Sub-50 mK electronic cooling with large-area superconducting tunnel junctions

TL;DR

This paper demonstrates a large-area superconducting tunnel junction cooler achieving a fivefold temperature reduction from 150 mK to 30 mK, with insights into limiting factors at ultra-low temperatures.

Contribution

It introduces an aluminium-based electronic cooler with suspended normal metal and quasi-particle drain, achieving significant cooling and analyzing factors limiting lower temperature performance.

Findings

Cooling from 150 mK to 30 mK achieved

Suspended normal metal enhances phonon bath cooling

Spatial non-uniformity in superconductor gap limits further cooling

Abstract

In electronic cooling with superconducting tunnel junctions, the cooling power is counterbalanced by the interaction with phonons and by the heat flow from the overheated leads. We study aluminium-based coolers that are equipped with a suspended normal metal and an efficient quasi-particle drain. At intermediate temperatures, the phonon bath of the suspended normal metal is cooled. At lower temperatures, by adjusting the junction transparency, we control the injection current, and thus the superconductor temperature. The device shows a strong cooling from 150 mK down to about 30 mK, a factor of five in temperature. We suggest that spatial non-uniformity in the superconductor gap limits the cooling toward lower temperatures.