Theory of polarization enhancement in epitaxial BaTiO3/SrTiO3 superlattices

The spontaneous polarization of epitaxial BaTiO3/SrTiO3 superlattices is studied as a function of composition using first-principles density-functional theory within the local density approximation. With the in-plane lattice parameter fixed to that of bulk SrTiO3, the computed superlattice polarization is enhanced above that of bulk BaTiO3 for superlattices with BaTiO3 fraction larger than 40%. In contrast to their bulk paraelectric character, the SrTiO3 layers are found to be tetragonal and polar, possessing nearly the same polarization as the BaTiO3 layers. General electrostatic arguments elucidate the origin of the polarization in the SrTiO3 layers, with important implications for other nanostructured ferroelectrics.