Localization of preformed Cooper-pairs in disordered superconductors

TL;DR

This study investigates how disorder induces a transition from superconducting to insulating states, providing direct evidence that localized Cooper pairs drive this transition in disordered superconductors.

Contribution

The paper presents the first direct experimental evidence that Cooper pair localization causes the superconductor-insulator transition in disordered systems.

Findings

Superconducting gap remains while coherence peaks disappear near transition

Localized Cooper pairs are present in the insulating state

Transition is driven by Cooper pair localization

Abstract

The most profound effect of disorder on electronic systems is the localization of the electrons transforming an otherwise metallic system into an insulator. If the metal is also a superconductor then, at low temperatures, disorder can induce a dramatic transition from a superconducting into an insulating state. An outstanding question is whether the route to insulating behavior proceeds via the direct localization of Cooper pairs or, alternatively, by a two-step process in which the Cooper pairing is first destroyed followed by the standard localization of single electrons. Here we address this question by studying the local superconducting gap of a highly disordered, amorphous, superconductor by means of scanning tunneling spectroscopy. Our measurements reveal that, in the vicinity of the superconductor-insulator transition, the coherence peaks in the one-particle density of states disappear while the superconducting gap remains intact indicating the presence of localized Cooper pairs. Our results provide the first direct evidence that the transition in our system is driven by Cooper pair localization.