Properties of two level systems in current-carrying superconductors

TL;DR

This paper demonstrates that in disordered superconductors, a dc supercurrent significantly amplifies the coupling of two-level systems to ac fields, affecting conductivity and current noise.

Contribution

It reveals a dramatic enhancement of TLS coupling to ac fields in the presence of supercurrent, impacting both linear and nonlinear superconducting phenomena.

Findings

Enhanced ac conductivity due to TLS-supercurrent interaction

Current fluctuations exhibit 1/f noise spectrum under broad relaxation times

Giant increase in ac response at low frequencies with supercurrent presence

Abstract

We show that in disordered superconductors, at sufficiently low frequencies $\omega$, the coupling of TLS to external ac electric fields increases dramatically in the presence of a dc supercurrent. This giant enhancement manifests in all ac linear and nonlinear phenomena. In particular, it leads to a parametric enhancement of the real part of the ac conductivity and, consequently, of the equilibrium current fluctuations. If the distribution of TLS relaxation times is broad, the conductivity is inversely proportional to $\omega$, and the spectrum of the equilibrium current fluctuations takes the form of 1/f noise.