Subgap conductivity in SIN-junctions of high barrier transparency

TL;DR

This paper studies the current-voltage behavior of high-transparency SIN junctions, revealing larger subgap currents than standard models predict, and proposes splitting junctions to reduce leakage for quantum circuit applications.

Contribution

It introduces a model explaining enhanced subgap currents in high-transparency SIN junctions and suggests a practical method to reduce leakage by dividing junctions into smaller parts.

Findings

Subgap leakage currents exceed standard tunnelling model predictions.

Splitting junctions into smaller sub-junctions reduces subgap current.

High-transparency SIN junctions can be optimized for quantum circuitry.

Abstract

We investigate the current-voltage characteristics of high-transparency superconductor-insulator-normal metal (SIN) junctions with the specific tunnel resistance below 30 kOhm per square micron. The junctions were fabricated from different superconducting and normal conducting materials, including Nb, Al, AuPd and Cu. The subgap leakage currents were found to be appreciably larger than those given by the standard tunnelling model. We explain our results using the model of two-electron tunnelling in the coherent diffusive transport regime. We demonstrate that even in the high-transparency SIN-junctions, a noticeable reduction of the subgap current can be achieved by splitting a junction into several submicron sub-junctions. These structures can be used as nonlinear low-noise shunts in Rapid-Single-Flux-Quantum (RSFQ) circuitry for controlling Josephson qubits.