Phase diagram of a strongly disordered s-wave superconductor, NbN, close to the metal-insulator transition

TL;DR

This study maps the phase diagram of disordered NbN thin films near the metal-insulator transition, revealing three regimes: conventional superconductivity, pseudogap behavior, and localized Cooper pairs in highly disordered states.

Contribution

It provides a detailed phase diagram of NbN under disorder, highlighting the evolution from BCS superconductivity to localized pairs with increasing disorder.

Findings

Superconductivity follows BCS theory at low disorder.

A pseudogap state persists above Tc at intermediate disorder.

Localized Cooper pairs survive in strongly disordered samples.

Abstract

We present a phase diagram as a function of disorder in three-dimensional NbN thin films, as the system enters the critical disorder for the destruction of the superconducting state. The superconducting state is investigated using a combination of magnetotransport and tunneling spectroscopy measurements. Our studies reveal 3 different disorder regimes. At low disorder the (k_{F}l~10-4), the system follows the mean field Bardeen-Cooper-Schrieffer behavior where the superconducting energy gap vanishes at the temperature where electrical resistance appears. For stronger disorder (k_{F}l<4) a "pseudogap" state emerges where a gap in the electronic spectrum persists up to temperatures much higher than Tc, suggesting that Cooper pairs continue to exist in the system even after the zero resistance state is destroyed. Finally, very strongly disordered samples (k_{F}l<1) exhibit a pronounced magnetoresistance peak at low temperatures, suggesting that localized Cooper pairs continue to survive in the system even after the global superconducting ground state is completely destroyed.