Electronic cooling of a TiW alloy normal-metal island using Nb-based superconducting tunnel junctions

Abstract

TiW thin films remain non-superconducting down to millikelvin temperatures and are compatible with large-scale CMOS manufacturing, making them attractive for cryogenic and quantum tunnel-junction devices, such as normal-metal-insulator-superconductor (NIS) thermal sensors and electron refrigerators. We demonstrate significant electronic cooling of a TiW-based normal-metal island and electron thermometry by using TiW-Al-AlOx-Nb NIS tunnel-junction stacks. The NIS thermometer enables local electron temperature measurements from 0.5 to 8.32 K. NIS cooling is observed between 0.6 and 3.5 K, with maximum absolute and relative temperature reductions of 217 mK and 27%, respectively. Calculations using tunnel-junction parameters obtained independently from current-voltage fits reproduce the measured optimum-voltage scale, which lies substantially below the ideal low-temperature prediction.