Frequency dependent superfluid stiffness in the pseudogap regime in strongly disordered NbN thin films

TL;DR

This study investigates how disorder affects the frequency-dependent superfluid stiffness in NbN thin films, revealing extended fluctuation regimes and phase fluctuations near the pseudogap temperature in strongly disordered samples.

Contribution

It provides the first detailed frequency-dependent measurements of superfluid stiffness in disordered NbN films, linking fluctuation dynamics to pseudogap phenomena.

Findings

Fluctuation regime extends above T_c in strongly disordered samples.

Superfluid stiffness becomes frequency-dependent and vanishes near T*.

Large-scale fluctuations suggest thermal phase fluctuations between domains.

Abstract

We measure the frequency dependence of the complex ac conductivity of NbN films with different levels of disorder in frequency range 0.4-20 GHz. Films with low disorder exhibit a narrow dynamic fluctuation regime above T_c as expected for a conventional superconductor. However, for strongly disordered samples, the fluctuation regime extends well above T_c, with a strongly frequency-dependent superfluid stiffness which disappears only at a temperature T* close to the pseudogap temperature obtained from scanning tunneling measurements. Such a finite-frequency response is associated to a marked slowing down of the superconducting fluctuations already below T*. The corresponding large length-scale fluctuations suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.