Metal versus insulator behavior in ultrathin SrTiO3-based heterostructures

Complex-oxide interfaces can give rise to two-dimensional electron gases (2DEGs) with extremely high densities: for SrTiO3/GdTiO3 (STO/GTO), a density of 1/2 electron per unit-cell area is found within the STO. In this work we use first-principles calculations to study GTO/STO/GTO heterostructures, where both interfaces contribute electrons to the STO. We find that for a thick STO layer the electrons from the interfaces delocalize over multiple TiO2 planes. Once the STO thickness is reduced below three layers, we find that the electrons localize on every other interfacial Ti atom, leading to an insulating phase. We attribute this localization to the combination of high electron density and distortions at the interface. This is further confirmed by a model of the transition based on electron doping of bulk STO, allowing for the same type of distortions as at the interface with GTO. These findings elucidate previous observations [P. Moetakef et al., Phys. Rev. B 86, 201102 (2012)], but our proposed physical mechanisms are general and should apply to other complex-oxide interfaces as well.