 40, (2006).


Poster: B132, March 6th, 15:30 - 17:30, 1st floor

CURRENT ADVANCES IN TEACHING AND LEARNING ATMOSPHERIC SCIENCES

A. Lauri1, L. Riuttanen1, T. M. Ruuskanen1

katja.lauri@helsinki.fi
1 Institute for Atmospheric and Earth System Research, University of Helsinki


There is a currently a lot happening in the online and intensive research-oriented education in atmospheric sciences.

In online education within atmospheric sciences in Finland, the first major advance was the introduction of the climate.now MOOC (Massive Open Online Course) in 2016, promoting transdisciplinarity, transformative and collaborative learning in multidisciplinary study groups (Lehtonen et al., 2018). In 2018, two new MOOCs Circular.now and Leadership for sustainable change were designed and produced. The MOOCs have been designed and produced in collaboration of several universities with the support of the Finnish Innovation Fund Sitra. The Leadership for sustainable change MOOC gained high interest and was started by some 300 students. The MOOCs done in collaboration with several universities have paved way for the start of Climate University project funded by the Ministry of Education and Culture. The Climate University, led by University of Helsinki, will offer open workshops for university teachers to meet and co-design future climate and sustainability education until the end of 2020, and produce new online learning materials for higher education.

With the funding by the Nordic Council of Ministers and the University of Helsinki digital leap programme, we are also preparing new MOOCs and other online education activities in the following topics: basics of meteorology, basics of oceanography, climate change in the arctic, air quality in the changing world, and statistical tools for atmospheric and climate scientists.

Apart from online education, Institute for Atmospheric and Earth System Research (INAR) has a long tradition of organizing short research-intensive courses for master and doctoral students, and currently we organize or contribute to 5-10 such courses annually. During the academic year 2017-2018, we organized two courses in analysis of atmosphere-surface interactions and feedbacks, two courses in atmospheric and biospheric measurement techniques, a course about atmosphere-biosphere modelling, a climate change course on science, art and activism in collaboration with the University of the Arts Helsinki, and in Nordic collaboration an interdisciplinary course about the effects of climate change in arctic livelihoods and living conditions.

These courses are not only an excellent way of promoting scientific approach in solving challenging interdisciplinary problems, but also an effective way of providing training in transferable skills such as advanced analysis of large datasets, teaching and learning skills, oral presentation skills, and academic writing (Ruuskanen et al., 2018).

References

A. Lehtonen, A. Salonen, H. Cantell and L. Riuttanen (2018). A pedagogy of interconnectedness for encountering climate change as a wicked sustainability problem. Journal of Cleaner Production 199, 860-867.

T. M. Ruuskanen, H. Vehkamäki, L. Riuttanen and A. Lauri (2018). An Exploratory Study of the Learning of Transferable Skills in a Research-Oriented Intensive Course in Atmospheric Sciences. Sustainability 10(5), 1385.


Poster: B141, March 6th, 15:30 - 17:30, 1st floor

TRACING VOLCANIC ERUPTIONS FROM TREE-RINGS BY PIXE

Anna Liski1

anna.liski@helsinki.fi
1 University of Helsinki


The age of organic material can be determined by radiocarbon dating. After a tree dies exchange of carbon with the environment stops, and the amount of radioactive $^{14}$C begins gradually to diminish. Time of death is determined from the ratio of radiocarbon and stable isotope $^{12}$C. Time data obtained with radiocarbon dating can be compared with those measured from tree-rings. The trees grow one ring every year and the wood within one ring remains unchanged. This way a yearly record of radiocarbon concentrations is created within the trunk. By using dead trees from the same area with overlapping ages a record can assembled for over ten thousand years into the past (cross-dating).


It may be possible to increase the accuracy of cross-dating by measuring traces of volcanic eruptions from tree-rings and using them as fixation points. Volcanic eruptions leave a chemical signature in the rings (C. Preston, 2005) . The signature contains some of the elements of volcanic gases that are absorbed into the tree. Ion beam analysis technique of PIXE, particle induced X-ray emission, was used to determine concentrations of elements in the tree-rings thus obtaining year by year data.


Modern tree samples were used as absolute dating standards and to validate the used method. In this work, the elemental concentrations of tree-rings from 1980 to 2015 were measured. Within this time period two major volcanic eruptions have occurred: Mount Pinatubo, 1991 in Philippines and Eyjafjallajökull, 2010 in Iceland. The tree-rings corresponding to the years of these volcanic eruptions were found to contain increased amounts of S and Cl. These elements are components of volcanic gases and are likely to originate directly from eruptions. The X-ray yields of S and Cl correlated with those of K, Ca, Al, Fe and to a lesser degree with those of Cu, Ni and Zn. The increase in concentrations of these elements may be linked to changes the volcanic gases bring to soil chemistry. Further studies could enlighten the processes involved.


Poster: B142, March 6th, 15:30 - 17:30, 1st floor

ICE NUCLEATION IN CONFINED GEOMETRY AND THE ROLE OF LATENT HEAT

O. H. Pakarinen1, C. Pulido Lamas2,1, H. Vehkamäki1

olli.pakarinen@helsinki.fi
1 INAR/Physics, University of Helsinki, Finland
2 Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, Spain


Understanding the way in which ice forms is of great importance to many fields of science.
Pure water droplets in the atmosphere can remain in the liquid phase to nearly -40 ºC. Crystallization of ice in the atmosphere therefore typically occurs in the presence of ice nucleating particles (INPs), such as mineral dust or organic particles, which trigger heterogeneous ice nucleation at clearly higher temperatures. Such active INPs can also be used for rain enhancement.
Experiments have shown in great detail what is the IN activity of different types of compounds, and recently also clarified the importance of small surface features such as surface defects. The molecular-scale processes responsible for ice nucleation are still not well known, however. In recent years, several computational studies have advanced our understanding of the details of ice nucleation in many materials, and also the role of defects. Recently simulations showed enhanced ice nucleation efficiency in confined geometry such as wedges or pits (Bi, Cao and Li, 2017).
We are studying these topics by utilizing the monatomic water model (Molinero and Moore, 2009) for unbiased molecular dynamics (MD) simulations, where a system including a defected surface, such as pyramidal pits, steps or surface cracks, immersed in water, is cooled continuously below the melting point over tens of nanoseconds of simulation time and crystallization is followed.
Results of simulations on pyramidal pits on Si (100) surfaces (Fig. 1), an experimentally realizable system, show a clear (∆T > 10 ºC) enhancement of ice nucleation compared to flat Si (100) or Si (111) surfaces, in agreement with initial experimental findings of preference of ice to nucleate at these sites. Understanding the enhanced activity in such confined geometry may lead to characterization of active sites on some ice nucleating materials, or lead to designs of new artificial materials optimized for cloud seeding applications.
With a combination of finite difference calculations and molecular dynamics simulations we also show that the release of latent heat from ice growth has a noticeable effect on both the ice growth rate and the initial structure of the forming ice. However, latent heat is found not to be as critically important in controlling immersion nucleation as it is in vapor-to-liquid nucleation [Tanaka et al. 2017].

This work was supported by the Academy of Finland Center of Excellence programme (grant no. 307331) and ARKTIKO project 285067 ICINA, by University of Helsinki, Faculty of Science ATMATH project, by the National Center for Meteorology (NCM), Abu Dhabi, UAE, under the UAE Research Program for Rain Enhancement Science, as well as ERC Grant 692891-DAMOCLES. Supercomputing resources were provided by CSC–IT Center for Science, Ltd, Finland.

References
Bi, Y., B. Cao and T. Li (2017). Nat. Commun. 8, 15372.
Molinero, V. and E. B. Moore (2009). J. Phys. Chem. B 113, 4008.
Tanaka, K. K et al. (2017). Phys. Rev. E 96, 022804.



Figure 1
Figure 1: Experimentally realizable etched Si (001) pits increase ice nucleation activity due to geometric confinement. Nucleated cubic ice is shown in purple.

Poster: B143, March 6th, 15:30 - 17:30, 1st floor

MONITORING GREENHOUSE GASES WITH EARTH-OBSERVING SATELLITES

H. Lindqvist1, R. Kivi2, J. Hakkarainen1, E. Kivimäki1, T. Karppinen2, A. Kauppi1, O. Lamminpää1, A. Mikkonen3

hannakaisa.lindqvist@fmi.fi
1 Finnish Meteorological Institute, Helsinki, Finland
2 Finnish Meteorological Institute, Sodankylä, Finland
3 Finnish Meteorological Institute, Kuopio, Finland


In this poster, we present an overview of the recent and ongoing research activities that take place at the Finnish Meteorological Institute related to satellite observations of greenhouse gases. The newly-established Greenhouse Gases and Satellite Methods group develops retrieval methods for both satellite and ground-based remote sensing, develops and applies mathematical methods for uncertainty quantification, and employs satellite-data-driven methods for greenhouse gas source-sink estimations, with a current special focus on anthropogenic CO2 emission areas. The group also maintains versatile validation measurement activities for greenhouse gases at Sodankylä, in Northern Finland. Most importantly, the FTIR instrument in Sodankylä has performed ground-based retrievals of column-averaged dry-air mole fractions of carbon dioxide (XCO2) and methane (XCH4) for 10 years, and thus has crucially contributed to the high-latitude validation of missions such as the Greenhouse Gases Observing Satellite (GOSAT), the Orbiting Carbon Observatory -2 (OCO-2), and the Tropospheric Monitoring Instrument (TROPOMI). Since 2013, also series of AirCore balloon launches have been performed to obtain accurate in-situ profiles of methane, carbon dioxide and carbon monoxide from the troposphere to lower stratosphere. In 2018, we started measuring
atmospheric profiles and spatial distribution of greenhouse gases with unmanned aerial vehicles (UAV). This year, we expand the measurements to vegetation photosynthesis by starting the measurements of Solar-Induced Fluorescence (SIF) radiation that has the potential to act as a proxy for the photosynthetic activity and therefore could be linked with CO2 fluxes.


Poster: B144, March 6th, 15:30 - 17:30, 1st floor

FRAGMENTATION OF ATMOSPHERIC CLUSTERS INSIDE A MASS SPECTROMETER

M. Passananti1,2, Evgeni Zapadinsky1, Tommaso Zanca1, Dina Alfaouri1, Juha Kangasluoma1, Nanna Myllys1,3, Matti P. Rissanen1,4, Theo Kurtén5, Mikael Ehn1, Michel Attoui6, Hanna Vehkamäki1

monica.passananti@helsinki.fi
1 Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Finland
2 Department of Chemistry, University of Turin, Italy
3 Department of Chemistry, University of California, Irvine, California 92697-2025, United States
4 Aerosol Physics, Faculty of Natural Sciences, Tampere University of Technology, Tampere, Finland
5 Institute for Atmospheric and Earth System Research / Chemistry, Faculty of Science, University of Helsinki, Finland
6 LISA, University Paris Est Creteil, Creteil, 94010, France


Atmospheric aerosol particles have a significant impact on climate, air quality and human health, therefore the knowledge of the processes that lead to formation of new particles in the atmosphere is crucial to predict and understand our climate. The development of Mass Spectrometers (MS) as the Atmospheric Pressure interface Time Of Flight (APi-TOF) and the Chemical Ionization APi-TOF (CI-APi-TOF) has revolutionized the study of atmospheric new particle formation. These instruments are able to detect molecules and small clusters, which are involved in the first stages of new particle formation, even at environmental low concentration. However, it has been shown that clusters can undergo transformations (fragmentation and/or evaporation) inside a mass spectrometer. This cause a problem to correctly determine the composition and the concentration of the detected clusters. Therefore, it is important to evaluate and quantify the collision induced cluster fragmentation (CICF) in order to retrieve the initial cluster distribution obtained in the experiments. In this work, we carried out a systematic study on the fate of clusters inside the APi-TOF and we developed a statistical model to predict the fragmentation of clusters inside the mass spectrometer.

In order to investigate the fate of clusters inside the APi-TOF we combined it with a high resolution Differential Mobility Analyser (DMA). The DMA allows us to measure the clusters size and separate them based on their size. Injecting into the APi-TOF only a mono-mobile size ions distribution it is easier to understand the fate of clusters inside the instrument because only one kind of clusters is studied at a time. We analysed sulfuric acid clusters produced by ElectroSpray Ionization (ESI) and we focused our study on sulfuric acid trimer fragmentation inside the APi. This latter is made by three vacuum chambers where an electric field is applied to guide the ions through the interface. We evaluated the effects of the voltages applied to the APi without changing the radio frequencies of the quadrupoles. We developed a model to describe the CICF inside the APi. In this model, the charged clusters move through the APi under applied constant and uniform electrical field (defined by the tuning of the instrument). We consider each cluster individually and its trajectory is simulated as a random process. Each trajectory can be affected by collisions, energy transfer and fragmentation. After simulating the trajectory (and the fate) of a statistically significant number of clusters we calculate the proportion of the fragmented clusters.
We experimentally quantified the fragmentation of sulfuric acid trimer (negatively charged) for 76 different voltage configurations and we compared the most significant results with the fragmentation obtained from our statistical model. The first results are very promising, we observed an excellent agreement between the experiments and the model. Moreover, we confirm that the voltages that have a significant impact on cluster fragmentation are at the end of the first chamber and at the beginning of the second one . However, we demonstrate through our model that the fragmentation of the clusters can happen until the end of the second chamber.
The development of this model and its validation by laboratory experiments is crucial to correctly interpret the experimental data obtained by APi-TOF instruments for simulating the atmospheric processes.


Poster: B151, March 6th, 15:30 - 17:30, 1st floor

ION BEAM PURIFICATION WITH A MULTI-REFLECTION TIME-OF-FLIGHT SEPARATOR AT JYFLTRAP

V. Virtanen1, O. Beliuskina1, L. Canete1, T. Eronen1, M. Hukkanen1, A. Jokinen1, A. Kankainen1, I. Moore1, D. Nesterentko1

ville.a.virtanen@student.jyu.fi
1 Department of Physics, P.O. Box 35, FIN-40014 University of Jyväskylä, Finland


Currently, Penning-trap mass spectrometry is the most accurate method for atomic mass measurements. A key challenge of the method is the removal of in-beam contaminants that can hinder the measurement. The origin of this contamination is commonly the ion source where the ions of interest are produced along with isobaric species which can be orders of magnitude more intense, in some cases. Thus high-precision mass measurements with a Penning trap require that the contaminant ions are removed with, for example, a multi-reflection time of flight (MR-TOF) separator. The MR-TOF offers many advantages [1], such as fast isobaric purification of the radioactive ion beam [2], and it can be utilized to remove contaminants with a mass resolving power on the order of $10^5$.$\\\\$ The JYFLTRAP Penning trap setup [3] at the IGISOL- facility [4] in the Accelerator Laboratory of the University of Jyväskylä, can be used as a high-resolution mass separator as well as for high-precision mass measurements. However, limitations prohibiting the measurement of certain light masses as well as the measurement of some fission isotopes exist. This is caused by the overlap with known contaminants in the buffer gas of IGISOL. To alleviate this issue, a multi-reflection time-of-flight separator is being installed to complement the Penning trap, with the effect of purifying larger quantities of beam in shorter time. This will additionally help with the measurement of short-lived isotopes. $\\\\$
The MR-TOF separates masses by trapping and reflecting ions in electric potential between electrostatic mirrors, for which the time-of-flight of individual ions depends on the ion's mass-to-charge ratio [2]. Consequently, the mass resolving power is sensitive to changes in the mirror voltages. To retain stable operating conditions, the voltages of the mirror electrodes need to be stable. Recently, a voltage stabilization system based on a PI-loop (proportional-integral) has been implemented [5] at ISOLTRAP's MR-TOF separator at CERN [2]. A similar PI-stabilization loop is now being implemented for JYFLTRAP's MR-TOF. In this contribution a general overview of the JYFLTRAP MR-TOF setup along with the preliminary results of the stabilization system will be given. $\\\\$

Refrences $\\\\$
[1] D. Lunney, J. Phys. G. Nucl. Partic. 44, 064008 (2017). $\\\\$
[2] R. Wolf et al., Int. J. Mass Spectrom. 349-350, 123–133 (2013). $\\\\$
[3] T. Eronen et al., Eur. Phys. J. A 48, 46 (2012).$\\\\$
[4] I. Moore et al., Nucl. Instrum. Methods Phys. Res. B 317, 208–213 (2013).$\\\\$
[5] F. Wienholtz, Private communication, Oct. 2018.


Poster: B152, March 6th, 15:30 - 17:30, 1st floor

RECENT DEVELOPMENTS ON FISSION ION GUIDE AND LASER IONIZATION AT IGISOL

A. Zadvornaya1, A. Kankainen1, I. D. Moore1, M. Reponen1

1 University of Jyväskylä, Department of Physics, P.O. Box 35, FI-40014, Jyväskylä, Finland


Experimental laser spectroscopy studies can give access to nuclear ground- and isomeric-state properties [1], which is essential to validate nuclear models applied to describe and to predict nuclear properties of the isotopes of different elements. The laser activities at Ion Guide Isotope Separation On-Line (IGISOL) laboratory at University of Jyväskylä cover a broad range of experiments, from in-source and collinear laser spectroscopy to atom trapping [2].

A part of the on-going development of the IGISOL laboratory will be presented in this talk, including development of IGISOL fission ion guide with extension enabling laser ionization [Figure 1, (a)]. Characterization of the extraction efficiencies and timings of the ion guide was performed using numerical calculations of subsonic compressible gas flows of argon and helium [Figure 1, (b)]. Finally, the status of the development of the injection-locked Titanium:Sapphire laser system [3] will be discussed.

[1] P. Campbell, I.D. Moore, and M.R. Pearson, Laser Spectroscopy for Nuclear Structure Physics, Prog. Part. Nucl. Phys. 86, 127 (2016).
[2] I. D. Moore, P. Dendooven, and J. Ärje (2013) The IGISOL technique—three decades of developments. In: Äystö J., Eronen T., Jokinen A., Kankainen A., Moore I.D., Penttilä H. (eds) Three decades of research using IGISOL technique at the University of Jyväskylä. Springer, Dordrecht
[3] M. Reponen et al., Towards in-jet resonance ionization spectroscopy: An injection-locked Titanium:Sapphire laser system for the PALIS-facility, Nucl. Instrum. Methods Phys. Res., Sect. B 908, 236-243 (2018)



Figure 1
Figure 1: IGISOL fission ion guide with laser extension. (a) Ion guide installed inside target chamber. (b) Numerical calculations of helium flow through the ion guide. The color bar indicates velocity magnitude. Red and green curves represent evacuation trajectories.

Poster: B153, March 6th, 15:30 - 17:30, 1st floor

SILICON-SCINTILLATOR DETECTORS AS A NOVEL CONCEPT FOR MEDICAL IMAGING AND DOSIMETRY

V. Litichevskyi1, E. Brücken1, A. Gädda1, J. Ott1, T. Naaranoja1, L. Martikainen1, S. Kirschenmann1, A. Karadzhinova-Ferrer2, M. Kalliokoski2, J. Tikkanen3, M. Golovlova1, P. Luukka1, J. Härkönen2

1 P.O.Box 64 (Gustaf Hällströmin katu 2), FI-00014 University of Helsinki, Finland
2 Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia
3 Radiation and Nuclear Safety Authority (STUK), Laippatie 4, 00880 Helsinki, Finland


Most of the conventional radiation detectors for wide spectra of medical imaging applications utilize either direct converting detectors (sensors based on pixelated Cadmium-Telluride (CdTe) and other wide band gap semiconductors), or scintillation panels directly coupled with photosensitive Si- matrix. Sensors, based on direct converting semiconductor materials, have a merit of better sensitivity to ionizing radiation and provide substantial dose reduction on the patients. On the other hand, expansion of such a technology to the market of medical imaging devices faces significant difficulties due to the limited availability of high-quality sensors (subsequently, highly expensive). Nowadays, more cost-efficient scintillator-based imaging systems have been widely deployed for medical imaging applications despite lower conversion efficiency.

Silicon based sensors for ionizing radiation are already well developed for a vast variety of applications where benefits of high conversion and manufacturing costs efficiencies are combined. But for the typical photon energy range of interest in most medical imaging applications (20-200 keV) the effective atomic number of Si is too low to ensure required contrast of taken images. This obstacle of low absorption efficiency for direct converting Si-based sensors can be resolved by attaching a conversion layer of scintillator materials with higher effective atomic numbers. The enhancement of Si-based sensors with scintillation material will provide the needed absorption efficiency for medical imaging applications and preserve high sensitivity of direct converting semiconductor detectors. Such novel sensors can be deployed not only for imaging but also for dosimetry applications. The R&D for such novel detectors has been started recently. First prototypes of a Si detector optically coupled with a conventional gadolinium oxysulfide scintillator (Figure 1) were developed, tested, and validated within the Multispectral Photon-counting for Medical Imaging and Beam characterization (MPMIB) project, funded by the Radiation Detectors for Health, Safety and Security (RADDESS) programme of the Academy of Finland.



Figure 1
Figure 1: Figure 1. Tb:GSO scintillator optically coupled with a Si detector.

Poster: B154, March 6th, 15:30 - 17:30, 1st floor

DIAMOND RADIATION HARDNESS

T. Naaranoja1, L. Forthomme1, F. Garcia1, M. Golovleva2, A. Gädda1, P. Koponen1, L. Martikainen1, F. Oljemark1, J. Ott1, H. Saarikko1, R. Turpeinen1, K. Österberg1

tiina.naaranoja@helsinki.fi
1 Helsinki Institute of Physics
2 Lappeenranta University of Technology


Diamond is used as a particle detector material in dedicated applications, where high radiation hardness or fast signals are required [1-3]. Although diamond is intrinsically radiation hard due to the high displacement energy[4], it does eventually suffer from radiation. The principal defects, which survive in room temperature for longer than one second, are neutral mono-vacancies and split self interstitial [5], which are both deep level traps. Radiation damage has two consequences: reduction in charge collection, and polarization i.e. modification of the electric field by trapped charge. The temporal properties of the signal change with irradiation as well. In certain conditions both the signal rise time and charge collection are reduced. This might lead to constant time resolution. Polarization, however, often leads to longer signal rise time and irregular signal shape. These adverse phenomena can be mitigated to certain degree by deploying methods such as switching bias voltage on-off or filling the trap states. In this way the sensor lifetime can be extended and stable operation achieved.

[1] CMS and TOTEM collaborations, (2014), CERN-LHCC-2014-021 TOTEM-TDR-003 CMS-TDR-13.
[2] TOTEM collaboration, (2017), JINST, 12 P03007.
[3] Leonard et al., (2014) Nuclear Instr.Meth. A 765, 235-239
[4] Venturi et al., (2018) Nuclear Instr.Meth. A, in press
[5] Pomorski, (2008), Doctoral dissertation


Poster: B155, March 6th, 15:30 - 17:30, 1st floor

HIGH ENERGY DIFFRACTION

M. Mieskolainen1

mikael.mieskolainen@helsinki.fi
1 University of Helsinki


Basic questions of any high energy interactions are unitarity, helicity structure and hidden geometric symmetries. At the LHC, one spectacular QCD but also electroweak playground to study these questions is diffraction - processes demonstrating a 'highly visible degree' of quantum mechanical coherence. Being rooted deeply in the non-perturbative Regge domain of QCD, obtaining high precision and maximally model independent LHC measurements of diffraction requires completely new methods. In this talk, I will introduce a new advanced computational engine: GRANIITTI, developed for high energy diffraction simulation and analysis, available at www.mv.helsinki.fi/home/mmieskol. In many aspects, it defines the current state-of-the-art.


Poster: B156, March 6th, 15:30 - 17:30, 1st floor

RESIDUAL MUON FLUX IN THE CANFRANC UNDERGROUND LABORATORY

P. Kuusiniemi1, T. Enqvist1, J. Joutsenvaara2, K. Loo1, M. Slupecki1, W.H. Trzaska1

pasi.kuusiniemi@gmail.com
1 University of Jyväskylä, Department of Physics, Finland
2 Kerttu Saalasti Institute, University of Oulu, Finland


Physics beyond the standard model is one of the cutting-edge fields of the modern physics. It includes the search for rare phenomena, such as neutrino properties, neutrino-less double beta decay or the study of dark matter, that require extremely low and well-understood background conditions. The only way to reduce the direct and induced cosmic-ray background is to place the experimental setup in one of the few underground laboratories around the world. One of those is the Canfranc Underground Laboratory (LSC) in the Aragonese Pyrenees.

In the present work the residual muon flux of high-energy cosmic muons together with its angular distribution were measured in two underground locations at the LSC. The experimental setup consisted of three layers of fast scintillation detector modules operating as 352 independent pixels. These plastic scintillation detectors were designed and constructed for the EMMA experiment placed below ground in the Pyhäsalmi mine, Finland [1].

The setup has flux-defining area of 1 m2, covers all azimuth angles and zenith angles up to 80 degrees. The measured integrated muon flux is (5.26 $\pm$ 0.21)$\times$10$^{-3}$ m$^{-2}$s$^{-1}$ in the Hall A of the LAB2400 and (4.29 $\pm$ 0.17)$\times$10$^{-3}$ m$^{-2}$s$^{-1}$ in LAB2500. These fluxes correspond to the depth of approximately 1500 m.w.e. (or 550 m of rock) with a flat overburden. The angular distribution is consistent with the known profile and rock density of the surrounding mountains. In particular, there is a clearly visible maximum in the muon flux entering from the direction of the Rioseta valley [2].


[1] P. Kuusiniemi et al., Performance of tracking stations of the underground cosmic-ray detector array EMMA. Astroparticle Physics, 102, 67–76, 2018.

[2] W.H. Trzaska et al., Cosmic-ray muon flux at Canfranc Underground Laboratory, to be published.


Poster: B157, March 6th, 15:30 - 17:30, 1st floor

DESIGN OF THE DETECTOR CONTROL SYSTEM OF THE FAST INTERACTION TRIGGER DETECTOR FOR THE UPGRADE OF THE ALICE EXPERIMENT

Heidi Rytkönen1

heidi.rytkonen@cern.ch
1 Department of Physics, University of Jyväskylä


The Large Hadron Collider at CERN is undergoing a major upgrade of the injectors. The physics case for the upgrade of ALICE (A Large Ion Collider Experiment) is presented in [1]. The implemented improvements will boost the collision parameters beyond the design specifications of the current ALICE detectors. To cope with the new operating conditions, ALICE will upgrade most of its sub-systems and introduce three new detectors [2]. One of them will be the Fast Interaction Trigger (FIT) [3]. FIT will consist of two Cherenkov array detectors called T0+ placed at both sides of the interaction point and a single large-size plastic scintillation ring called V0+.

One of the major challenges in the design and operation of a collider experiment is the need for full remote control and safe operation of all the key elements of the setup. This the role of the Detector Control System (DCS) [4]. DCS allows to configure, monitor and control the equipment from a single workplace - the ALICE experimental control room at CERN’s Point 2 site. Communication with devices such as high voltage sources and different sensors is accomplished by sending commands through control units to device units and receiving and handling states or alarms from these devices. The core software of the ALICE control system is based on a commercial solution provided by WinCC OA package. It will be used for the FIT detector and for every other sub-detector in the ALICE experiment. The control panels for FIT will be created using WinCC OA as well as the connections of the devices to the controls. The main challenge for FIT DCS will be to integrate the new digital electronics and the new, fully digital trigger generator. In my presentation, I will outline the conceptual design of the project

[1] B.Abelev et al., Upgrade of the ALICE Experiment: Letter Of Intent, Journal of Physics G: Nuclear
and Particle Physics, 41(8)087001, 2014
[2] M.Slupecki, ALICE Upgrade for LHC Run 3 and Run 4, in this book of abstracts
[3] W.H.Trzaska, New Fast Interaction Trigger for ALICE, Nuclear Instruments and Methods in Physics Research A 845 (2017) 463–466s, 2016
[4] C.W.Fabjan et al., ALICE trigger data-acquisition high-level trigger and control system:
Technical Design Report Geneva: CERN, 2004


Poster: B158, March 6th, 15:30 - 17:30, 1st floor

NEUTRINO PROPERTIES AND VACUUM STABILITY OF SMASH

T. J. Kärkkäinen1, C. R. Das2, K. Huitu1,3

timo.j.karkkainen@helsinki.fi
1 Helsinki Institute of Physics
2 JINR
3 University of Helsinki


SMASH (Standard Model + Axion + Seesaw mechanism + Higgs portal inflation) is a minimal framework to explain several of the current problems of particle physics and cosmology, that is, strong CP problem, neutrino mass, cosmic inflation, dark matter, metastability of vacuum and the baryonic asymmetry of the universe at one unified energy scale. It expands the particle content of the Standard Model with three heavy right-handed sterile Majorana neutrinos, quark-like colour triplet and a complex scalar field. The hypercharge is promoted to Peccei-Quinn charge, with the corresponding gauge boson leading to invisible axion after the breaking of Peccei-Quinn symmetry. We show that the minimal model is in excellent agreement with the present experimental constraints on neutrino masses. Also, we perform a renormalisation group analysis and obtain the scale dependence of neutrino masses and mass squared differences. We also show that the scalar potential is stable with a small portal coupling and acquire the necessary conditions for stability. Gauge unification is partially realised.


Poster: B159, March 6th, 15:30 - 17:30, 1st floor

SEARCH FOR CHARGED HIGGS BOSONS WITH THE CMS EXPERIMENT

S. Laurila1, J. Havukainen1, M. Lotti1, S. Lehti1

santeri.laurila@helsinki.fi
1 Helsinki Institute of Physics


A search for charged Higgs bosons is presented in the H$^+ \to \tau \nu$ decay mode in hadronic and leptonic final states. The search is based on 35.9 fb$^{-1}$ of proton-proton collision data recorded by the CMS experiment in 2016 at a centre-of-mass energy of 13 TeV. The results agree with the expectation from the standard model. Upper limits at the 95% confidence level are set on the production cross section times branching fraction to $\tau\nu$ for a charged Higgs boson in the mass range $80-3000$ GeV, including the mass region near the top quark mass. The limit is interpreted in the context of the MSSM $m_\mathrm{h}^{\mathrm{mod-}}$ scenario.


Poster: B160, March 6th, 15:30 - 17:30, 1st floor

MULTISPECTRAL PHOTON-COUNTING FOR MEDICAL IMAGING AND BEAM CHARACTERIZATION

Erik Brücken1, M. Golovleva2, A. Gädda1, R. Hostettler3, S. Kirschenmann1, V. Litichevskyi1, P. Luukka1, L. Martikainen1, J. Ott1, Z. Purisha3, T. Siiskonen4, S. Särkkä3, J. Tikkanen4,1, T. Tuuva2, A. Winkler5

erik.brucken@iki.fi
1 Helsinki Institute of Physics, Finland
2 Lappeenranta University of Technology, Finland
3 Department of Electrical Engineering and Automation, Aalto University, Finland
4 Radiation and Nuclear Safety Authority, Finland
5 Detection Technology Plc, Finland


Next generation detection systems operating in multispectral mode have potential to revolutionize diagnostic capabilities in medical imaging in terms of efficiency, image quality and lower patient dose.

We present our approach that utilizes direct conversion radiation detectors operating in \textit{photon counting} (PC) mode. This work is conducted within a consortium of research groups from Helsinki Institute of Physics, Aalto University, Lappeenranta University of Technology and Radiation and Nuclear Safety Authority (STUK) under the RADDESS program of Academy of Finland.

Our strategy is to use several different detector technologies including thick silicon, high-Z semiconductor materials (CdTe/CdZnTe) and silicon enhanced by scintillator (SiS) material together with \textit{read-out chips} (ROC) running in PC mode. Due to our involvement in high energy physics, in particular in the CMS Tracker at CERN, we have access to existing solutions of ROCs that are capable of working in the PC mode.

After producing a first successful prototype for the proof-of-concept, we are now focusing on the processing of the CdTe crystals and thick Si wafers at Micronova Nanofabrication Centre in Espoo. Processed sensors will then be flip-chip bonded with the ROCs. A critical part will be the flip-chip bonding of the CdTe sensors, due to intrinsic material properties. This has to happen at lower temperatures compared to flip-chip bonding of Si type sensors, thus usual bump bonding materials cannot be used. A feasible approach is to use Indium based bumps that allow bonding at low temperatures.

In addition to detector development, other crucial tasks related to this project are: the evolution from single module to detector arrays and its electronic readout; the advanced data and imaging reconstruction, and prototype testing in respect of repeatability and long term stability.



Figure 1
Figure 1: Photograph of processed CdTe crystal matching the pixel structures of the ROC (left) and SEM image of Indium based bumps on the ROC (right).

Poster: B171, March 6th, 15:30 - 17:30, 1st floor

ON THE ROLE OF LIPIDS IN THE FUNCTION AND MECHANISM OF PULMONARY SURFACTANT PROTEIN B

J. Liekkinen1, M. Javanainen2, G. Enkavi1, J. Perez-Gil3, I. Vattulainen1,4

juho.liekkinen@helsinki.fi
1 Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland
2 Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Prague 6, Czech Republic
3 Department of Biochemistry, Faculty of Biology, Complutense University, Madrid, Spain
4 Computational Physics Laboratory, Tampere University, P.O. Box 692, FI-33014 Tampere, Finland


The air–liquid interface of the alveoli is lined by a complex mixture of lipids and surfactant proteins called the pulmonary surfactant. The pulmonary surfactant reduces the surface tension of the essential gas exchange interface, which prevents the alveoli from collapsing and lowers the work of breathing [1]. Rapid spreading of the surfactant into a monomolecular film is essentially dependent on the sufficient interplay of the surface-active lipids and the surfactant associated proteins, in particular the hydrophobic proteins SP-B and SP-C [2]. Disruptions in the formation of the surfactant complex may lead to fatal conditions, such as respiratory distress syndrome. Hence, understanding the roles of the lipid and protein components of the pulmonary surfactant is not only necessary to improve the treatment of such disorders, but also to uncover the molecular mechanisms by which gas exchange occurs effectively at the air–liquid interface.

One essential component of the pulmonary surfactant is the surfactant protein SP-B. SP-B has an integral role in the initial formation, stability, and maintenance of the surfactant film at the air–liquid interface. Despite its importance, the structure and the molecular mechanism of SP-B are not well known. However, recent studies [3,4] have provided the first low-resolution structural models for ring-shaped oligomeric assemblies of SP-B. Molecular dynamics (MD) simulations are often the computational method used to study the structure and dynamic behavior of proteins and lipid layers at an atomistic or near-atomistic level, mostly unattainable by any other means. Here, we examined several oligomeric structures of SP-B through extensive coarse-grained MD simulations.

We show that the novel oligomeric structure of SP-B supports the proposed mechanism of function of the protein [3]. Additionally, our simulations demonstrate specific affinity of certain lipid types for distinct interaction sites in the SP-B structure. These interactions cause lateral reorganization of lipids in surfactant layers in contact with the SP-B oligomers. The results are discussed in terms of understanding how SP-B participates in the transfer of lipids between adjacent surfactant assemblies, and how different lipid types are affecting this process by interacting with SP-B.

Acknowledgments: We thank the Academy of Finland (Centre of Excellence project) and the European Research Council (Advanced Grant project) for financial support, and CSC – IT Center for Science for computational resources.
References

[1] E. Parra and J. Pérez-Gil. Chem. Phys. Lipids 185, 153-175 (2015)
[2] A. G. Serrano and J. Pérez-Gil. Chem. Phys. Lipids 141, 105-118 (2006)
[3] B. Olmeda, B. García-Álvarez, M. J. Gómez, M. Martínez-Calle, A. Cruz, and J. Pérez-Gil. FASEB J. 5, 4236–4247 (2015)
[4] E. Cabré, M. Martínez-Calle, M. Prieto, A. Fedorov, B. Olmeda, L. M. S. Loura, and J. Pérez-Gil. J. Biol. Chem. (2018)


Poster: B172, March 6th, 15:30 - 17:30, 1st floor

COMMISSIONING OF THE MULTIRAD 350 CELL AND SMALL ANIMAL X-RAY IRRADIATION SYSTEM

A. Saikkonen1,2, J. Niemelä2, P. Sipilä3, J. Keyriläinen2

aleksi.saikkonen@tyks.fi
1 Department of Physics and Astronomy, Turku University, Vesilinnantie 5, FIN-20521 Turku, Finland
2 Department of Medical Physics & Department of Oncology and Radiotherapy, Turku University Hospital, Hämeentie 11, FIN-20521 Turku, Finland
3 Radiation and Nuclear Safety Authority (STUK), Laippatie 4, FIN-00880 Helsinki, Finland


Purpose: The main objective of this study was to commission a commercial x-ray irradiation system to be used for cell and small animal studies.

Materials and methods: Evaluated characteristics of an x-ray irradiator included dose linearity and dose repeatability with respect to time, x-ray beam profiles, light field to irradiation field agreement and absolute radiation dose. Radiochromic films, ionization chambers and radiophotoluminescence dosimeters were used for dosimetry and the maximum settings of the irradiator, i.e. 350 kV and 10 mA, were applied.

Results: Tests showed that the dose was linear with respect to time (tested also with the settings 250 kV / 14 mA and 350 kV / 5 mA) and the dose repeatability versus time was within 5% beyond 15 s of irradiation time. The x-ray beam profiles were acceptable, flatness being less than 4%. The light field to irradiation field agreement appeared to have a maximum difference of 0.5 cm, the irradiation field being closer to the irradiator's door than the light field. Based on the absolute dosimetry, a specific dose-to-time table was created in order to help users to calculate irradiation times for desired dose.

Conclusions: In general, the investigated type of an x-ray irradiation system can be used in a safe and controlled manner for irradiating cells and small animals.


Poster: B173, March 6th, 15:30 - 17:30, 1st floor

STUDYING THE DIMERIZATION OF PULMONARY SURFACTANT PROTEIN SP-C THROUGH MOLECULAR DYNAMICS SIMULATIONS

H. Korolainen1, W. Kulig1,2, F. Lolicato1, I. Vattulainen1,2,3

hanna.korolainen@helsinki.fi
1 Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland
2 Computational Physics Laboratory, Tampere University, P.O. Box 692, FI-33014 Tampere, Finland
3 MEMPHYS - Center for Biomembrane Physics


A healthy pulmonary surfactant (PSurf) is essential for breathing and normal functioning of lungs, and abnormalities in its structure and function are linked to various diseases. The PSurf is a multilayered structure located inside the alveoli of the lungs, and it consists of roughly 90\% of phospholipids and 10\% of proteins. The main functions of the PSurf are the reduction of the surface tension and its role as a part of the immune defense system. Despite this importance, there is a significant knowledge gap in the understanding of the PSurf composition and its mechanisms of action.

Proteins have a major role in structure and functioning of the PSurf. One of the main proteins is called surfactant protein C (SP-C), and its primary functions include the transfer of lipids from lipid monolayers to multilayered structures, the enhancement of the adsorption of surface active molecules into the air-liquid interface, and the maintenance of the integrity of the multilayered structure. Deficiency of SP-C is known to lead to severe chronic respiratory pathologies, such as idiopathic pulmonary fibrosis (IPF) and interstitial lung disease (ILD).

In this study we focus on the dimerization of SP-C. Oligomeric proteins, in general, are abundant in nature, constituting around 35\% of cellular proteins. The oligomeric form allows the existence of more complex structures that can enhance the function of the protein. In the case of SP-C, the dimeric form might, for example, enhance the interactions of SP-C with the PSurf phospholipids or be otherwise beneficial to the function of the protein.

Extensive molecular dynamics simulations, both coarse-grained and atomistic, were performed. Our preliminary results from coarse-grained simulations show that SP-Cs form dimers in PSurf membranes.


Poster: B174, March 6th, 15:30 - 17:30, 1st floor

SMALL- AND WIDE-ANGLE X-RAY SCATTERING MEASUREMENTS ON TROPICAL HARDWOODS

M. Viljanen1, P. Ahvenainen1, P.A. Penttilä2, K. Svedström1

mira.viljanen@helsinki.fi
1 Department of Physics, University of Helsinki
2 Department of Bioproducts and Biosystems, Aalto University


Cellulose is the most abundant biopolymer on earth, providing the structural framework for all the species of the plantae. Wood is commonly used as construction material around the world and it is also important source for many industrial products such as chemical pulping and biofuel applications. The demand for forest biomass is growing annually and especially in the Asian market, the need for tropical hardwood timber is increasing, resulting in more aggressive, and often illegal, logging in tropical regions. [1]
Over the years, this has led to some commonly used tree species (e.g. mahogany, ebony and rosewoods) becoming vulnerable and even critically endangered, bearing a high risk of vanishing from the nature. Research to find wood species with the same or similar desired properties matching the currently overharvested species is therefore essential. [2]

On the ultrastructural scale of wood, cellulose is assembled into microfibrils embedded in a lignin-hemicellulose matrix in the cell walls of wood cells. The properties of these microfibrils, such as crystallinity, crystallite size and orientation, affect the mechanical properties of wood. Reaction wood is a special type of tissue generated in tree stems as a response for mechanical stresses and it has different composition and cellular and nanoscale structure compared to normal wood. The presence of reaction wood is commercially significant as its fibers can influence the chemical pulping parameters as well as workability and stability of hard timber wood. [3]

Besides offering crucial biological knowledge, information on the cell wall ultrastructure enables solving of the solubility issues of cell wall components and leads thus to more effective biomass processes in the industry. One of the most effective, non-destructive ways to acquire nanoscale information of the cellulose microfibrils of the cell walls is the x-ray scattering technique. With wide-angle x-ray scattering (WAXS), it is possible to determine nano- and atomic scale order in materials while small-angle x-ray scattering (SAXS) describes the order of tens of nanometers and above, offering thus multiscale information on the structure of biological samples.

In this study, both SAXS and WAXS techniques were applied on temperate and tropical hardwood samples to reveal properties of cellulose microfibrils and to track the presence of possible reaction wood. The measurements were conducted at a high-brilliance synchrotron in the European Synchrotron Radiation Facility in Grenoble, France at beamline BM02.

Different parameters of wood were successfully quantified with both WAXS and SAXS measurements. For example, a variety of crystallite sizes was observed in the samples and reaction wood was detected in some of the studied species based on these results.

[1] Shearman, P., Bryan, J., & Laurance, W. F. (2012). Are we approaching ‘peak timber’in the tropics?. Biological Conservation, 151(1), 17-21.
[2] Ahvenainen, P. (2018). Anatomy and mechanical properties of woods used in electric guitars. IAWA Journal, 1(aop), 1-S6.
[3] Barnett, J., Gril, J., & Saranpää, P. (2014). The Biology of Reaction Wood Introduction.


Poster: B175, March 6th, 15:30 - 17:30, 1st floor

4D MRI MODELLING OF LIVER FOR SBRT TREATMENT OF LIVER METASTASIS

H. Kavaluus1, T. Seppälä1, E. Salli2, L. Koivula1, K. Saarilahti1, J. Collan1, M. Tenhunen1

Henna.Kavaluus@hus.fi
1 HUH Cancer Center
2 HUH Medical Imaging Center


Purpose of the research was to localize liver metastasis using magnetic resonance imaging (MRI) and develop a deformable four-dimensional (4D) model of liver to presume the movement of liver metastasis for stereotactic radiotherapy (SBRT) use. The SBRT is based on delivery of a single high dose radiation fraction (≥ 6Gy) to a small internal target volume (ITV) (≤ 6cm) with steep dose gradient. The ITV consist of a clinical target volume (CTV) and an additional outer marginal which covers the motion of the target during a respiratory motion. The model improves the accuracy of marginals from CTV to ITV which improves the accuracy of the SBRT treatments of liver metastasis.
1.5T MRI was utilized with features required by radiotherapy planning for imaging a T2-weighted 4D MRI of abdomen of volunteers. The MRI parameters as imaging direction, overall imaging time and slice thickness were optimized for 4D imaging of liver.
The respiratory movement of the volunteers was observed by linear navigator echoes interleaved with the liver images. The navigator data was used retrospectively to sort the image data into multiple respiratory phases (bins). The retrospectively sorted images will be employed for deformable model of liver to predict the movement of liver metastasis.
5 mm image slices did not meet the requirements of radiotherapy use. Therefore, the slice thickness was decreased. The relative data loss was decreased due to increased overall imaging time. According to literature, it is not appropriate to increase the imaging time too much when there is a small drift of liver over time which changes continuously the position of the liver.
The number of bins affected to the amount of data loss. Generally, 10 bins are utilized when the 4D is carried out with computed tomography (CT), therefore the number of bins was 10 at first. The results showed that there occurred more data loss with 10 bins than with 8 bins.
According to results, the number of images per slice should cover at least one breathing cycle to avoid the data loss in the 4D MR image. In addition, the data loss requires either interpolating slices, increasing of imaging time or decreasing number of bins.


Poster: B176, March 6th, 15:30 - 17:30, 1st floor

THE ROLE OF MEMBRANE LIPIDS IN CELL SIGNALING

M. Creutz1, S. Paajanen1, P. Kaurola1, G. Enkavi1, W. Kulig1,2, I. Vattulainen1,2,3

santeri.e.paajanen@helsinki.fi
1 Department of Physics, University of Helsinki, P.O. Box 64, 00014 University of Helsinki, Finland
2 Computational Physics Laboratory, Tampere University, P.O. Box 692, FI-33014 Tampere, Finland
3 MEMPHYS-Center for Biomembrane Physics


Cell surface receptors translate external signals to trigger specific cellular functions within the cell. G-protein coupled receptors (GPCRs) is the largest family of cell surface receptors comprised of over 800 different receptors. They all share a conserved mechanism. Ligand binding to the receptor acts as an external signal leading to conformational changes in the receptor. These conformational changes, in turn, alter the interactions between the receptor and the guanine nucleotide-binding protein (G-protein) in the cytoplasmic side of the membrane, which relays the signal deeper into the cell.

Even though about a third of all marketed drugs target GPCRs, their signaling mechanism remain poorly understood. The activation and deactivation mechanism of GPCRs is integral to developing better-targeted and more reliable drugs. Moreover, the membrane lipid composition of the native environment of GPCRs has been suggested to modulate their function. On the other hand, the role of lipids in the process is even less known. In this study, we aim to elucidate the role of lipids on the activation and deactivation mechanism of GPCRs.

The structural characterization of GPCR members in both active and inactive conformations has allowed us to study the GPCR activation and deactivation processes in atomistic detail using molecular dynamics simulations. Earlier work performed in our group has established that cholesterol exerts an effect on the receptor binding to specific annular binding sites. In this study, we performed 20 2-3 $\mu$s long simulations of the human $\beta_2$ adrenergic receptor (B2AR), each starting with the protein in the active or inactive state embedded in membranes with various lipid compositions. In particular, we focus on the effects of the polyunsaturated lipids and their crosstalk with cholesterol in regulating the GPCR.


Poster: B177, March 6th, 15:30 - 17:30, 1st floor

INTERACTIONS OF COMMON PROTEIN GLYCANS WITH THE GM3 GANGLIOSIDE

Mykhailo Girych1, Tomasz Róg1, Iipo Vattulainen1,2

mykhailo.girych@helsinki.fi
1 University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
2 Computational Physics Laboratory, Tampere University, P.O. Box 692, 33014 Tampere, Finland


Over the past decade, membrane protein glycosylation has attracted ever-growing attention in a variety of research areas ranging from biomedicine to biotechnology [1]. Abnormal glycosylation of membrane receptors has been shown to be associated with a number of pathological conditions, including neurological diseases and cancer [2,3]. Intriguingly, the emerging data suggest that the glycan-driven modulation of receptor activity is linked to the cellular membrane interface. In particular, it was recently demonstrated that several cell receptors are modulated by the membrane glycolipid GM3 [4].

One of the prevalent hypotheses suggests that GM3 binds to specific functionally relevant termini of protein glycans and maintains their specific functional conformation at the membrane interface [5]. In experiments, it was shown that the affinity of the protein-GM3 binding strongly depends on the sequence of the protein glycans. This affinity was reported to be the highest for proteins with GlcNAc termini and much lower in cases where the glycan termini were of different type [6]. However, due to the limitations of experimental approaches, the details as to how the protein glycans interact with GM3 remain unclear.

In this work, using atomistic molecular dynamics simulations of membrane systems with typical functional protein glycan termini, we clarified the molecular-scale details of the interaction between specific protein glycan structures and GM3. The results are in very good agreement with the experimental data and show that the glycans with sialylated termini have the lowest affinity to GM3. On the other hand, protein glycans that end with the Man and GlcNac termini show a pronounced affinity for GM3. Overall, this study provides new knowledge to better understand the mechanisms used by glycans to control the functions of membrane receptors and their impairment.

Acknowledgments: This work was supported by the European Research Council (Advanced Grant 290974 CROWDED-PRO-LIPIDS), the Academy of Finland (Center of Excellence projects 272130 and 307415), and the Sigrid Juselius Foundation. CSC – IT Center for Science (Espoo, Finland) is thanked for computational recourses.

References

[1] H.-J. Gabius, The Sugar Code, WILEY-VCH Verlag Gmbh & Co. KGaA, Weinheim (2009).
[2] H. Freeze, E. Eklund, B. Ng, M. Patterson, Annu Rev Neurosci, 38, 105-125 (2015).
[3] S. Stowell, T. Ju, R. Cummings, Annu Rev Pathol, 10, 473-510 (2015).
[4] A. Ernst, B. Brügger, Biochim Biophys Acta, 1841(5), 665-670 (2014).
[5] N. Kawashima N, S. Yoon, K. Itoh, K. Nakayama, J Biol Chem, 284(10), 6147-6155 (2009).
[6] S. Yoon, K. Nakayama, N. Takahashi, H. Yagi, N. Utkina, H. Wang, K. Kato, M. Sadilek, S. Hakomori, Glycoconj J, 23, 639-49 (2006).


Poster: B181, March 6th, 15:30 - 17:30, 1st floor

LEARNING 3D-FEATURES OF MOLECULES FROM ATOMIC FORCE MICROSCOPE IMAGES USING MACHINE LEARNING

B. Alldritt1, P. Hapala1, F. Urtiev1, N. Oinonen1, O. Krejci1, F. F. Canova1,2, F. Schulz3, J. Kannala4, P. Liljeroth1, A. S. Foster1,5,6

niko.oinonen@aalto.fi
1 Department of Applied Physics, Aalto University, P.O. Box 11100, 00076 Aalto, Espoo, Finland
2 Nanolayers Research Computing Ltd ,1 Granville Court, Granville Road, London, England, N12 0HL
3 IBM Research − Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland
4 Department of Computer Science, Aalto University, P.O. Box 11100, 00076 Aalto, Espoo, Finland
5 Graduate School Materials Science in Mainz, Staudinger Weg 9, 55128, Germany
6 WPI Nano Life Science Institute (WPI-NanoLSI), Kanazawa University , Kakuma-machi, Kanazawa 920-1192, Japan


Non-contact atomic force microscopy (AFM) utilizing a functionalized carbon monoxide tip has been proven an invaluable tool in probing atomic-scale features of individual molecules [1]. However, the use of these methods outside of relatively flat geometries remains difficult, due to the probe-sample interaction easily causing distortions when imaging non-planar molecules, and the interaction being mostly limited to atoms near the surface.

In recent years, artificial neural networks have gained much attention for solving problems that involve finding complex patterns in large amounts of data [2]. In large part this is due to increased computational capacity provided by general-purpose computing on graphics processing units enabling the training of networks of increasing complexity. Particularly, convolutional neural networks have been tremendously successful in recognizing features in images with minimal preprocessing [3].

We have designed a convolutional neural network model which can extract from AFM images useful features, such as molecule height profile, and atomic positions in an easily interpretable format. The model is trained on simulated AFM images. The advantage of the simulation approach is twofold: firstly, the amount of available data compared to experiments is several orders of magnitude greater, and secondly, the desired output labels can be generated automatically, eliminating the need for costly and potentially inaccurate human labour. Our simulation software can process several dozen molecules per second, and our database of molecules consist of tens of thousands of organic molecules, relaxed using density functional theory calculations. We apply the model on both simulated and experimental data, and make comparisons.

The ultimate goal would be the ability to recognize the chemical identity of individual atoms and a partial or complete reconstruction of the 3D molecule structure from experimental AFM data. Our current work takes us closer to that goal.

[1] L. Gross, et al., "The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy", Science, 325, pp. 1110-1114, 2009.

[2] Yann LeCun, et al., "Deep learning", Nature, 521, pp. 436-444, 2015.

[3] K. He, et al., "Deep Residual Learning for Image Recognition", CVPR 2016, pp. 770-778, 2016.



Figure 1
Figure 1: Schematic of the pipeline.

Poster: B182, March 6th, 15:30 - 17:30, 1st floor

DESCRIPTOR DESIGN AND SELECTION FOR HYDROGEN EVOLUTION REACTION

Yashasvi S. Ranawat1, Marc O. J. Jager1, Eiaki V. Morooka1, Adam Foster1

1 Department of Applied Physics, Aalto University


Catalyst design is a crucial aspect of hydrogen evolution reaction (HER). It is aided by rigorous simulations of catalytic action on suitable candidates. However, this is non-trivial due to an infinite search space, and computationally intensive DFT simulations. Here, machine-learning aided approaches can be applied to augment the database of catalyst, and also select the relevant catalyst. Since computers cannot inherently understand the rotational, translational, or permutational invariance of atomic representation, various representations of chemical environment — descriptors — are introduced[1]. The study compares the descriptors: SOAP[2], an electronic description based on SOAP and local adaptation of MBTR[3], to outline their applicability in predicting adsorption-energy for HER.

1. Jäger, M. O., Morooka, E. V., Canova, F. F., Himanen, L. & Foster, A. S. Machine learning hydrogen adsorption on nanoclusters through structural descriptors. npj Comput. Mater. 4, 37 (2018).
2. Bartók, A. P., Kondor, R. & Csányi, G. On representing chemical environments. Phys. Rev. B 87, 184115 (2013).
3. Huo, H. & Rupp, M. Unified representation for machine learning of molecules and crystals. arXiv preprint arXiv:1704.06439 (2017).


Poster: B183, March 6th, 15:30 - 17:30, 1st floor

SPEEDING UP MOLECULAR DYNAMICS BY IGNORING THERMALLY EQUILIBRATED REGIONS

Christoffer Fridlund1, Risto Toijala1, Kai Nordlund1, Flyura Djurabekova1,2

christoffer.fridlund@helsinki.fi
1 Department of Physics, University of Helsinki
2 Helsinki Institute of Physics, University of Helsinki


When high energy ions or recoiling atoms in solid materials hit each other, collision cascades start developing. During these collision cascades a lot of atoms receive large amounts of energy, causing them to further collide rapidly. The cascades are typically fractal-shaped, with multiple levels of sub-cascades. Compared to the surrounding matter, regions containing cascades feature higher than average forces, energies, and temperatures. The heat from the cascade is gradually transported away by the surrounding matter.

There are two fundamentally different ways to simulate the movement of energetic ions hitting materials: molecular dynamics (MD) and binary collision approximation (BCA). MD can simulate the time evolution of nanoscale systems at pico- and nanosecond time scales, by integrating the equations of motion at small discrete time steps. The total force of every single atom is calculated from the distance to the neighbouring atoms and an interatomic potential model. BCA, on the other hand, uses statistics from the classical scattering integral when simulating the high-energy impacts. BCA interactions only involve two atoms, and all many-body terms and collisions between low energy atoms are ignored. MD simulations are orders of magnitude more computationally expensive than BCA simulations, however, can describe dynamic features totally ignored by the BCA approach.

The vast majority of computational time in MD is spent computing many-body forces between atoms. However, only a small fraction of atoms are ``interesting'' at any given time during the cascade simulation. The rest are performing equilibrium vibrations that are not affecting the results of the simulation. Here we present an MD speed-up algorithm (MDSA), that identifies and excludes ``uninteresting'' atoms from the simulation, implemented in PARCAS$^{1,2}$. The goal of the MDSA is to reduce the computational expense by introducing BCA-like approximations when the cascades are spreading, and normal MD for the cascades themselves. The uninteresting atoms are located based on the forces they experience and the total kinetic energy of their surrounding. Based on these parameters, MDSA can ``freeze'' the uninteresting atoms in place, excluding them from the computationally heavy force calculation, saving time.

Static (frozen) atoms are activated when the cascade reaches them, which is identified by the magnitude of the force they experience. Since the forces caused by frozen atoms are not computed, the activation force can only be caused by active atoms. When the cascade has cooled down in the region, the atoms will become static again. In order to keep the approximation at an acceptable level, a skin region of activated (non-frozen) atoms around the interesting atoms is required. This region functions as the heat bath, removing temperature from the hot cascade, as heat waves can not propagate through the frozen regions. The MDSA is a trade-off between computational accuracy and computational time. Freezing atoms at lower threshold values will cause the algorithm to approach the same computational accuracy as traditional MD at the cost of simulation time.

The initial tests of the MDSA have proven very successful in comparison with benchmark runs. The cascades develop in the same fashion and have similar penetration depths, at only one fourth of the normal simulation time. Defects are not recombining as well as in normal MD, but this can easily be compensated by running a shorter equilibrium simulation with normal MD after the time-consuming cascade.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688072. We thank the IT Center for Science, CSC, for granted computational resources.

[1] K. Nordlund et al., Phys. Rev. B 57, 7556 (1998)
[2] K. Nordlund et al., Nature 398, 49 (1999)


Poster: B184, March 6th, 15:30 - 17:30, 1st floor

NEW PATHS FOR FIRST-PRICIPLES METHODS

Tapio T. Rantala1

1 Physics, Tampere University, Finland


Electronic structure is the key issue behind materials properties, those of atoms, molecules and solids. The conventional approaches to this many-body problem target on finding either the wave function or the density of the many-electron system, the latter one being called density functional theory (DFT). Both of these approaches have their pros and cons, and consequently, somewhat different subfields of applications. But also, both of these approaches suffer from laborious or insufficient description of many-body effects and they are restricted to zero-Kelvin temperature and Born–Oppenheimer approximation.

Feynman path integrals (PI) provide an alternative approach to quantum theory and derivation of Schrödinger equation. Furthermore, with the PI the time evolution of the wave function or the density matrix can be simulated, and also, stationary eigenstates can be found. These approaches allow exact account of many-body effects, finite temperature and account of nuclear dynamics in numerical quantum Monte Carlo (QMC) realizations. In this presentation we demonstrate a few such PI approaches to quantum chemistry.

The PI formalism in imaginary time and Monte Carlo simulation (PIMC) [1] provides the stationary state (free energy minimum) density matrix of the many-particle system at the given finite temperature. With PIMC we have simulated a few small atoms and molecules [2], and carried out the first ab initio simulation of an equilibrium chemical reaction [3]. Recently, we have evaluated static and frequency dependent electric polarizabilities of small atoms and molecules, while introducing a new method [4].

The real time PI formalism and its numerical realization (RTPI) can be used to simulate time evolution of a system of electrons and nuclei, i.e., the time-dependent wave function, but also, the ground and excited eigenstates. This novel method has passed some test cases like Hooke's atom [5] and further testing is underway. The RTPI may also turn out to be a relief for other QMC methods suffering from the fermion sign problem (FSP).

Recently, we have demonstrated one more novel and robust method based on the real time PI formalism. This approach simulates kind of "real time diffusion" of the Monte Carlo walkers in a similar way as the good old diffusion Monte Carlo (DMC) does in imaginary time. The proof-of-concept of this tDMC will become published, soon [6].

ACKNOWLEDGEMENTS
Markku Leino, Ilkka Kylänpää, Ilkka Ruokosenmäki, Juha Tiihonen and David M. Ceperley.

References
1. D.M. Ceperley, Rev.Mod.Phys 67, 279 (1995).
2. I. Kylänpää et al., J.Chem.Phys. 133, 044312 (2010); I. Kylänpää, Phys.Rev.A 86, 052506 (2012).
3. I. Kylänpää et al., J.Chem.Phys. 135, 104310 (2011).
4. J. Tiihonen et al., Phys.Rev.A 94, 032515 (2016); J. Tiihonen et al., J.Chem.Phys. {\bf 147}, 204101 (2017); J. Tiihonen et al., J.Chem.Theor.Comput. {\bf 14}, 5750 (2018).
5. I. Ruokosenmäki et al., Comm.Comput.Phys. 18, 91 (2015); I. Ruokosenmäki et al., Comp.Phys.Comm. 210, 45 (2017).
6. I. Ruokosenmäki et al., Comm.Comput.Phys. 25, 347 (2018).


Second Floor

Time: March 6th, 15:30 - 17:30

Poster: B201, March 6th, 15:30 - 17:30, 2nd floor

ACCURATE PROJECTIVE TWO-BAND DESCRIPTION OF TOPOLOGICAL SUPERFLUIDITY IN SPIN-ORBIT-COUPLED FERMI GASES

J. Brand1, L. A. Toikka1,2,3, U. Zülicke4

lauri.toikka@gmail.com
1 Massey University, Auckland, New Zealand
2 Center for Theoretical Physics of Complex Systems, Institute for Basic Science, Daejeon, South Korea
3 University of Innsbruck, Innsbruck, Austria
4 Victoria University of Wellington, Wellington, New Zealand


The interplay of spin-orbit coupling and Zeeman splitting in ultracold Fermi gases gives rise to a topological superfluid phase in two spatial dimensions that can host exotic Majorana excitations. Theoretical models have so far been based on a four-band Bogoliubov-de Gennes formalism for the combined spin-1/2 and particle-hole degrees of freedom. Here we present a simpler, yet accurate, two-band description based on a well-controlled projection technique that provides a new platform for exploring analogies with chiral p-wave superfluidity and detailed future studies of spatially non-uniform situations [1].

[1] J. Brand, L. A. Toikka, and U. Zülicke SciPost Phys. 5, 016 (2018)


Poster: B202, March 6th, 15:30 - 17:30, 2nd floor

ROLE OF ALLOYING ELEMENTS ON THE PROPERTIES OF AN FE(1-X)AX SIGMA 3[111]/(10-1) GRAIN BOUNDARY

Assa Aravindh Sasikala Devi1, W.Cao1, M.Alatalo1, M.C.Somani2, S. Pallaspuro2, J. Kömi2, M. Huttula1

Assa.Sasikaladevi@Oulu.fi
1 Nano and Molecular Systems Research Unit, Centre for Advanced Steels Research, University of Oulu,Finland
2 Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu,Finland


The occurrence of grain boundaries (GBs) and the manifestation of impurity atoms along the GBs are known to affect the chemical and physical properties of materials in general [1]. In particular, the presence of alloying elements such as Mn and Al along with C at the GBs in steel are known to exhibit high strength combined with excellent corrosion resistance. Besides, the formation of precipitates combining C and Mn is known to occur in high Al-steel [2].  In this scenario, we studied the properties of an Fe(1-x)Ax  ∑3[111]/(10-1)GB (Fig.1) in presence of alloying elements such as C, Mn and Al employing density functional theory (DFT) as implemented in the Vienna Abinitio Simulation Package (VASP) within the GGA approximation. The optimized lattice parameter of bcc Fe, 2.85 Å, was used to construct the GB supercell with dimensions of 6.98x16.11x2.47 Å3. This supercell was subjected to optimization and the calculated formation energy of the GB, 1.79 Jm-2, was found to be in reasonable agreement with previous first-principles studies on Fe GBs [3].  Furthermore, the stability of the solute atoms at the GB was analyzed by calculating the cohesive and segregation energies, which in turn would help us to understand the preference in respect of strengthening or embrittlement of the GB. The magnetic and electronic properties have also been analyzed to gain insight into the observed properties.

References:


 



  1.        Assa Aravindh S, U. Schwingenschloegl and Iman S Roqan, Chem. Phys, 143, 224703 (2015); Assa Aravindh et al., RSC Adv., 8, 13850 (2018).

  2.       Shangping Chen, R. Rana, A. Haldar, R. K. Ray, Progress in Materials Science, 89, 345 (2017).

  3.       Wachowicz et al. Phys. Rev. B. 81, 094104 (2010).


Acknowledgement: Academy of Finland (No. 311934). CSC computing resources.



Figure 1
Figure 1:

The Fe(1-x)Ax  ∑3[111]/(10-1)grain boundary. The dashed lines serve as guide for the reader to distinguish the two grains constituting the GB.


Poster: B203, March 6th, 15:30 - 17:30, 2nd floor

CHARACTERIZATION OF CADMIUM TELLURIDE FOR NOVEL X-RAY DETECTORS IN MEDICAL APPLICATIONS

S. Kirschenmann1, E. Brücken1, A. Gäddä1, M. Kalliokoski2, V. Litichevskyi1, P. Luukka1, J. Ott1, J. Tikkanen1,3, A. Winkler4

stefanie.kirschenmann@helsinki.fi
1 Helsinki Institute of Physics
2 Ruđer Bošković Institute
3 Radiation and Nuclear Safety Authority (STUK)
4 Detection Technology Plc


Current state-of-the-art X-ray detectors, which are used e.g. for computed tomography, acquire image information by integrating the absorbed radiation along a line. Thus, they only acquire summed intensity values on this line, and the energy information of individual photons is lost. A promising alternative for a detector material is the high-Z material Cadmium Telluride (CdTe). Its photon radiation absorption properties outperform those of Silicon-based semi-conductors. Another advantage is its operability at room temperature, due to the larger energy band gap. Our novel CdTe detectors, which are processed using techniques such as surface passivation via the atomic layer deposition (ALD) method, are aimed to be operable in photon counting, “multispectral”, mode. This allows to obtain the energy information of the absorbed radiation through which the extraction of different material, radiation, or tissue types becomes possible.
The crystal growth, the characterization, as well as the processing of CdTe is substantially more complicated than e.g. for silicon. Its mechanical properties make it more fragile and large concentrations of extended crystallographic defects, such as grain boundaries, twins, and tellurium (Te) inclusions, restrict the processing temperature of CdTe to below 140$^\circ$C. Furthermore, the Te inclusions can act as charge traps for electron-hole-pairs and therefore, reduce the spectroscopic performance. A quality assessment of the material prior to the complex fabrication process is therefore deemed essential. Three-dimensional (3D) infrared (IR) microscopy is a non-obtrusive method with which the CdTe crystals can be scanned and defects categorized. This provides us with important information on the defect density and positions of Te inclusions and thus, the quality of the material. Another method of studying defects in CdTe crystals is ion beam induced current (IBIC) analysis. Improved IR scans in combination with IBIC analysis are planned and expected to give more insight on the relation between defects and charge collection efficiency.


Poster: B204, March 6th, 15:30 - 17:30, 2nd floor

HYDROGEN ISOTOPE EXCHANGE EXPERIMENTS IN TUNGSTEN WITH PRECISE DEPTH PROFILING

Tomi Vuoriheimo1, Pasi Jalkanen1, Kenichiro Mizo­hata1, Anna Liski1, Tommy Ahl­gren1, Kalle Heinola1,2, Jyrki Räisänen1

tomi.vuoriheimo@helsinki.fi
1 University of Helsinki Department of Physics, PO Box 43, FI-00014 Helsinki, Finland
2 International Atomic Energy Agency IAEA, PO Box 100, A-1400 Vienna, Austria


Future fusion reactors use a deuterium-tritium (D-T) gas mixture as hydrogen fuel for their plasma. A fraction of hydrogen species can escape the plasma confinement and hit the first wall. The retention of radioactive T in the wall materials causes both operational and safety problems and therefore must be minimized. One way to remove T from the walls is the use of hydrogen isotope exchange, an effect that replaces trapped T atoms with the lighter isotopes of hydrogen D or H. Typically in the isotope exchange experiments using D as a proxy for T makes the experiments safer and easier to do. Exposing studied samples to D plasmas or ion beams creates D trapping in the material. Subsequent exposure to H has induced the isotope effect, which has been seen as a decrease in the D amount due to replacement by H, but the underlying physics has mostly remained unresolved. Our previous work with 30 keV D-implanted tungsten (W) showed the samples being fully emptied of D after annealing at 300 ˚C in H2 atmosphere. [1] Present work continues this systematic research in understanding the physics of isotope exchange by reversing the experimental setup – by implanting H ions into W samples, and subsequently annealing in D2 atmosphere. Elastic Recoil Detection Analysis (ERDA) measured the resulted H and D concentrations.

A 500 keV particle accelerator was used to implant H into polycrystalline tungsten samples with 15 keV/ion energy and 1017 ions/cm² fluence. Annealing the H-implanted samples at 200-400 °C in D2 atmosphere of 1 bar or in vacuum produced comparable samples for the experiment. ERDA with 25 MeV chlorine beam measured depth profiles of the retained H and D simultaneously. The simultaneous measurement allowed determination of H and D depth profile concentrations to see how implanted H gets replaced by D.

Comparing the amounts of H after heating showed the isotope exchange effect clearly taking place in the temperature range of 200-400 °C. Heating the samples in D2 gas reduced the amount of the trapped H more than heating in vacuum. With isotope exchange the removal of trapped H occurred more efficiently and in lower temperatures. Vacuum heating requires much longer heating times or temperatures over 400 °C to remove all hydrogen from the sample.

The results show that the isotope exchange effect takes place regardless of the mass of the acting hydrogen isotope. This indicates that the exchange phenomenon is a statistical effect in which the abundance of the neighboring hydrogen near trapped hydrogen dictates the efficiency of the effect. T removal in future fusion reactors may take place using isotope exchange-based heating.

[1] T. Ahlgren, et al., Nucl. Fusion 59, 026019 (2019)


Poster: B205, March 6th, 15:30 - 17:30, 2nd floor

ADSORPTION AND AGGREGATION MECHANISMS IN BIOBASED OILS

Maisa Vuorte1, Sampsa Vierros1, Maria Sammalkorpi1

maisa.vuorte@aalto.fi
1 Department of Chemistry and Materials Science, Aalto University


Renewable, bio-based oils have attracted significant attention as alternative raw materials to replace conventional fossil-based crude oils in the energy, chemical, and pharmaceutical industries. Bio-based oils are dominantly mixtures of triglycerides but they also contain a number of species that act as surfactants and naturally occurring moisture (water). While surfactant aggregation and adsorption in aqueous solutions are relatively well-understood topics in soft materials physics, for surfactant assembly in apolar media major open questions remain about factors determining surfactant assembly characteristics; even the fundamental assembly mechanisms remain elusive.

Here, we focus on resolving the physical mechanisms governing adsorption and aggregation in especially bio-based oils. We employ molecular modelling techniques to predict surfactant adsorption as a function of surfactant net charge, hydrogen bonding capability, surfactant concentration, and adsorbent surface morphology. We extract trends and generalize these to assembly mechanisms and adsorption preferences. Finally, we conclude by discussing the results in terms of fundamental understanding contributions to surfactant assembly in apolar media but also for engineering reverse micellar and emulsion systems for synthesis or pharmaceutical purposes and design of more effective oil purification adsorbents.


[1] S. Vierros, M. Österberg, and M. Sammalkorpi, Aggregation response of triglyceride hydrolysis products in cyclohexane and triolein, Physical Chemistry Chemical Physics 20, 27192-27204 (2018).

[2] M. Vuorte, S. Vierros, and M. Sammalkorpi, Adsorption and aggregation characteristics of impurities in vegetable oil: a molecular modelling study, under preparation (2018).


Poster: B206, March 6th, 15:30 - 17:30, 2nd floor

EFFECT OF CARBON ON THE EVOLUTION OF EARLY-STAGE IRRADIATION DAMAGE IN COCRFEMNNI HIGH ENTROPY ALLOYS

E. Lu1, K. Mizohata2, Z. Li3, I. Makkonen1, F. Tuomisto1

eryang.lu@aalto.fi
1 Department of Applied Physics, Aalto University, P.O. Box 15100, FI-00076, Espoo Finland
2 Department of Physics, University of Helsinki, P.O. Box 43, FI-00014, Helsinki Finland
3 Max-Planck-Institut für Eisenforschung, Max-Planck-Straße 1, 40237 Düsseldorf, Germany


Due to the excellent fracture toughness, irradiation and corrosion tolerance, High-Entropy alloys (HEAs), or concentrated multicomponent alloys have been drawing much attention and have the potential to be developed as new structural materials in extreme condition. Recent research showed that carbon addition improved the strength and ductility simultaneously in face center cubic (fcc) HEAs [1, 2].
In order to understand the carbon effect on microstructure evolution in early stage irradiation damaged HEAs, 150 keV hydrogen ions were implanted in C-containing equi-atomic CoCrFeMnNi high entropy alloys at room temperature. The irradiation fluence were ranged from 2.5×1014 up to 2.5×1017 ions/cm2 and the damage dose were estimated between 0.001 dpa and 1dpa by using SRIM calculation. Doppler broadening spectroscopy based on slow positron beam [3] was utilized to characterize the radiation defects and the microstructure evolution in as-irradiated samples. The results indicate that mono-vacancies generated in samples at early stage irradiation process, and the irradiation induced point defects migrated and accumulated to vacancy clusters as the irradiation dose increasing. The addition of carbon interstitials interact with irradiation vacancies to form C-vacancy complexes, which suppress the migration and aggregation of point defects and furthermore enhanced the irradiation tolerance of CoCrFeMnNi high entropy alloys.

References:
[1] D.B. Miracle, et al., Acta Materialia, 122 (2017), 448-511.
[2] Z. Wang, et al., Acta Materialia, 120 (2016), 228-239.
[3] F. Tuomisto and I. Makkonen, Reviews of Modern Physics, 85 (2013), 1583-1631.


Poster: B207, March 6th, 15:30 - 17:30, 2nd floor

SUPER-LANDAU SUPERFLOW IN THE POLAR PHASE OF 3HE

J. Rysti1, V. B. Eltsov, G. E. Volovik, J. T. Mäkinen

1 Aalto University


According to the Landau criterion, superfluid ceases to exist if the flow velocity exceeds a critical value, which is determined by the energy spectrum of the excitations in the system. In Fermi superfluids, however, the flow velocity can exceed the Landau critical velocity. [1]

The polar phase of $^3$He is created in a system, where the liquid is confined in nafen, commercially produced nano-structured material with nearly parallel Al$_2$O$_3$ strands. It is a topological spin-triplet superfluid with a Dirac node line in the energy spectrum of Bogoliubov quasiparticles. When the flow velocity exceeds the Landau critical velocity, particle and hole Fermi pockets are formed and type-II Weyl points appear connecting them.

We use nuclear magnetic resonance (NMR) to probe the properties of the system. We have found evidence for flow velocities exceeding the Landau critical velocity using Bose-Einstein condensate of magnons as a detector. We have also measured the response of the NMR signal to the non-thermal normal component in the superfluid system when the negative energy states, which are formed by the super-critical flow, are filled.

When the superfluid moves faster than the Landau critical velocity, it offers an analog of a black hole event horizon including Hawking radiation. The Hawking radiation is emitted when the pocket with negative energy is being filled. [3]

The polar phase of $^3$He belongs to the same class of topological matter as nodal line semimetals. Topological semimetals may provide technological applications in electrical devices. [4] Helium-3 offers the possibility to study a material with similar topological properties.


[1] Bradley et al., Nature Physics 12, 1017 (2016).
[2] Dmitriev et al., Phys. Rev. Lett. 115, 165304 (2015).
[3] Volovik, JETP Lett. 104 645 (2016).
[4] Burkov, Nat. Mater. 15, 1145 (2016).



Figure 1
Figure 1: Fermi pockets and type-II Weyl points formed when the flow velocity exceeds the critical velocity.

Poster: B208, March 6th, 15:30 - 17:30, 2nd floor

GREEN PHOTODEPOSITION OF RUO2 CO-CATALYST NANOPARTICLES ONTO TIO2 SUBSTRATES

E. See1, C. Tossi1, L. Hällström1, I. Tittonen1

erich.see@aalto.fi
1 Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University


Titanium dioxide (TiO$_2$) is an attractive photocatalytic material due to its low cost, low toxicity, and general stability. Further, TiO$_2$ has a broad range of uses without modification, which include water splitting and the catalysis of various pollutants. Unfortunately, TiO$_2$ has a wide bandgap and high charge-carrier recombination rate. The addition of metallic and semiconductor co-catalysts can greatly mitigate many of these issues, and result in improvements such as lowering the activation energy of the catalysis, suppressing photocorrosion, improving charge separation, acting as nucleation sites for relevant precursors, and in the case of noble metal co-catalysts, providing plasmonic photocatalytic enhancement.
$\\$
Ruthenium(IV) oxide (RuO$_2$) has shown significant promise as a co-catalyst with TiO$_2$, due in part to its low cost and synergy with the bandgap of TiO$_2$. Combined with TiO$_2$, RuO$_2$­/TiO$_2$ catalysts have been demonstrated to promote catalyses such as water splitting, the production of hydrogen by photoreforming, the photodecomposition of organic dyes, and even industrial chlorine evolution. However, many of the methods of depositing RuO$_2$ onto TiO$_2$ surfaces involve toxic precursors (such as RuCl$_3$) or precursors that are otherwise difficult to handle.
$\\$
Herein, investigate an existing method of Ru photodeposition utilizing the less toxic potassium peruthunate (KRuO$_4$), briefly described by Mills et al. [1] involving the photodeposition of KRuO$_4$ onto a TiO$_2$ nanopowder. We expand the investigation of this photodeposition process onto TiO$_2$ films, and characterize the deposition process as a function of time. We also further explore the importance of the crystallinity of the TiO$_2$ film on the photodeposition process by comparing deposition on anatase and amorphous TiO$_2$ films. The results provide a more in-depth understanding of the photodeposition parameters and process, and provide a firm starting point for future investigation into this less-toxic method of photodeposition.$\\$
This work was done as a part of a joint project of Aalto University and VTT aiming to increase the efficiency of storing the energy from sunlight in specific fuel components. The project is funded by the Academy of Finland, Aalto University, and VTT. The present work was conducted in the Micronova Nanofabrication Center at Aalto University.
$\\$
[1] Mills, Andrew, Paul A. Duckmanton, and John Reglinski. "A simple, novel method for preparing an effective water oxidation catalyst." \(Chemical\) \(Communications\) 46.14 (2010): 2397-2398.


Poster: B209, March 6th, 15:30 - 17:30, 2nd floor

PARAMETRIC STUDY OF PLATINUM NANOPARTICLE PHOTODEPOSITION ON TITANIUM DIOXIDE

C. Tossi1, L. Hällström1, J. Selin1, M. Vaelma1, E. See1, I. Tittonen1

lassi.hallstrom@aalto.fi
1 Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University, FI-00076 Espoo, Finland


Photodeposition has been demonstrated to be a reliable method for the growth of co-catalyst nanoparticles on titanium dioxide. However, the deposition process includes a multitude of different parameters, the effects of which are only partially understood. In order to maximize the effective surface area for photocatalytic applications densely formed small particles are preferred. The presented work analyzes the effects of different parameters and identifies the most influential of them affecting the size and distribution of the deposited particles.

A statistical analysis on the effects of chosen parameters on the size and distribution of metallic platinum on titanium dioxide substrate is presented. Seven parameters were simultaneously tested utilizing an orthogonal design of experiments, which allows distinguishing the effects of individual parameters. Clear differences between different parameter sets are observed as shown in fig. 1. Software image analysis and statistical methods were then used to identify the main factors responsible of the variation.

The results further establish photodeposition as simple, viable and controllable method for the self-assembly of metal nanoparticles from liquid precursors and provide understanding on the role of process parameters such as the concentration of sacrificial reagents, the illumination power or the deposition time, in the structural properties of the deposited nanoparticles. The results determine the most important parameters affecting the size and density of the deposited particles, enabling controlled synthesis of co-catalysts for different applications.

This work was done as a part of a joint project of Aalto University and VTT aiming to increase the efficiency of storing the energy from sunlight in specific fuel components. The project is funded by the Academy of Finland, Aalto University, and VTT. The work was conducted in the Micronova Nanofabrication Center at Aalto University.



Figure 1
Figure 1: SEM images of two samples prepared with different sets of parameters. The image on the left shows considerably larger amount of deposited platinum nanoparticles than the one on the right.

Poster: B210, March 6th, 15:30 - 17:30, 2nd floor

CLASSIFICATION OF VACUUM ARC BREAKDOWNS IN A PULSED DC SYSTEM

Anton Saressalo1,2, Iaroslava Profatilova3, Walter Wuensch3, Flyura Djurabekova1,2

anton.saressalo@helsinki.fi
1 University of Helsinki
2 Helsinki Institute of Physics
3 CERN


Understanding the microscopical phenomena behind vacuum arc generation is crucial for being able to control the breakdown rate, thus improving the efficiency of many high-voltage applications where breakdown generation is a limiting factor.

Statistical properties, such as pulses between breakdowns, breakdown locations, waveforms and crater shapes are studied with almost identical pulsed DC systems in Helsinki and in CERN. In the systems, copper electrodes separated by a 20-100 μm gap, are placed in near ultra high vacuum. Electric field up to 100 MV/m is pulsed across the gap, resulting in electric discharges - breakdowns.

Statistics are collected over thousands of breakdown events with varying parameters such as electric field strength, pulse length and pulsing frequency. The statistical analysis is compared to the to the breakdown-induced features on the electrode surfaces which are imaged using various imaging and surface analysis techniques.

The resulting statistics are used to classify the breakdown events in order to understand the underlying processes leading to them.


Poster: B211, March 6th, 15:30 - 17:30, 2nd floor

DEVELOPMENTS ON REAL-SPACE APPROACHES IN ELECTRONIC STRUCTURE THEORY

Susi Lehtola1

susi.lehtola@helsinki.fi
1 Department of Chemistry, University of Helsinki, Finland


Real-space approaches allow for electronic structure calculations at an arbitrary level of accuracy, in contrast to the commonly used linear combination of atomic orbitals (LCAO) approach that is in general unreliable. The need for real-space approaches is especially pressing for applications that involve extreme environments, as these may break the fundamental assumptions made in the LCAO approach.

In addition to reproducing benchmark quality numbers, real-space approaches can also be used to fashion more accurate initial guesses for self-consistent field LCAO calculations [1]. However, the real-space guesses discussed in [1] should yield even better results when used in real-space or plane-wave calculations on large molecules or crystals.

I will discuss new finite element approaches for atoms [2] and diatomic molecules [3] in the HelFEM program [4] that can be used to obtain results at sub-microhartree accuracy at the Hartree-Fock or density functional levels of theory.

The approach used in HelFEM affords stable convergence, allowing calculations even for systems in extreme environments. In addition to strong electric fields [2,3], atoms and diatomic molecules can also be calculated in extremely strong magnetic fields [5], found e.g. in the atmospheres of white dwarfs and magnetars, which change not only the ground state geometry of a molecule, but also have significant effects on its ground spin state.

HelFEM can even handle uniquely challenging geometries i.e. extremely small internuclear distances $R \ll 1$ Å; such cases are encountered in the calculation of repulsive potentials for e.g. the calculation of range parameters for irradiation processes of practical interest [6].


[1] S. Lehtola, J. Chem. Theory Comput., in press (2019). DOI: 10.1021/acs.jctc.8b01089. arXiv:1810.11659.
[2] S. Lehtola, submitted. arXiv:1810.11651.
[3] S. Lehtola, submitted. arXiv:1810.11653.
[4] S. Lehtola, HelFEM -- Finite element methods for electronic structure calculations on small systems. https://github.com/susilehtola/HelFEM
[5] S. Lehtola, M. Dimitrova, and D. Sundholm, submitted. arXiv:1812.06274.
[6] To be published.


Poster: B212, March 6th, 15:30 - 17:30, 2nd floor

CHARACTERIZATION OF VMCO RESISTIVE SWITCHING MEMORY DEVICES WITH POSITRON ANNIHILATION SPECTROSCOPY (PAS)

Afrina Khanam1, Jonatan Slotte1

afrina.khanam@aalto.fi
1 Department of Applied Physics, Aalto University, P.O. Box 15100, FI-00076, Aalto, Finland


Vacancy Modulated Conductive Oxide (VMCO) is a low switching current device and a very promising candidate for high-density memory applications, in which the conductance of a tunnel barrier is electrically modulated to store the memory states [1]. VMCO devices include gradual set/ reset operations, switching currents in the range of a few μA. These devices are compatible with ON- and OFF- state resistances along with indicative of a non-filamentary switching mechanism as well.

VMCO-like sample epilayers consisted of a TiN layer (10 nm), an amorphous-Si layer (8 nm), and a 15 nm layer of TiO2 (anatase), grown at different temperatures and a top TiN layer (10 nm). A 50 nm thick TiO2 layer was used as a reference. The TiO2 layer plays the role of a switching layer in the VMCO stack [2]. The positron annihilation spectroscopy (PAS) technique, in Doppler and coincidence Doppler mode, was utilized for sample characterization. The PAS technique is based on the detection of the radiation created in the annihilation of positron-electron pairs and is a useful tool for studying open volume defects in solids.

In the Doppler experiment, the momentum of the annihilating electron-positron pair is detected as broadening of the 511 keV annihilation line. The low momentum parameter S, is the fraction of counts in the central region of the annihilation line. Fig. 1 shows the S-parameter as a function of positron implantation energy. The S-parameter minimum at approximately 1.2 keV coincides with the positron implantation profile centered in the TiO2 layer. Slight differences in the S-parameter can be observed for the TiO2 layers grown at different temperatures. This is an indication of differences in the amount of open volume defects in the layers. In order to deepen understanding of the coincidence Doppler spectra, electronic structure calculations (DFT) of different vacancy clusters in TiO2 are planned.


Acknowledgement:

We thank Subhali Subhechha, Ludovic Goux, Mihaela Ioana Popovici, Attilio Belmonte, Valeri Afanasiev, Gouri Sankar Kar from Imec vzw, kapeldreef 75, 3001 Leuven, Belgium for sample preparation.

Citations:

[1] H.-S. Philip Wong, Heng-Yuan Lee, Shimeng Yu, Yu-Sheng Chen, Yi Wu, Pang-Shiu Chen, Byoungil Lee, Frederick T. Chen, Ming-Jinn Tsai, Metal–Oxide RRAM, (Proceedings of the IEEE, June 2012).

[2] Subhali Subhechha, Bogdan Govoreanu, Yangyin Chen, Sergiu Clima, Kristin De Meyer, Jan Van Houdt, and Malgorzata Jurczak, Extensive reliability investigation of a-VMCO nonfilamentary RRAM: Relaxation, retention and key differences to filamentary switching , DOI: 10.1109/IRPS.2016.7574568



Figure 1
Figure 1: S vs E plot for sample epilayers grown at different temperatures

Poster: B213, March 6th, 15:30 - 17:30, 2nd floor

SUPERFLUID WEIGHT OF TWISTED BILAYER GRAPHENE SYSTEMS

A. Julku1, L. Liang, P. Törmä

aleksi.julku@aalto.fi
1 Department of Applied Physics, Aalto University School of Science


Twisted bilayer graphene (TBG) has recently gained widespread attention due to its intriguing phase diagram which consists of strongly correlated insulating and superfluid domains in proximity of each other [1, 2]. These phases appear when two graphene sheets are twisted relative to each other by the so-called magic angles. When two graphene layers are twisted, their interlayer coupling and thus single-particle dispersions are modified. At magic angles nearly dispersionless flat bands appear and, due to reduced kinetic energy, interaction processes play more prominent role than in case of dispersive bands. Emergence of flat bands are likely the reason for the superconductivity and strongly correlated insulating phases discovered in TBG systems.

Interesting theoretically open questions of TBG superconductivity are related to the interaction mechanism between electrons that ultimately leads to the formation of Cooper pairs and thus superfluidity. The nature of the underlying interaction processes in TBG system remains, however, still poorly characterized. As the phase diagram of TBG possesses similar domains than high temperature superconductors [3], understanding TBG superconductivity could potentially shed light to currently unanswered questions of physical phenomena of high temperature superconductivity in general.

In this work we study theoretically the superfluidity of TBG systems by computing the superfluid weight $D^s$ which is responsible for the fundamental properties of superconductors, namely the dissipationless electric current and the Meissner effect [4,5]. We choose the interaction potential to be attractive and non-local in real space - in earlier theoretical studies such pairing interaction has been argued to emerge in TBG [6,7] and in single layer graphene [8]. We study for which particle densities and interaction strengths the superfludity emerges in TBG system. Furthermore, based on the results for $D^s$, we are able to compute the so-called Berezinskii-Kosterlitz-Thouless transition temperature and find that it is very close to the experimental values of around $1$ Kelvin [1]. This indicates that our choice for the interaction mechanism is feasible and that our results could be of practical importance in understanding the superconcuting phases of TBG systems.

[1] Y. Cao et al., Nature 556, 4350 (2018)
[2] Y. Cao et al., Nature 556, 8084 (2018)
[3] P. A. Lee et al., Rev. Mod. Phys. 78, 17 (2016)
[4] S. Peotta et al., Nat. Comm. 6, 8944 (2015)
[5] A. Julku et al., Phys. Rev. Lett. 117, 045303 (2016)
[6] Y. Su et al. Phys. Rev. B 98, 195101 (2018)
[7] F. Wu et al. arXiv:1805.08735 (2018)
[8] M. L. Kiesel et al., Phys. Rev. B 86, 020507 (2012)


Poster: B221, March 6th, 15:30 - 17:30, 2nd floor

PHASE FIELD CRYSTAL MODELING OF GRAPHENE/HEXAGONAL BORON NITRIDE INTERFACES

P. Hirvonen1, S. S. Channe1, T. Ala-Nissilä1,2

shrikant.channe@aalto.fi
1 Department of Applied Physics and QTF Center of Excellence, Aalto University, School of Science, P.O. Box 11000, FI-00076 Aalto, Espoo, Finland
2 Interdisciplinary Centre for Mathematical Modelling and Department of Mathematical Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK


Two-dimensional materials such as graphene and hexagonal boron nitride (h-BN) are an important class of materials that have enhanced structural and electronic properties in comparison to their bulk counterparts. However, the limited length and time scales of the traditional modeling methods, such as the molecular dynamics (MD) and the quantum mechanical density functional theory (QMDFT) methods poses a severe challenge to study the underlying mechanism of various properties of these materials and their heterostructures. The phase field crystal (PFC) model [1, 2] can reach diffusive time scales (relevant e.g. in nucleation and growth of crystallites [3], relaxation of strain-driven 2D monolayers [4], and thermal conduction [5]) that are much larger in comparison to MD and QMDFT methods while retaining atomic resolution. The model also incorporates an atomic length scale and elastic and plastic deformations in a natural manner [1, 2]. Various PFC models have been used to study topological defects, such as pentagon-heptagon (5|7) defects and inversion grain boundaries formed in the graphene [6] and h-BN [7] monolayers, respectively. In this work, we generalize the one-mode PFC model [8] to study the formation of the topological defects at the interface of in-plane graphene/h-BN heterostructures. Likewise, we also use the model to determine the equilibrium shape of crystal of h-BN embedded in a graphene monolayer.

References:
[1] K. R. Elder $\textit{et al.}$, Phys. Rev. Lett. $\textbf{88}$, 245701 (2002).
[2] K. R. Elder $\textit{et al.}$, Phys. Rev. E. $\textbf{70}$, 051605 (2004).
[3] R. Backofen $\textit{et al.}$, J. Phys. Condens. Matter. $\textbf{21}$, 464109 (2009).
[4] K. R. Elder $\textit{et al.}$, Phys. Rev. B $\textbf{88}$, 075423 (2013).
[5] H. Dong $\textit{et al.}$, Phys. Chem. Chem. Phys. $\textbf{20}$, 24602 (2018).
[6] P. Hirvonen $\textit{et al.}$, Phys. Rev. B $\textbf{94}$, 035414 (2016).
[7] D. Taha $\textit{et al.}$, Phys. Rev. Lett. $\textbf{118}$, 255501 (2017).
[8] P. Hirvonen $\textit{et al.}$ (unpublished).


Poster: B222, March 6th, 15:30 - 17:30, 2nd floor

THERMAL RUNAWAY OF METAL NANO-TIPS DURING INTENSE ELECTRON EMISSION

A. Kyritsakis1, M. Veske1, K. Eimre2, V. Zadin2, F. Djurabekova1

andreas.kyritsakis@helsinki.fi
1 University of Helsinki, Helsinki Institute of Physics
2 Intelligent Materials and Systems Lab, Institute of Technology, University of Tartu


When an electron emitting tip is subjected to very high electric fields, plasma forms even under ultra high vacuum conditions. This phenomenon, known as vacuum arc, causes catastrophic surface modifications and constitutes a major limiting factor in a wide range of devices that involve metal surfaces exposed to high electric fields. Such devices span from micro- and nano-electronic capacitors, macroscopic high-voltage devices such as vacuum interrupters and X-ray tubes, up to large-scale apparatuses such as fusion reactors and particle accelerators.
Although vacuum arcs have been studied thoroughly and it is widely accepted that are initiated at electron emitting spots, the physical mechanisms that lead from intense electron emission to plasma ignition are still unclear.

In this work we give insights to the atomic scale processes taking place in metal nanotips under intense field emission conditions. We use multi-scale multi-physics simulations that concurrently incorporate various effects into classical molecular dynamics. Such effects include field-induced forces, electron emission with finite-size and space-charge effects, Nottingham and Joule heating.

Figure 1 shows the evolution of a Cu nano-tip of 93nm height and 3nm cap radius, as obtained from our calculations. The tip was subjected to an applied far field of 0.8GV/m. Our results showed that the emission-generated heat partially melts its top, with the field-induced forces elongating and sharpening it.
This initiates a positive feedback thermal runaway process, which eventually causes evaporation of large fractions of the tip.

The average evaporation rate we obtain is in agreement with the minimum neutral evaporation required to ignite plasma, as found by previous Particle-In-Cell calculations.
This initial neutral evaporation is a key process required to explain the plamsa onset of vacuum arc. Therefore, the runaway mechanism found here, which gives a plausible explanation to this evaporation process, is a crucial step towards understanding the physics behind the vacuum arc ignition.



Figure 1
Figure 1: Evolution of the tip as obtained from our simulations. The red curve (left axis) shows the evolution of its total height over time. The black curve (left axis) shows the corresponding evaporation rate. The inset images show snapshopts of the atomistic shape of the tip and its temperature distribution (color coding) in the 7 crucial stages, designated on the graphs by the corresponding letters.

Poster: B223, March 6th, 15:30 - 17:30, 2nd floor

CLUSTER TEMPERATURES IN NON-ISOTHERMAL VAPOR-LIQUID NUCLEATION

V. Tikkanen1, R. Halonen, B. Reischl, H. Vehkamäki

1 Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Finland.


Keywords: Non-isothermal nucleation, molecular dynamics simulations.

During a vapor-liquid phase transition, latent heat release during cluster growth can significantly increase the temperature of post-critical clusters above the average temperature of the surroundings. The original work describing the framework of this non-isothermal nucleation (Feder et al., 1966) also predicted cold sub-critical clusters below the bath temperature. Whether these cold sub-critical clusters exist, or not, has been the subject of debate ever since.

We performed molecular dynamics (MD) simulations using the LAMMPS code (Plimpton, 1995) to study the non-isothermal vapor-liquid nucleation of supersaturated Lennard-Jones fluid and determine average temperatures of clusters of different sizes. The mean first-passage time method (Wedekind et al., 2007) was used to study the growth of an individual cluster in a small simulation box in many uncorrelated simulations. Temperature control was achieved by using carrier gas to remove latent heat via non-sticking collisions. Three different carrier gas to condensable particles ratios (1, 10 and 100) were used in order to test the effect of thermalization on the temperature of the growing clusters. The cluster temperatures were calculated by considering the mean kinetic energy per atom in each cluster.

Poor thermalization increases the temperature of growing post-critical clusters significantly, compared to the bath temperature. The highest carrier gas to condensable particles ratio of 100 almost completely eliminated this effect in our simulations. Moreover, sub-critical clusters below the bath temperature were observed.

Feder, J., Russell, K., Lothe, J., and Pound, G. (1966). Homogeneous nucleation and growth of droplets in vapours. Adv. Phys., 15(57):111–178.
Plimpton, S. (1995). Fast parallel algorithms for short-range molecular dynamics. J. Comput. Phys., 117(1):1–19.
Wedekind, J., Strey, R., and Reguera, D. (2007). New method to analyze simulations of activated processes. J. Chem. Phys., 126(13):134103.


Poster: B224, March 6th, 15:30 - 17:30, 2nd floor

DESIGN AND FABRICATION OF NANOSCALE PHONONIC CRYSTAL MEMBRANES FOR LOW TEMPERATURE DETECTOR APPLICATIONS

T. A. Puurtinen1, T. Loippo1, K. Rostem2, P. J. de Visser3, I. J. Maasilta1

tuomas.a.puurtinen@jyu.fi
1 Nanoscience Center, Department of Physics, University of Jyväskylä, 40014, Finland
2 NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20881, USA
3 SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands


Nanoscale phononic crystals (PnC) are promising components for several low temperature detector devices, such as bolometers, transition edge sensors and kinetic inductance detectors (KID). Recent calculations and measurements demonstrate a wide range of tunability for thermal properties of PnCs with correct choice of the PnC geometry. [1-2] Low temperature thermal applications of PnCs rely on modifications in the phonon band structure, which affects density of states and velocity of heat carrying phonons. These changes can either reduce or improve thermal conductance, which both can be taken advantage of in low temperature detector design.

For instance, reducing thermal conductance and improving heat capacity is important in optimizing responsivity of bolometric detectors. Furthermore, PnCs could also be used to improve sensitivity of KIDs by restricting escape of the quasiparticle recombination phonons, thus increasing quasiparticle lifetime in the superconductor. [3] While acting as a high frequency notch filter tuned to the specific superconducting energy gap, it would be advantageous if the PnC would improve thermal conductance between the superconductor and thermal bath to reduce the negative effect of readout heating of the KID. [4]

In this work, we discuss theoretical and experimental design process of a PnC that acts as a notch filter for the recombination phonons. We develop a phonon scattering model based on scattering of elastic waves from the PnC, and use it to estimate phonon escape rates in several PnC design candidates. We also explore neon ion-beam milling process for fabrication of nanoscale PnC structures on thin SiN membranes, and measure their geometric features so that good correspondence with the model and samples are obtained.

Our calculations show that obtaining a wide band gap at ~35 GHz is possible, and such PnC samples can be produced with novel nanofabrication technologies. Full band gap leads to high reflection probability for the recombination phonons, and overall, these modifications do not have significant effect on the thermal conductance at the operation temperature of the device. We also demonstrate that a full band gap is not compulsory, and high reflection levels can be obtained with less extensive modifications to the spectrum by combining several matched PnCs in series (see Fig. 1).

References:
[1] N. Zen, et al. Nat. Commun. 5, 3435 (2014).
[2] T. A. Puurtinen and I. J. Maasilta, AIP Adv. 6, 121902 (2016).
[3] K. Rostem, P. J. de Visser and E. J. Wollack, Phys. Rev. B 98, 014522 (2018).
[4] P. J. de Visser, S. Withington and D. J. Goldie, J. Appl. Phys. 108 (11) (2010).



Figure 1
Figure 1: A matched pair of phononic crystal structures on a SiN membrane, that generate a reflection band at $\sim 32$ GHz.

Poster: B225, March 6th, 15:30 - 17:30, 2nd floor

ADSORPTION OF WATER ON MANGANESE AND SILVER DOPED TITANIUM DIOXIDE

T. Rytkönen1, O. Mansikkamäki2, S. Posysaev2, O. Miroshnichenko2, M. Alatalo2, R. Keiski1

olli.mansikkamaki@oulu.fi
1 Environmental and Chemical Engineering, University of Oulu
2 Nano- and Molecular Systems Research Unit, University of Oulu


Many of the photocatalytic applications of titanium dioxide happen in an aqueous environment, and hydroxyl groups tend to play an important role in the reactions. Therefore, the behaviour of water, and particularly the likelihood of water molecules splitting, on the surfaces of titanium dioxide has been extensively studied for decades. One way of dealing with the inconveniently large bandgap of titanium dioxide is doping it with transition metals, such as manganese or silver. We have performed density functional theory (DFT) based calculations relating to the adsorption of water on manganese doped rutile (110) and silver doped anatase (101) surfaces, and computed the reaction barriers for the splitting of water using the climbing image nudged elastic band method (CI-NEB). The computational results are compared to diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of doped and non-doped titanium dioxide samples under synthetic air and humid nitrogen gas flows. The computational and experimental results both indicate that doping rutile with manganese increases its hydrophilicity and results in an increased number of adsorbed hydroxyl groups. The DRIFTS spectra for the silver doped samples show an increase in hydrophilicity as well, but only a small difference in the presence of dissociated water compared to the non-doped sample.


Poster: B231, March 6th, 15:30 - 17:30, 2nd floor

DIELECTRIC NANOPARTICLE ARRAYS – TUNING THE LIGHT-MATTER INTERACTION

R. Heilmann1, A. I. Väkeväinen1, P. Törmä1

1 Department of Applied Physics, Aalto University, Finland


Nanoparticles interacting with light provide possibilities to enhance the light-matter interaction due to high local field confinement and sensitivity to their local environment. If nanoparticles are arranged in periodic lattices and interact with light, they form the so-called surface lattice resonances (SLRs). The SLRs are combinations of the single particle resonances of the nanoparticles and the diffracted orders (DOs) caused by the periodic structures. These SLRs can couple to emitters, e.g. dye molecules and even be in the strong coupling regime.

Typically, nanoparticle arrays consist of metals which, however, suffer from very high ohmic losses and heating. In contrast, high-index dielectrics provide a possibility to overcome these losses due to their low-dissipative nature while still providing strong scattering. This can lead to narrower SLRs and therefore to longer lifetimes.

We study the SLRs of dielectric nanoparticle arrays made out of germanium and amorphous silicon as well as the strong coupling between the nanoparticle arrays and dye molecules. For this, we measure the transmission through the arrays with dye molecules of different concentrations on top. We observe a Rabi splitting, which increases linearly with the square root of the concentration of the dye molecules. Because this linear dependence is a property of a system being in the strong coupling regime, we observe an indication of strong coupling. Since the SLRs of dielectric nanoparticle arrays provide longer lifetimes and are able to strongly couple with dye molecules, this can provide new developments and improvements in lasing applications, e.g. towards lower thresholds.



Figure 1
Figure 1: Dispersion measurements of an a-Si nanoarray with dye concentrations of a) 0 b) 100 and c) 200 mM. a): The dashed lines depict the uncoupled lattice DOs and the solid lines the SLRs obtained by a coupled mode fitting of the extinction maxima (dots). b) and c): The horizontal dashed line depicts the dye absorption line, the diagonal one the fitted SLRs from a). The solid lines depict the coupled mode fitting of the extinction maxima. The Rabi splitting increases with increasing dye concentration.

Poster: B232, March 6th, 15:30 - 17:30, 2nd floor

ENHANCED NONLINEAR OPTICAL PROCESSES IN PLASMONIC METASURFACES USING SURFACE LATTICE RESONANCES

T. Stolt1, M. J. Huttunen1, P. Rasekh2, A. Kiviniemi1, M. Kauranen1, R. W. Boyd2,3,4, K. Dolgaleva2,3

timo.stolt@tuni.fi
1 Laboratory of Photonics, Tampere University, Korkeakoulunkatu 3, FI-33720 Tampere, Finland
2 School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
3 Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
4 The Institute of Optics and Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA


Development of coherent and intense light sources is an important application of nonlinear optics. An everyday challenge in nonlinear optics is that material nonlinearities are intrinsically very weak, necessitating the use of strong excitation fields. Alternatively, use of nonlinear fibers, waveguides or resonators can result in efficient nonlinear processes due to coherent build of the up signal over long interaction lengths [1]. Recently, nonlinear plasmonic metasurfaces and metamaterials have shown potential for enabling nanoscale nonlinear processes [2], such as generation of terahertz (THz) radiation [3]. Despite such progress, conversion efficiencies of nonlinear metasurfaces have not yet rivalled traditional nonlinear materials [4]. Here, we numerically demonstrate how nonlinear responses of metasurfaces can be dramatically enhanced by utilizing collective responses of periodic lattices, known as surface lattice resonances (SLRs). We apply nonlinear discrete-dipole approximation (NDDA) approach [4] to predict the nonlinear response of metasurface consisting of periodically arranged gold nanoprisms. Particularly, we design an array to enhance the process of difference-frequency generation (DFG) resulting in THz signal generation.

The designed THz-emitting metasurface is illustrated in Fig. 1(a). A square lattice of equilateral triangular nanoprisms (thickness \(h = 25 nm\) and width \(w = 60 nm\)) with a lattice constant \(p_x = p_y = 526 nm\) was excited with two normally incident x-polarized plane waves oscillating at wavelengths \(\lambda_1\) and \(\lambda_2\). The simulated DFG dipole moment amplitudes and the calculated spectral amplitude of the THz signal are shown in Figs. 1(b) and 1(c), respectively. An over 2400-fold enhancement of the DFG dipole moment close to the SLR was predicted. We estimate an up to 1000-fold enhancement of the THz field amplitude per particle in comparison with the response of an individual nanoprism.

\(\textbf{References}\)
[1] S. Saeidi, P. Rasekh, K. M. Awan, A. Tüğen, M. J. Huttunen, and K. Dolgaleva, "Demonstration of Optical Nonlinearity in InGaAsP/InP Passive Waveguides," Opt. Mater., 84, 524–530 (2018).
[2] M. Kauranen and A. Zayats, "Nonlinear Plasmonics," Nat. Photonics, 6, 737–748 (2012).
[3] L.Luo, I. Chatzakis, J. Wang, F. B. P. Niesler, M. Wegener, T. Koschny and C. M. Soukoulis, “Broadband terahertz generation from metamaterials," Nat. Commun., 5, 3055 (2014)
[4] M. Nezmi, D. Yoo, G. Hajisalem, "Gap Plasmon Enhanced Metasurface Third-Harmonic Generation in Transmission Geometry," ACS Photonics, 3, 1461–1467 (2016).
[5] M. J. Huttunen, P. Rasekh, R. W. Boyd, and K. Dolgaleva, "Using surface lattice resonances to engineer nonlinear optical processes in metal nanoparticle arrays," Phys. Rev. A, 97, 053817 (2018).



Figure 1
Figure 1: (a) Metasurface consisting of equilateral triangular nanoprisms with periods \(p_x = p_y = 526 nm\), embedded in medium with refractive index n = 1.5. (b) Incident fields (\(\lambda_1\approx\lambda_2\approx800 nm\)) result in an up to 2400-fold enhancement in the DFG dipole moment. (c) Calculated THz spectral amplitude reveals a broadband response ranging from 0.1 to 6 THz with an enhancement factor of 30–1000 per particle in comparison to the response of an individual particle.

Poster: B233, March 6th, 15:30 - 17:30, 2nd floor

TAILORING VERTICAL-EXTERNAL-CAVITY SURFACE-EMITTING LASERS FOR QUANTUM TECHNOLOGY APPLICATIONS

M. Mäki1,2, J-P. Penttinen1,2, E. Kantola1,2, S. Ranta1,2, M. Guina1,2

mika.maki@tuni.fi
1 Optoelectronics Research Centre, Physics Unit, Tampere University, Korkeakoulunkatu 3, 33720 Tampere, Finland
2 Vexlum Ltd, Korkeakoulunkatu 3, 33720 Tampere, Finland


Many quantum technology applications such as quantum information processing, atomic clocks and quantum sensing rely on lasers at many different wavelengths with demanding characteristics such as high power, narrow linewidth, excellent beam and low intensity noise. The lasers are typically used to detect or change the atomic states of neutral atoms and ions, and the availability of suitable inexpensive lasers has been slowing the advances in this field.

Vertical-External-Cavity Surface-Emitting Lasers (VECSELs) are a relatively new type of optically pumped semiconductor lasers with wide wavelength coverage and good adjustability of the laser properties, and are expected to address this need. The external cavity geometry of a VECSEL enables single-frequency operation with diffraction-limited output beam and a tuning range of tens of nanometers. Direct emission between 390 nm and 5 µm is enabled by material engineering of the semiconductor gain mirror. The wavelength coverage can be further expanded with efficient intracavity and/or external frequency conversion. [1]

We present VECSEL systems tailored for quantum technology applications. These lasers are sealed in compact packages for long-term stable operation (see the photo below), and require only electrical and cooling water inputs. Recent examples of applications include the generation and manipulation of trapped magnesium ions for quantum computing using the wavelength of 279.6 nm for Doppler cooling and 285.3 nm for photoionization. [2] Our current focus is in tailoring the VECSELs for similar use with several other ions such as beryllium, for which the corresponding wavelengths are 313 nm and 235 nm.

[1] M. Guina, A. Rantamäki, and A. Härkönen, “Optically pumped VECSELs: review of technology and progress,” Journal of Physics D: Applied Physics, vol. 50, no. 38, p. 383001, Sep. 2017.
[2] S. C. Burd et al., “VECSEL systems for the generation and manipulation of trapped magnesium ions,” Optica, vol. 3, no. 12, p. 1294, Dec. 2016.



Figure 1
Figure 1: VALO SF, a single-frequency VECSEL system developed by our research group, and example characteristics of a unit built for the repumping of Mg+ ions.

Poster: B234, March 6th, 15:30 - 17:30, 2nd floor

EFFICIENT NONLINEAR METASURFACES USING MULTIRESONANT HIGH-Q PLASMONIC ARRAYS

M. J. Huttunen1, O. Reshef2, T. Stolt1, K. Dolgaleva2,3, R. W. Boyd2,3,4, M. Kauranen1

mikko.huttunen@tuni.fi
1 Photonics Laboratory, Physics Unit, Tampere University, Tampere, Finland
2 Department of Physics, University of Ottawa, Ottawa, Canada
3 School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa, Canada
4 The Institute of Optics and Department of Physics and Astronomy, University of Rochester, Rochester, USA


Nonlinear optical processes are important in many fields of photonics, ranging from biomedical imaging to ultrafast spectroscopy [1]. Due to recent progress in nanophotonics and metamaterials, there is a growing demand for smaller and more efficient nonlinear optical components. Unfortunately, it is very challenging to answer this demand by using traditional materials, motivating the search for alternative approaches. Nonlinear plasmonics has recently emerged as a potential solution for enabling more efficient nanoscale nonlinear optics [2]. However, it is not yet clear how nonlinear responses of metamaterials can be enhanced sufficiently to enable practical nonlinear applications. Recent work to solve this issue has focused on two enhancement strategies, which are both based on resonance engineering. First, nanoparticle arrays associated with collective and narrow plasmon resonances with relatively high quality factors (Q-factors) known as surface lattice resonances (SLRs) have been utilized [3–5]. Second, multiply-resonant nanostructures have been investigated where the resonance enhancement occurring both at the input and output wavelengths results in strong nonlinear response [6]. Here, we combine these two concepts to enable even higher overall enhancement.

We numerically study the enhancement of second-harmonic generation (SHG) in multiply-resonant plasmonic metasurfaces consisting of L-shaped aluminium nanoparticles (see Fig. 1a). The array is designed to support two simultaneous high-Q SLRs, termed as SLR$_\omega$ and SLR$_{2\omega}$, peaking at the fundamental and SHG wavelengths, respectively. The Q-factors of SLR$_\omega$ and SLR$_{2\omega}$ were estimated to be 800 and 250, respectively (see Fig. 1b). We performed SHG simulations using an approach based on the nonlinear discrete-dipole approximation [3], and predict an enhancement of over six orders-of-magnitude of the emitted SHG intensity at doubly-resonant conditions (see Fig. 1c).

To conclude, we have numerically demonstrated how nonlinear responses of plasmonic metasurfaces can be enhanced by utilizing periodic arrays supporting multiply-resonant surface lattice resonances with high quality factors. Our results open new avenues for using nonlinear metasurfaces in realistic nonlinear applications, such as in frequency conversion.

References
[1] R. W. Boyd, Nonlinear optics (Academic Press, San Diego, 2003).
[2] M. Kauranen and A. Zayats, "Nonlinear plasmonics," Nat. Photonics, 6, 737–748 (2012).
[3] M. J. Huttunen, P. Rasekh, R. W. Boyd, and K. Dolgaleva, "Using surface lattice resonances to engineer nonlinear optical processes in metal nanoparticle arrays," Phys. Rev. A, 97, 053817 (2018).
[4] R. Czaplicki et al., "Less is more: Enhancement of second-harmonic generation from metasurfaces by reduced nanoparticle density," Nano Lett., 18, 7709–7714 (2018).
[5] L. Michaeli et al., "Nonlinear surface lattice resonance in plasmonic nanoparticle arrays," Phys. Rev. Lett., 118, 243904 (2017).
[6] M. Celebrano et al., "Mode matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation," Nat. Nanotechnol., 10, 412–417 (2015).



Figure 1
Figure 1: a) Multiresonant metasurface consisting of L-shaped aluminium nanoparticles arranged into an array with periods $p_x$ = 395 nm and $p_y$ = 793 nm. b) Two SLRs peaking at the SHG and fundamental wavelengths are visible in the simulated transmission spectra. c) An over million-fold enhancement of SHG emission intensity is simulated for the doubly-resonant array (black solid line), whereas decreased SHG emission occurs for singly-resonant cases (red and blue dotted lines).

Poster: B235, March 6th, 15:30 - 17:30, 2nd floor

THREE-DIMENSIONAL MICROSCOPIC MAPPING OF LONGITUDINAL ELECTRIC FIELDS OF A SPATIALLY PHASE-SHAPED HIGHER-ORDER BEAM

L. Kallioniemi1, L. Turquet1, X. Zang1, H. Lipsanen2, G. Bautista1, M. Kauranen1

leevi.kallioniemi@tuni.fi
1 Photonic Laboratory, Physics Unit, Tampere University
2 Department of Electronics and Nanoengineering, Aalto University


There has been much interest about the creation and application of spatially phase-shaped optical beams. To date, such beams have found extensive use in a wide range of disciplines including optical trapping, microscopy, nanofabrication, nonlinear optics, quantum optics, information processing, and so on. More importantly, the tight focusing and manipulation of such beams are expected to unravel new ways to control light‐matter interactions in the focal volume. To control precisely the spatial distribution of these beams at the focus, it is essential to develop optical techniques to verify their three-dimensional (3D) spatial distribution. So far, this capability is hindered by the shortage of techniques to probe directly and reliably these beams or their particular field components in three dimensions. In this work, we present a general approach to visualize the spatial distribution of the longitudinal electric fields of a phase-shaped higher-order beam at the focal volume. The technique is based on 3D microscopic mapping of second-harmonic generation from a single semiconductor nanowire that is extremely sensitive to the orientation of the electric field. Using the technique, the spatial distribution of the longitudinal electric fields of a phase-shaped Hermite-Gaussian beam of order (1,0) or HG10 beam was visualized in three dimensions for the first time.


Poster: B241, March 6th, 15:30 - 17:30, 2nd floor

ANALOG MACH-ZEHNDER INTERFEROMETRY IN A CIRCUIT QED SETUP

M. Haataja1, D. Lan1, S. Dogra1, G. S. Paraoanu1

miika.haataja@aalto.fi
1 Aalto University


We demonstrate a circuit QED analog to Mach-Zehnder (MZ) interferometry [1] using a superconducting transmon [2] qubit. The MZ interferometer is a device for precision phase measurements, where a beam of light is split into two parts and later recombined in another beam splitter. The qubit is an artificial atom, and its ground $|0\rangle$ and first excited $|1\rangle$ states correspond to the two spatial paths of the MZ interferometer. A ($\pi/2$)-pulse microwave excitation acts as a beam splitter, creating the desired superposition of the paths. A phase shifter can be included in one of the paths by adjusting the phase of the second "beam splitter" pulse. The relative phase difference between the pulses modulates the population of the excited state, which is calculated from repeated measurements of the qubit. A more complicated nested MZ setup can also be constructed, using higher excited states $|n\rangle$ of the qubit. We show experimental results for MZ analogs consisting of two and three separate paths.

References:
[1] E. Hecht, Optics, 5th ed., 429-430, Pearson (2017)
[2] J. Koch, T. M. Yu, J. M. Gambetta, A. A. Houck, D. I. Schuster, J. Majer, A. Blais, M. H.
Devoret, S. M. Girvin, and R. J. Schoelkopf, Charge Insensitive Qubit Design from Optimizing
the Cooper-Pair Box, Phys. Rev. A 76, 042319 (2007).


Poster: B242, March 6th, 15:30 - 17:30, 2nd floor

IMPROVEMENT OF THE CURRENT QUANTIZATION IN SINIS TURNSTILE BY QUASIPARTICLE EXTRACTION THROUGH BIASED SIS JUNCTIONS

M. Marín Suárez1, J. T. Peltonen1, J. P. Pekola1

marco.marinsuarez@aalto.fi
1 Low Temperature Laboratory (OVLL), Aalto University School of Science


We have enhanced the performance of superconductor-insulator-normal metal-insulator-superconductor (SINIS) single-electron turnstiles by lowering the (effective) temperature of one of its leads. This is done by extracting quasiparticles from one superconducting lead using biased SIS junctions. We show that it is possible to tune the accuracy of the turnstile without modifying its physical structure. As a result an improvement on the current quantization due to this bias is possible even in gate signal frequencies as high as 160 MHz, for which we reached low quasiparticle densities. We estimate the quasiparticle density approximating the (effective) temperature of the lead by means of sequential tunnelling and Andreev reflection numerical models, finally we illustrate generally how the lead temperature varies with the bias voltage of the SIS junction.



Figure 1
Figure 1: (a) Current plateaus at 20 MHz of gate signal frequency. Closed diamonds are data measured with unbiased SIS junctions, and open circles are with biased SIS junctions. The solid black lines are simulations including first and second order tunneling. (b) similar as (a) but with a signal frequency of 100 MHz. (c) SEM and experimental set of one of the studied single-electron turnstile corresponding to the plots in (a) and (b).