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

TL;DR

This study investigates how phase fluctuations affect superconductivity in strongly disordered 3D NbN thin films, revealing a link between disorder, quantum fluctuations, and the superconductor-insulator transition.

Contribution

It provides detailed experimental insights into the role of phase fluctuations near the superconductor-insulator transition in disordered NbN films.

Findings

Superconducting T_c decreases with increasing disorder.

Superfluid density is suppressed near critical disorder.

A pseudogap state emerges due to quantum phase fluctuations.

Abstract

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 (k_Fl~10.12) to the very dirty limit (k_Fl~1.24). The superconducting transition temperature (T_c) decreases from ~17K to less than 350mK with increasing disorder. The T_c and conductivity at the lowest temperature both asymptotically approach zero as k_Fl tends to 1, indicating a coincidence of the metal-insulator transition (MIT) and the superconductor-insulator transition. 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 (n_s) and a pseudogap state where the resistance is no longer zero but the energy gap remains finite