Fluctuation-driven excess noise near superconducting phase transition

TL;DR

This paper analyzes how fluctuation-driven conductivity variations near the superconducting transition lead to significant excess current noise, especially when quasiparticle relaxation is slow, using a microscopic nonequilibrium approach.

Contribution

It introduces a detailed microscopic model of nonequilibrium excess noise near the superconducting transition, highlighting the role of correlated conductivity fluctuations.

Findings

Fluctuations of conductivity can become correlated near the transition.

The noise spectral density has a Lorentzian shape and scales with the inelastic relaxation time.

This mechanism can dominate over temperature fluctuation noise in certain regimes.

Abstract

We discuss intrinsic mechanisms of nonequilibrium excess noise in superconducting devices and transition edge sensors. In particular, we present an overview of fluctuation-driven contributions to the current noise in the vicinity of the superconducting transition. We argue that sufficiently close to the critical temperature fluctuations of conductivity may become correlated provided that the rate of quasiparticle relaxation is slow as compared to dynamics of superconducting fluctuations. In this regime, fluctuations of conductivity adiabatically follow the fluctuations of the electron distribution. This leads to a substantial enhancement of current noise. The corresponding spectral power density of noise has a Lorentzian shape in the frequency domain while its magnitude scales proportionally to the inelastic relaxation time. It also sensitively depends on the dephasing and Ginzburg-Landau timescales. Further estimates suggest that this mechanism dominates over the conventional temperature fluctuations in the same range of parameters. To describe these effects microscopically, we use the nonequilibrium Keldysh technique in the semiclassical approximation of superconductivity with Boltzmann-Langevin random forces to account for correlations of fluctuations.