InAs nanowire superconducting tunnel junctions: Quasiparticle spectroscopy, thermometry, and nanorefrigeration

We demonstrate an original method based on controlled oxidation for creating high-quality tunnel junctions between superconducting Al reservoirs and InAs semiconductor nanowires (NWs). We show clean tunnel characteristics with a current suppression by >4 orders of magnitude for a junction bias well below the Al gap of $Delta$0≈ 200 $mu$eV. The experimental data agree well with the Bardeen--Cooper--Schrieffer theoretical expectations for a superconducting tunnel junction. The studied devices employ small-scale tunnel contacts functioning as thermometers as well as larger electrodes that provide proof-of-principle active cooling of the electron distribution in the NWs. A peak refrigeration of approximately $delta$T = 10 mK is achieved at a bath temperature of $T_{bath}$ ≈ 250--350 mK for our prototype devices. This method introduces important perspectives for the investigation of the thermoelectric effects in semiconductor nanostructures and for nanoscale refrigeration.