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

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.