  We present an ab initio study of the (001) interfaces between two insulating
perovskites, the polar LaAlO3 and the nonpolar SrTiO3. We observe an
insulating-to-metallic transition above a critical LaAlO3 thickness. We explain
that the high conductivity observed at the TiO2 /LaO interface and the lack of
similar conductivity at the SrO/AlO2 interface are inherent in the atomic
geometry of the system. A large interfacial hopping matrix element between
cations causes the formation of a bound electron state at the TiO2 /LaO
interface. This mechanism for the formation of interfacial bound states
suggests a robust means for tuning conductivities at various oxide
heterointerfaces.
