Correlation between Superconductivity, Band Filling and Electron Confinement at the LaAlO$_{3}$-SrTiO$_{3}$ Interface

TL;DR

This study investigates how different gating methods influence superconductivity at the LaAlO3-SrTiO3 interface, revealing the importance of electron confinement and subband structure in controlling superconducting properties.

Contribution

It demonstrates that top- and backgating have distinct effects on superconductivity, linking these effects to band filling, confinement potential, and Lifshitz transitions through combined experimental and theoretical analysis.

Findings

Maximum Tc coincides with a kink in band filling at high topgate voltage.

Lifshitz transition of the second d_{xy} subband is related to the observed kink.

Gating controls the relative energy of subbands, affecting superconductivity.

Abstract

By combined top- and backgating, we explore the correlation of superconductivity with band filling and electron confinement at the LaAlO$_3$-SrTiO$_3$ interface. We find that the top- and backgate voltages have distinctly different effects on the superconducting critical temperature, implying that the confining potential well has a profound effect on superconductivity. We investigate the origin of this behavior by comparing the gate-dependence of $T_c$ to the corresponding evolution of the band filling with gate voltage. For several backgate voltages, we observe maximum $T_c$ to consistently coincide with a kink in tuning the band filling for high topgate voltage. Self-consistent Schr\"odinger-Poisson calculations relate this kink to a Lifshitz transition of the second $d_{xy}$ subband. These results establish a major role for confinement-induced subbands in the phase diagram of SrTiO$_3$ surface states, and establish gating as a means to control the relative energy of these states.