Mesoscopic transition in the shot noise of diffusive S/N/S junctions

TL;DR

This paper experimentally studies shot noise in diffusive S/N/S junctions, revealing a transition at the Thouless energy from coherent to incoherent regimes, with significant noise enhancement and potential evidence of multiple charge transfer.

Contribution

It provides the first detailed experimental analysis of shot noise transition in diffusive S/N/S junctions across the Thouless energy, supported by semi-classical theory.

Findings

Minimum in noise density at Thouless energy

Noise enhancement above Thouless energy

Divergence of shot noise at low voltage below Thouless energy

Abstract

We experimentally investigated the current noise in diffusive Superconductor/Normal metal/Superconductor junctions with lengths between the superconducting coherence length xi_Delta and the phase coherence length L_Phi of the normal metal (xi_Delta < L < L_Phi). We measured the shot noise over a large range of energy covering both the regimes of coherent and incoherent multiple Andreev reflections. The transition between these two regimes occurs at the Thouless energy where a pronounced minimum in the current noise density is observed. Above the Thouless energy, in the regime of incoherent multiple Andreev reflections, the noise is strongly enhanced compared to a normal junction and grows linearly with the bias voltage. Semi-classical theory describes the experimental results accurately, when taking into account the voltage dependence of the resistance which reflects the proximity effect. Below the Thouless energy, the shot noise diverges with decreasing voltage which may indicate the coherent transfer of multiple charges.