Evidence for Superconducting Phase Coherence in the Metallic/Insulating Regime of the LaAlO3-SrTiO3 Interface Electron System

TL;DR

This study provides evidence that phase-coherent Cooper pairs persist in the metallic/insulating regime of the LaAlO3-SrTiO3 interface, indicating a complex transition from superconductivity to a non-superconducting state.

Contribution

It demonstrates the presence of macroscopic phase coherence and fluctuating vortex-antivortex pairs in the metallic/insulating phase, revealing new insights into the superconducting transition mechanism.

Findings

Presence of phase-coherent Cooper pairs in the metallic/insulating state

Identification of vortex-antivortex fluctuations

Carrier-density dependence of coherence length

Abstract

A superconducting phase with an extremely low carrier density of the order of 10^13 cm^-2 is present at LaAlO3-SrTiO3 interfaces. If depleted from charge carriers by means of a gate field, this superconducting phase undergoes a transition into a metallic/insulating state that is still characterized by a gap in the spectral density of states. Measuring and analyzing the critical field of this gap, we provide evidence that macroscopically phase-coherent Cooper pairs are present in the metallic/insulating state. This is characterized by fluctuating vortex-antivortex pairs, and not by individual, immobile Cooper pairs. The measurements furthermore yield the carrier-density dependence of the superconducting coherence length of the two-dimensional system.