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

Abstract

Multiple experiments have observed a sharp transition in the band structure of LaAlO3/SrTiO3 (001) interfaces as a function of applied gate voltage. This Lifshitz transition, between a single occupied band at low electron density and multiple occupied bands at high density, is remarkable for its abruptness. In this work, we propose a mechanism by which such a transition might happen. We show via numerical modeling that the simultaneous coupling of the dielectric polarization to the interfacial strain ("electrostrictive coupling") and strain gradient ("flexoelectric coupling") generates a thin polarized layer whose direction reverses at a critical density. The Lifshitz transition occurs concomitantly with the polarization reversal and is first-order at T = 0. A secondary Lifshitz transition, in which electrons spread out into semiclassical tails, occurs at a higher density.LaAlO 3 (LAO) and SrTiO 3 (STO) are band insulators; however, when four or more monolayers of LAO are grown on top of a STO substrate, a mobile twodimensional electron gas (2DEG) forms on the STO side of the interface [1, 2]. One compelling feature of these interfaces is that the character of the 2DEG changes dramatically with the application of a gate voltage. Indeed, for (001) interfaces there is a narrow doping range over which the superconducting transition temperature [3][4][5][6][7], the spin-orbit coupling [7-10], and the metamagnetic response [11] change by an order of magnitude. Furthermore, the superconducting gap and resistive transition appear at different temperatures at low electron densities, n 2D , but track each other closely at high n 2D [12]. 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]. Indeed, we showed previously that, because of STO is close to a ferroelectric (FE) quantum critical point [32], electrons become deconfined from an ideal interface as n 2D → 0, and form a dilute quasi-threedimensional (quasi-3D) gas extending far into the STO substrate [33]. This result is incompatible with experiments and raises the question, why is only a single band occupied at low n 2D ? Furthermore, t...

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Modified transverse Ising model for the dielectric properties of SrTiO3 films and interfaces

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Anomalous Scaling of Parasitic Capacitance in FETs with a High-K Channel Material

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