Effect of Phase Fluctuations on the Superconducting Properties of Strongly Disordered 3D NbN Thin Films

We present transport, Hall effect, electronic tunnelling and penetration depth studies in 3D homogeneously disordered epitaxial NbN thin films with disorder ranging from the moderately clean limit (KFl~10.12) to the very dirty limit (KFl~1.24). The superconducting transition temperature (Tc) decreases from ~17K to less than 350mK with increasing disorder. The Tc and conductivity at the lowest temperature (σ0) both asymptotically approach zero as KFl →l indicating a coincidence of the metal-insulator transition (MIT) and the superconductor-insulator transition (SIT). Close to critical disorder there is spatial inhomogeneity in the superconducting density of states (DOS) and the superconducting state is governed by quantum phase fluctuations. This results in suppression of the superfluid density (ns) and a pseudogap state where the resistance is no longer zero but the energy gap (Δ) remains finite.