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

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.

4 pages, 4 figures

References in corpus (4)

• Controlling the spontaneous emission of a superconducting transmon qubit
• Andreev Current-Induced Dissipation in a Hybrid Superconducting Tunnel Junction
• Characterization of a fabrication process for the integration of superconducting qubits and RSFQ circuits
• Subgap conductivity in SIN-junctions of high barrier transparency

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