Evidence for a proximity-induced energy gap in Nb/InAs/Nb junctions

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Abstract
We investigate the subharmonic energy gap structure (SGS) in coplanar Nb/p-type InAs(2DEG)/Nb junctions. The junctions are produced by employing a technique which enables us to achieve distances between the superconducting electrodes considerably below 100 nm. In the coplanar geometry of our junctions, a smearing of the SGS is expected when a voltage drop along the Nb/InAs interfaces is present. This behavior is indeed observed in junctions with interfaces of minor quality. In junctions with high-quality interfaces, the SGS is well developed despite the coplanar geometry. We interpret the absence of smearing, together with a temperature-dependent shift of the SGS, in terms of a recent theory by Aminov et al. Within this theory, the influence of the proximity effect on the InAs inversion layer under the Nb electrodes is taken into account, leading to the opening of a gap in the density of states. Because of the narrow inversion layer and the high interface transparency, the gap is comparable in magnitude to the superconducting energy gap Δ of the Nb. For our junction, we find from the shift of the SGS a gap of 0.81Δ at temperature T=0 induced in the inversion layer.