Vortex excitations in the Insulating State of an Oxide Interface

TL;DR

This study investigates the superconductor-insulator transition at a 2D oxide interface, revealing vortex excitations and Cooper pair localization as key factors, with superconducting fluctuations persisting into the insulating phase.

Contribution

It provides the first continuous mapping of the SIT in a crystalline 2D system, highlighting vortex excitations' role in the insulating state.

Findings

Identification of a new magnetic field scale H_{pairing} where superconducting fluctuations are suppressed.

Observation of vortex excitations and Cooper pair localization deep into the insulating phase.

Strong superconducting features in a crystalline oxide interface previously seen only in amorphous systems.

Abstract

In two-dimensional (2D) superconductors an insulating state can be induced either by applying a magnetic field, $H$, or by increasing disorder. Many scenarios have been put forth to explain the superconductor to insulator transition (SIT): dominating fermionic physics after the breaking of Cooper pairs, loss of phase coherence between superconducting islands embedded in a metallic or insulating matrix and localization of Cooper pairs with concomitant condensation of vortex-type excitations. The difficulty in characterizing the insulating state and its origin stems from the lack of a continuous mapping of the superconducting to insulating phase diagram in a single sample. Here we use the two-dimensional (2D) electron liquid formed at the interface between the two insulators (111) SrTiO$_3$ and LaAlO$_3$ to study the superconductor to insulator transition. This crystalline interface surprisingly exhibits very strong features previously observed only in amorphous systems. By use of electrostatic gating and magnetic fields, the sample is tuned from the metallic region, where supeconductivity is fully manifested, deep into the insulating state. Through examination of the field dependence of the sheet resistance and comparison of the response to fields in different orientations we identify a new magnetic field scale, H$_{pairing}$, where superconducting fluctuations are muted. Our findings show that vortex fluctuations excitations and Cooper pair localization are responsible for the observed SIT and that these excitations surprisingly persist deep into the insulating state.