Tunneling studies in a homogeneously disordered s-wave superconductor: NbN

TL;DR

This study investigates how disorder affects superconducting properties in NbN thin films, revealing that increased disorder leads to phase-fluctuation-driven loss of superconductivity without closing the energy gap.

Contribution

It provides experimental evidence that in highly disordered NbN, superconductivity is lost through phase fluctuations rather than gap suppression, expanding understanding of disorder effects.

Findings

Superconducting transition temperature remains finite despite broadening of the density of states.

Disorder induces phase fluctuations that suppress superconductivity without closing the energy gap.

The Ioffe-Regel parameter varies significantly, correlating with changes in superconducting behavior.

Abstract

We report the evolution of superconducting properties as a function of disorder in homogeneously disordered epitaxial NbN thin films grown on (100) MgO substrates, studied through a combination of electrical transport, Hall Effect and tunneling measurements. The thickness of all our films are >50nm much larger than the coherence length ~5nm. The effective disorder in different films encompasses a large range, with the Ioffe-Regel parameter varying in the range kFl~1.38-8.77. Tunneling measurements on films with different disorder reveals that for films with large disorder the bulk superconducting transition temperature (Tc) is not associated with a vanishing of the superconducting energy gap, but rather a large broadening of the superconducting density of states. Our results provide strong evidence of the loss of superconductivity via phase-fluctuations in a disordered s-wave superconductor.