Investigation of InAs–based devices for topological applications

Abstract

Hybrid superconductor/semiconductor devices constitute a powerful platform to investigate the emergence of new topological state of matter. Among all possible semiconductor materials, InAs represents a promising choice, owing to its high quality, large g-factor and spin-orbit component. Here, we report on InAs-based devices both in one-dimensional and two-dimensional configurations. In the former, low-temperature measurements on a suspended nanowire are presented, inspecting the intrinsic spin-orbit contribution of the system. In the latter, Josephson Junctions between two Nb contacts comprising an InAs quantum well are investigated. Supercurrent flow is reported, with Nb critical temperature up to T c ∼ 8 K. Multiple Andreev reflection signals are observed in the dissipative regime. In both systems, we show that the presence of external gates represents a useful knob, allowing for wide tunability and control of device properties, such as spin-orbit coherence length or supercurrent amplitude.Among all semiconductor materials, InAs plays a prominent role due to its low effective mass, strong spinorbit coupling, and high Landé g-factor. 16-21 Moreover, hybrid superconductor/semiconductor devices require high quality contacts with low normal/superconductor (N/S) interface resistance, to guarantee a robust proximity effect, and eventually large electron mean free path (MFP). 20,22 It has been shown that InAs represents the ideal choice for building hybrid devices, owing to the lack of a Schottky barrier at the interface with the metal, combined with the small effective mass and large spin-orbit coupling of this semiconductor. 16-21 Epitaxial Al/InAs heterostructures were realized that show an exceptionally transparent superconductor-semiconductor

Gate-controlled Supercurrent in Ballistic InSb Nanoflag Josephson Junctions

“…The critical temperature of the superconducting leads was determined to be T c = 8.44 K, from which the bulk gap can be computed using the theory by Bardeen, Cooper, and Schrieffer (BCS): 35 ∆ = 1.76k B T c = 1.28 meV, consistent with values of Nb superconducting contacts previously reported in literature. 28,[36][37][38][39][40][41] The induced superconducting coherence length 13,27,42,43 is ξ s = hv F /∆, with v F the Fermi velocity in the N region (v F = 1.5×10 6 m/s) and ∆ the gap in the superconductor. Here, ξ s ∼ 750 nm > L, so the device is in the short junction regime.…”

“…The positions of these MAR singularities follow the equation eV n = 2∆ * /n, with n = 1, 2, 3, ... and ∆ * the induced gap in the N region. Most commonly, the position of the maxima in the differential conductance has been analyzed, 6,12,13,18,27,28,36,41,64,65 but recently it was pointed out that for highly transparent junctions, the MAR resonances appear as minima in the differential conductance. In order to estimate the junction transparency and the induced gap, we used a simple scattering model that assumes fully-coherent transport across a multimode JJ (see Ref.…”

“…The measured critical temperature T c = 8.44 K is close to the reported value for bulk Nb and the resulting value of the gap ∆ = 1.28 meV is in good agreement with values reported for JJs with Nb contacts. 28,[36][37][38][39][40][41]65 Thus, we attribute the observed critical temperature to a switching of the Nb film from the superconducting to the normal state.…”