On the character of the insulator-metal transition at polar-nonpolar perovskite interfaces

A point that is still unclear relating to the two-dimensional electron gas (2DEG) observed at the interfaces between a polar perovskite thin film and a non-polar perovskite substrate, is the character of the insulator to metal transition defining its onset. It is relevant, since, in addition to film thickness, the transition can in principle be induced by an electric across the film for a thickness close to critical, and its being continuous or discontinuous can have important implications. Both kinds of switching have been proposed. Although octahedral tilts have been discussed in systems like LAO/STO, their effect on the 2DEG onset has not been discussed. We propose a theory of 2DEG formation at a polar-nonpolar interface which includes coupling to tilts. We find that they change the character of the transition, in addition to altering the critical value of thickness or electric field. Five distinct transition scenarios are obtained as possible, depending only on the energetics of the tilts, their coupling with the polar mode, and the polar discontinuity, and irrespective of whether the transition occurs via thickness or electric field. We will present first-principles results constraining the picture arising.