Proximity effect in a Nb-InAs-Nb nanowire junction*

Proximity effect superconductivity in semiconductor-superconductor hybrid devices contains rich physics and could be key to the realization of topological quantum information processing. We have performed a series of low temperature electronic transport measurements on an InAs nanowire contacted with Niobium leads. The channel length ($\sim4$ times the nanowire diameter) is shorter than the electronic phase coherence length, but longer than the elastic mean free path, leading to behaviour that can be modelled by a superconductor-normal-superconductor junction in the diffusive transport regime. A supercurrent is observed below a critical current $I_c$ of up to $\sim$50 nA. The critical current varies with local gate voltages and correlates with the normal state conductance, producing modulation of $I_c$ related to universal conductance fluctuations. An applied magnetic field produces a Gaussian decay of $I_c$, consistent with known theory. Analysis of multiple Andreev reflection corrections to conductance indicates a contact transparency $\approx$0.6. The full results help to shed light on the nature of proximity effect superconductivity in a quasi-one-dimensional semiconductor in the quasi-diffusive regime.