Accepted as a poster
in Poster Session A
in Floor 2 on March 5th, 16:00

# 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: An example of our cooler platform. The diameter of the sub-chips varied between c. 300 $\mu$m and 1 mm.