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

摘要

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