Correlation between superconductivity, band filling, and electron confinement at the LaAlO3/SrTiO3 interface

By combined top- and backgating, we explore the correlation of superconductivity with band filling and electron confinement at the LaAlO3/SrTiO3 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 Tc to the corresponding evolution of the band filling with gate voltage. For several backgate voltages, we observe maximum Tc to consistently coincide with a kink in tuning the band filling for high topgate voltage. Self-consistent Schrödinger-Poisson calculations relate this kink to a Lifshitz transition of the second dxy subband. These results establish a major role for confinement-induced subbands in the phase diagram of SrTiO3 surface states, and establish gating as a means to control the relative energy of these states.