Subgap current in superconducting tunnel junctions with diffusive electrodes

TL;DR

This paper investigates the subgap current in superconducting tunnel junctions with diffusive electrodes, revealing how geometry and scattering influence the current and offering explanations for anomalously high subgap currents observed experimentally.

Contribution

It introduces a model showing subharmonic gap structure scaling with an effective transparency that can greatly exceed the junction transparency, explaining experimental anomalies.

Findings

Subharmonic gap structure scales with an effective transparency

Effective transparency can be up to 100 times the junction transparency

Finite quasiparticle lifetime further enhances multiparticle tunneling

Abstract

We calculate the subgap current in planar superconducting tunnel junctions with thin-film diffusive leads. It is found that the subharmonic gap structure of the tunnel current scales with an effective tunneling transparency which may exceed the junction transparency by up to two orders of magnitude depending on the junction geometry and the ratio between the coherence length and the elastic scattering length. These results provide an alternative explanation of anomalously high values of the subgap current in tunnelling experiments often ascribed to imperfection of the insulating layer. We also discuss the effect of finite lifetime of quasiparticles as the possible origin of additional enhancement of multiparticle tunnel currents.