Avoiding the polarization catastrophe in LaAlO3 overlayers on SrTiO3(001) through a polar distortion

TL;DR

This study reveals that a uniform polar distortion in LaAlO3 overlayers on SrTiO3(001) prevents the polarization catastrophe, maintaining insulating behavior up to several monolayers through a novel ionic stabilization mechanism.

Contribution

It uncovers a previously unrecognized polar distortion mechanism that stabilizes LaAlO3 overlayers and prevents electronic reconstruction, based on density functional theory calculations.

Findings

Polar distortion extends over several unit cells.

The system remains insulating up to about five monolayers.

The band gap decreases with increasing LaAlO3 thickness before transition.

Abstract

A pronounced uniform polar distortion extending over several unit cells enables thin LaAlO3 overlayers on SrTiO3(001) to counteract the charge dipole and thereby neutralize the "polarization catastrophe" that is suggested by simple ion-counting. This unanticipated mechanism, obtained from density functional theory calculations, allows several unit cells of the LaAlO3 overlayer to remain insulating (hence, fully ionic). The band gap of the system, defined by occupied O $2p$ states at the surface and unoccupied Ti 3d states at the interface in some cases $\sim$20 \AA distant, decreases with increasing thickness of the LaAlO3-film before an insulator-to-metal transition and a crossover to an electronic reconstruction occurs at around five monolayers of LaAlO3.