Polar catastrophe and electronic reconstructions at the LaAlO3/SrTiO3 interface: evidence from optical second harmonic generation

TL;DR

This study uses optical second harmonic generation to investigate electronic reconstructions at the LaAlO3/SrTiO3 interface, revealing two abrupt electronic rearrangements associated with film thickness and conductivity onset.

Contribution

It provides direct optical evidence of electronic and ionic reconstructions at the interface, highlighting two critical thicknesses linked to electronic and conductive transitions.

Findings

Electronic rearrangements occur at 3 and 4-6 unit cells of LaAlO3.

Two physical scenarios proposed: localized electronic states transfer and ionic relaxations.

Reveals correlation between film thickness, electronic structure, and conductivity.

Abstract

The so-called "polar catastrophe", a sudden electronic reconstruction taking place to compensate for the interfacial ionic polar discontinuity, is currently considered as a likely factor to explain the surprising conductivity of the interface between the insulators LaAlO3 and SrTiO3. We applied optical second harmonic generation, a technique that a priori can detect both mobile and localized interfacial electrons, to investigating the electronic polar reconstructions taking place at the interface. As the LaAlO3 film thickness is increased, we identify two abrupt electronic rearrangements: the first takes place at a thickness of 3 unit cells, in the insulating state; the second occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for which the samples become conducting. Two possible physical scenarios behind these observations are proposed. The first is based on an electronic transfer into localized electronic states at the interface that acts as a precursor of the conductivity onset. In the second scenario, the signal variations are attributed to the strong ionic relaxations taking place in the LaAlO3 layer.