Modeling charge-imbalanced NaNbO3/SrTiO3 superlattices: Lattice relaxation and metallicity

Abstract

The electronic and structural properties of different charge-imbalanced perovskite oxide NaNbO3/SrTiO3 superlattices are investigated with density-functional theory (local density approximation and local spin density approximation+U) methods. Metallic or insulating behavior of such a superlattice depends on the types of interfaces present: nonstoichiometric composition of a superlattice introduces holes to O p orbitals or extra electrons to Nb/Ti d orbitals. Lattice parameters, superlattice volume, and the extent of conduction electron or hole states are found to depend on interface type. The extent of the metallic state may also depend on the NaNbO3/SrTiO3 ratio. Octahedral rotations and other low-symmetry phases increase the gap between p and d orbitals but do not affect metallicity. Adding a Hubbard U to account for possible electronic correlations does not affect electron localization. Within LSDA+U, the delocalized holes align ferromagnetically.