Ferroelectric PbTiO ₃/SrRuO ₃ superlattices with broken inversion symmetry

We have fabricated PbTiO ₃/SrRuO ₃ superlattices with ultrathin SrRuO ₃ layers. Because of the superlattice geometry, the samples show a large anisotropy in their electrical resistivity, which can be controlled by changing the thickness of the PbTiO ₃ layers. Therefore, along the ferroelectric direction, SrRuO ₃ layers can act as dielectric, rather than metallic, elements. We show that, by reducing the concentration of PbTiO ₃, an increasingly important effect of polarization asymmetry due to compositional inversion symmetry breaking occurs. The results are significant as they represent a new class of ferroelectric superlattices, with a rich and complex phase diagram. By expanding our set of materials we are able to introduce new behaviors that can only occur when one of the materials is not a perovskite titanate. Here, compositional inversion symmetry breaking in bicolor superlattices, due to the combined variation of A and B site ions within the superlattice, is demonstrated using a combination of experimental measurements and first principles density functional theory.