Survival of superconducting correlations across the 2D superconductor-insulator transition: A finite frequency study

TL;DR

This study investigates the persistence of superconducting correlations in disordered 2D films across the superconductor-insulator transition using finite-frequency measurements, revealing quantum fluctuations and localized Cooper pairs.

Contribution

It provides direct experimental evidence of superconducting fluctuations and localized Cooper pairs in the insulating regime of disordered 2D superconductors.

Findings

Finite-frequency superfluid stiffness persists into the insulating regime.

Quantum superconducting fluctuations are observed around the insulating ground state.

A phase diagram including superconducting, Bose insulator, and Fermi insulator states is established.

Abstract

The complex ac conductivity of thin highly disordered InOx films was studied as a function of magnetic field through the nominal two-dimensional superconductor-insulator transition. We have resolved a significant finite-frequency superfluid stiffness well into the insulating regime, giving direct evidence for quantum superconducting fluctuations around an insulating ground state and a state of matter with localized Cooper pairs. A phase diagram is established that includes the superconducting state, a transition to a "Bose" insulator, and an eventual crossover to a "Fermi" insulating state at high fields. We speculate on the consequences of these observations, their impact on our understanding of the insulating state, and its relevance as a prototype for other insulating states of matter that derive from superconductors.