Current noise in diffusive SNS junctions in the incoherent MAR regime (a review)

TL;DR

This paper develops a theory for current shot noise in long diffusive SNS junctions, revealing universal noise features at low voltages and temperatures, and explaining the effects of non-equilibrium quasiparticles and electron scattering.

Contribution

It provides a comprehensive theoretical framework for understanding shot noise in diffusive SNS structures in the incoherent MAR regime, including universal noise levels and crossover behaviors.

Findings

Universal finite noise at zero voltage and temperature, S=4Δ/3R.

Enhanced effective charge of carriers, q_eff=2Δ/V.

Crossover to hot electron regime with decreasing effective temperature.

Abstract

We present a theory for the current shot noise in long diffusive SNS structures with low-resistive interfaces at arbitrary temperatures. In such structures, the noise is mostly generated by normal electron scattering in the N-region. Whereas the $I$-$V$ characteristics are approximately described by Ohm's law, the current noise reveals all characteristic features of the MAR regime: "giant" enhancement at low voltages, pronounced SGS, and excess noise at large voltages. The most spectacular feature of the noise in the incoherent MAR regime is a universal finite noise level at zero voltage and at zero temperature, $S= 4\Delta/3R$. This effect can be understood as the result of the enhancement of the effective charge of the carriers, $q^{\it eff}=2\Delta/V$, or, alternatively, as the effect of strongly non-equilibrium quasiparticle population in the energy gap region with the effective temperature $T_0=\Delta/3$. Under the condition of dominant electron-electron scattering, the junction undergoes crossover to the hot electron regime, with the effective temperature of the subgap electrons decreasing logarithmically with the voltage. Calculation of the noise power has been done on the basis of circuit theory of the incoherent MAR.