Supertetragonal BaSnO3 induced giant ferroelectricity in SrTiO3/BaSnO3 superlattices

Abstract: Perovskite BaSnO3 has an Sn s-orbital conduction band minimum, which makes it of interest as a transparent-conducting oxide parent compound but also contraindicates the ferroelectric instability characteristic of the related compound BaTiO3. In this work, we studied the effect of (001) compressive strain on BaSnO3 using first-principles methods. We found that, with low compressive strain, symmetry breaking takes cubic BaSnO3 to a nonpolar tetragonal state, with a first-order phase transition to a hidden highly-polarized ferroelectric supertetragonal state at about -5%. Based on the facts that the mismatch of lattice constant in experiment between BaSnO3 and SrTiO3 is about -5.2% and coherent growth of BaSnO3 on SrTiO3 has been experimentally realized for BaSnO3 layers thinner than 3 unit-cells, we studied a series of SrTiO3/BaSnO3 superlattices with one or two unit-cells of BaSnO3 and several unit-cells of SrTiO3. We found that the superlattices are ferroelectric with large polarizations. We propose that the origin of ferroelectricity in the superlattices is the mechanical and electrical coupling of the BaSnO3 and SrTiO3 layers, with polarized supertetragonal state of BaSnO3 induced by compressive-strain from the SrTiO3 layers and polarization of the SrTiO3 layers by the polar BaSnO3 layers. Due to the distinctive electronic states in the BaSnO3 layers, the realization of ferroelectricity holds promise for the design of novel electronic devices.