Two-dimensional electron gas at the LaAlO$_3$/SrTiO$_3$ inteface with a potential barrier

TL;DR

This paper models the electronic properties of the LaAlO$_3$/SrTiO$_3$ interface using a tight binding approach, revealing a potential-dependent localized energy band and a valve effect that controls interfacial conductivity.

Contribution

It introduces a tight binding model incorporating an interface potential to explain charge localization and conductivity suppression at the LAO/STO interface, including a novel valve effect at a critical potential.

Findings

Charge carriers can form localized energy bands within the band gaps.

A critical potential strength causes the interface to become dispersionless, suppressing conductivity.

The model predicts a transition between quasielectron and quasihole regimes.

Abstract

We present a tight binding description of electronic properties of the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO). The description assumes LAO and STO perovskites as sets of atomic layers in the $x$-$y$ plane, which are weakly coupled by an interlayer hopping term along the $z$ axis. The interface is described by an additional potential, $U_0$, which simulates a planar defect. Physically, the interfacial potential can result from either a mechanical stress at the interface or other structural imperfections. We show that depending on the potential strength, charge carriers (electrons or holes) may form an energy band which is localized at the interface and is within the band gaps of the constituting materials (LAO and STO). Moreover, our description predicts a {\it valve effect} at a certain critical potential strength, $U_{0cr}$, when the interface potential works as a valve suppressing the interfacial conductivity. In other words, the interfacial electrons become dispersionless at $U_0= U_{0cr}$, and thus cannot propagate. This critical value separates the {\it quasielectron} ($U_0<$ $U_{0cr}$) and {\it quasihole} ($U_0>$ $U_{0cr}$) regimes of the interfacial conductivity.