Direct observation of quantum superconducting fluctuations in an insulating groundstate

TL;DR

This paper presents the first direct measurement of quantum superconducting fluctuations in an insulating ground state of thin InO_x films, revealing finite frequency superfluid stiffness deep into the insulating regime.

Contribution

It provides the first experimental evidence of quantum superconducting fluctuations in an insulator, using complex AC conductivity measurements at finite frequency.

Findings

Finite frequency superfluid stiffness observed in insulating regime

Quantum fluctuations are temperature independent as T approaches 0

Superconducting fluctuations occur in highly resistive samples

Abstract

We review our recent measurements of the complex AC conductivity of thin InO_x films studied as a function of magnetic field through the nominal 2D superconductor-insulator transition. These measurements - the first of their type to probe nonzero frequency - reveals a significant finite frequency superfluid stiffness well into the insulating regime. Unlike conventional fluctuation superconductivity in which thermal fluctuations give a superconducting response in regions of parameter space that don't exhibit long range order, these fluctuations are temperature independent as T --> 0 and are exhibited in samples where the resistance is large (greater than 10^6 Ohms/Square) and strongly diverging. We interpret this as the direct observation of quantum superconducting fluctuations around an insulating ground state. This system serves as a prototype for other insulating states of matter that derive from superconductors.