Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces

Abstract

Polar discontinuities occurring at interfaces between two materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates, or charge transfer. In the latter case, one may expect a local electronic doping of one material: one example is the two‐dimensional electron liquid (2DEL) appearing in SrTiO3 once covered by a polar LaAlO3 layer. Here, it is shown that tuning the formal polarization of a (La,Al)1− x(Sr,Ti)xO3 (LASTO:x) overlayer modifies the quantum confinement of the 2DEL in SrTiO3 and its electronic band structure. The analysis of the behavior in magnetic field of superconducting field‐effect devices reveals, in agreement with ab initio calculations and self‐consistent Poisson–Schrödinger modeling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on the interface total charge densities. These results strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO3/SrTiO3 interface and demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.

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

“…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].…”

Manifold field effects at a complex oxide interface

“…The most direct comparison can be made to other recent gating experiments (55,113,129,132,173,204,240), with which our experimental results and conclusions are in good agreement. In parallel with and following the publication of Chapter 4, the redistribution of carriers among bands above the d xz,yz Lifshitz transition has been quite firmly established, and seems to be a universal effect in SrTiO 3 -based heterostructures (132,204,240).…”

“…Center-of-mass (nm) and even on superconductivity (113). We observe that the COM stays fairly constant with topgating, whereas with backgating it increases by about a factor of 3 over the gate voltage range studied.…”

“…the ones discussed in (104)(105)(106)(107)(108)(109)(110). By using solid solutions like La x Sr 2y Al x+y Ta y O 3 or (LaAlO 3 ) 0.5 (SrTiO 3 ) 0.5 , the polar discontinuity can be weakened, leading to a higher critical thickness and an alteration of interface properties (111)(112)(113). All these systems may have different properties emerging from the interaction of the SrTiO 3 surface with the (polar) overlayer, although their electronic structure is remarkably similar (114).…”

“…Carrier density of the surface states must be equally dependent on temperature (134). In an attempt to unite superconductivity in bulk SrTiO 3 and at its interface, Li et al recently employed Schrödinger-Poisson calculations to predict when the three-dimensional carrier density is high enough to trigger superconductivity (113).…”