Non-equilibrium current noise in mesoscopic disordered SNS junctions

TL;DR

This paper analyzes current noise in SNS junctions, revealing subharmonic gap singularities, effective charge behavior, and noise divergence, advancing understanding of mesoscopic superconducting systems.

Contribution

It provides a detailed theoretical analysis of current noise in SNS junctions, including effects of multiple Andreev reflections and disorder, with new insights into noise features and effective charge.

Findings

Subharmonic gap singularities at $eV=2\Delta/n$ observed in noise spectra.

Effective transferred charge $q^*$ increases as bias voltage decreases.

Divergence of noise $S_I(0) \\propto V^{-1/2}$ at small voltages.

Abstract

Current noise in superconductor-normal metal-superconductor (SNS) junctions is calculated within the scattering theory of multiple Andreev reflections (MAR). It is shown that the noise exhibits subharmonic gap singularities at $eV=2\Delta/n$, $n=1,2,... $ both in single-mode junctions with arbitrary transparency $D$ and in multi-mode disordered junctions. The subharmonic structure is superimposed with monotonic increase of the effective transferred charge $q^*=S_I(0)/2I$ with decreasing bias voltage. Other features of the noise include a step-like increase of $q^*$ in junctions with small $D$, and a divergence $S_I(0) \propto V^{-1/2}$ at small voltages and excess noise $S_{ex} = 2eI_{ex}$, where $I_{ex}$ is the excess current, at large voltages, in junctions with diffusive transport.