Tunneling studies in a homogeneously disordered<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>s</mml:mi></mml:math>-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.