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

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 kFl~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.