Microwave response and electrical transport studies of disordered s wave superconductor: NbN thin films

TL;DR

This study investigates the temperature-dependent microwave response of disordered NbN thin film superconductors, revealing deviations from BCS theory and suggesting inhomogeneity and pseudogap phenomena.

Contribution

It provides new insights into the microwave response of disordered NbN films at higher temperatures, highlighting deviations from BCS predictions and potential pseudogap and BKT-like behavior.

Findings

$f_0$ vanishes slower than BCS predictions between 0.4 and 0.8$T_c$

Possible evidence of pseudogap-like features in disordered NbN

Indications of a BKT-like transition near $T_c$

Abstract

It is now well known that the properties of the disordered s wave superconductors can deviate significantly from the prediction of standard Bardeen-Cooper-Schrieffer (BCS) theory. By measuring the temperature dependence of the resonance frequency, $f_0$, of microwave resonators made from disordered NbTiN and TiN thin films, at low temperatures, below half of the superconducting critical temperature, $T_c$, Driessen et al. \cite{driessen2012PRL} demonstrated that $f_0$ vanishes faster than predicted from the BCS theory. Here, we report on the temperature dependence of $f_0$ of microwave resonators made from disordered NbN thin films at higher temperatures--- typically from 0.4 to 0.8$T_c$. In this temperature range, we demonstrate that $f_0$ vanishes slower than predicted from the BCS theory. We discuss the possible role of electronic inhomogeneity and possibility of appearing a pseudogap-like feature. We also discuss the possibility of a faster downturn of the superfluid density, $n_s$, near $T_c$, resembling a Berezinski--Kosterlitz--Thoules (BKT) type transition.