Tail States below the Thouless Gap in SNS junctions: Classical Fluctuations

TL;DR

This paper investigates the origin of subgap quasiparticle states in SNS junctions, showing that classical impurity fluctuations create tail states below the Thouless gap, with results applicable across different effective dimensions.

Contribution

It provides a comprehensive calculation of subgap density of states caused by classical impurity fluctuations in SNS junctions, comparing them with mesoscopic fluctuation effects.

Findings

Impurity fluctuations induce subgap states below the Thouless gap.

The subgap DOS tails depend on the effective dimensionality of the system.

Classical and mesoscopic fluctuation contributions are related by dimensional reduction.

Abstract

We study the tails of the density of states (DOS) in a diffusive superconductor-normal metal-superconductor (SNS) junction below the Thouless gap. We show that long-wave fluctuations of the concentration of impurities in the normal layer lead to the formation of subgap quasiparticle states, and calculate the associated subgap DOS in all effective dimensionalities. We compare the resulting tails with those arising from mesoscopic gap fluctuations, and determine the dimensionless parameters controlling which contribution dominates the subgap DOS. We observe that the two contributions are formally related to each other by a dimensional reduction.