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

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 Schrödinger-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.

Possible flexoelectric origin of the Lifshitz transition in LaAlO3/SrTiO3 interfaces

“…This qualitative distinction between low and high doping has also been seen in quantum dot transport experiments, which reveal a crossover from attractive to repulsive pairing interactions with increasing n 2D [13]. While there is general agreement that the sensitivity to doping is connected to an observed Lifshitz transition [5, 14-17] between a single occupied band at low density and multiple occupied bands at high density [6,[17][18][19][20][21][22][23][24], the mechanism by which this transition happens is not established.Density functional theory (DFT), while instrumental in establishing fundamental interface properties [25][26][27][28], finds electron densities that are an order of magnitude larger than the Lifshitz transition density n L ∼ 0.02-0.05 electrons per 2D unit cell, and cannot easily be tuned through the transition. Schrödinger-Poisson calculations, for which n 2D can be continuously tuned, persistently find multiple occupied bands even for n 2D n L [5,15,[29][30][31].…”

“…This qualitative distinction between low and high doping has also been seen in quantum dot transport experiments, which reveal a crossover from attractive to repulsive pairing interactions with increasing n 2D [13]. While there is general agreement that the sensitivity to doping is connected to an observed Lifshitz transition [5, 14-17] between a single occupied band at low density and multiple occupied bands at high density [6,[17][18][19][20][21][22][23][24], the mechanism by which this transition happens is not established.…”

“…The effects of gating have not been reported; however, one may infer from Fig. 4(a) that if the strain were to relax with increasing n 2D , the slope above the transition would be negative, as found in some experiments [15][16][17]. Figure 4(e) shows that the inclusion of intra-atomic interactions has a small quantitative effect on n 1xy , but does not change qualitative features of the transition.…”

“…In this instance, the quartic terms in Eq. (17) are required to stabilize the polarization. We find that the numerics are most easily controlled if we work in a basis in whichD is diagonal.…”

“…Furthermore, the evolution of the band structure with n 2D is highly unusual. Early work [14] showed that the filling of the lowest band is constant for n 2D > n L , and recent experiments have found geometries in which its filling decreases [15][16][17]. In the latter case, the first band actually empties itself into higher energy bands with increasing chemical potential.…”