Andreev reflection and enhanced subgap conductance in NbN/Au/InGaAs-InP junctions

TL;DR

This paper reports the fabrication of highly transparent NbN/Au/InGaAs-InP junctions, observing Andreev reflection and enhanced subgap conductance, with analysis showing persistent effects under high magnetic fields.

Contribution

It introduces a new fabrication process for transparent superconductor/2DEG junctions and reports the first observation of specific subgap conductance features in such systems.

Findings

Observation of decreased differential resistance with double-dip structure

Subgap resistance remains high under strong magnetic fields

Analysis confirms proximity effect consistent with quasiclassical models

Abstract

We report on the fabrication of highly transparent superconductor/normal metal/two-dimensional electron gas junctions formed by a superconducting NbN electrode, a thin (10nm) Au interlayer, and a two-dimensional electron gas in a InGaAs/InP heterostructure. High junction transparency has been achieved by exploiting of a newly developed process of Au/NbN evaporation and rapid annealing at 400C. This allowed us to observe for the first time a decrease in the differential resistance with pronounced double-dip structure within the superconducting energy gap in superconductor-2DEG proximity systems. The effect of a magnetic field perpendicular to the plane of the 2DEG on the differential resistance of the interface was studied. It has been found that the reduced subgap resistance remains in high magnetic fields. Zero-field data are analyzed within the previously established quasiclassical model for the proximity effect.