Are pinholes the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

TL;DR

This study investigates the cause of excess subgap current in highly resistive superconducting tunnel junctions, providing evidence that pinholes are not responsible, and highlights the role of Andreev transport mechanisms.

Contribution

The paper demonstrates through experimental and theoretical analysis that excess current is due to Andreev processes, not microscopic pinholes, in highly resistive superconducting junctions.

Findings

Transition from SIS to SIN reduces current by two orders of magnitude.

Coherent diffusive Andreev transport observed in junctions with homogeneous barriers.

Good agreement between theory and experiment over a wide range of transparencies.

Abstract

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic "pinholes" in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.