Tunneling studies in a homogeneously disordereds-wave superconductor: NbN

Abstract

We report the evolution of superconducting properties as a function of disorder in epitaxial NbN thin films grown on (100) MgO substrates. The effective disorder in these films encompasses a large range, with ${k}_{F}l\ensuremath{\sim}1.38--8.77$. Tunneling measurements reveal that for films with large disorder the superconducting transition temperature is not associated with a vanishing of the superconducting energy gap but with a large broadening of the superconducting density of states. Our results provide strong evidence of the loss of superconductivity via phase fluctuations in a homogeneously disordered $s$-wave superconductor.