Understanding polarization bias in PbTiO3/SrTiO3 and PbTiO3/SrRuO3 superlattices

We use first principles density functional theory to explain how Pb-O divacancies (DV) and other symmetry breaking factors create a preferred polarization state leading to a bias as seen in experiments. Our results for PbTiO$_3$/SrTiO$_3$ (PTO/STO) indicate that Pb-O DV prefer to form on one side of the interface than the other during growth, leading to a preferred polarization state. For PbTiO$_3$/SrRuO$_3$ (PTO/SRO) the situation is more complicated due to intrinsic compositional inversion symmetry breaking, which competes with the tendency of DV to form at different sites within the superlattice structure.