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

Abstract

We demonstrate an original method -based on controlled oxidation -to create high-quality tunnel junctions between superconducting Al reservoirs and InAs semiconductor nanowires. We show clean tunnel characteristics with a current suppression by over 4 orders of magnitude for a junction bias well below the Al gap ∆0 ≈ 200 µeV. The experimental data are in close agreement with the BCS theoretical expectations of a superconducting tunnel junction. The studied devices combine small-scale tunnel contacts working as thermometers as well as larger electrodes that provide a proof-of-principle active cooling of the electron distribution in the nanowire. A peak refrigeration of about δT = 10 mK is achieved at a bath temperature T bath ≈ 250 − 350 mK in our prototype devices. This method opens important perspectives for the investigation of thermoelectric effects in semiconductor nanostructures and for nanoscale refrigeration.The control over the heat flow and the local electron distribution in a nanodevice represents a crucial experimental challenge [1][2][3] with an important impact both on the solution of key open problems in fundamental physics and on development of future device applications [4,5]. In particular, the recent progress of thermoelectric physics in nanostructured materials offers fascinating new perspectives for the realization of more efficient solid-state heat pumps for energy conversion [6][7][8] and/or for the creation of self-cooling nanodevices where the electron or the phonon system in the active region can be refrigerated below the phonon bath [3,4]. The progress in these fields calls for the development of novel methods to reliably control heat and measure the device thermoelectric parameters with a nanometer-scale precision [9][10][11][12]. Local electronic cooling can be relevant for improving the device performance either in terms of noise, sensitivity or decoherence [4] and for finding a role in advanced applications, including topological quantum computation [13][14][15][16] and ultrasensitive radiation detection [17,18]. In addition, the manipulation of heat is at the basis of the emerging field of coherent caloritronics [1,[19][20][21] and it could be crucial in solving important standing fundamental problems in condensed matter physics, including quantum thermodynamics and the study of the elusive Majorana fermions in solid-state systems [13,22,23].Hybrid architectures combining superconductive elements with normal metals represent a bright example of refined technology to locally measure and manipulate heat at low temperatures and have been the subject of an intense research effort [3][4][5]. In particular, devices integrating normal-insulator-superconductor (NIS) tunnel junctions between a normal metal (N) and Al [24] or other superconductors (S) [25,26] have yielded the demonstration of significant nanorefrigeration in the milliKelvin regime. A similar technology has been so far hard to achieve in the context of semiconductor nanostructures, given the notorious technical challenges th...

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