Theory of polarization enhancement in epitaxial BaTiO₃/SrTiO₃ superlattices

The spontaneous polarization of epitaxial BaTiO$_3$/SrTiO$_3$ 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 SrTiO$_3$, the computed superlattice polarization is enhanced above that of bulk BaTiO$_3$ for superlattices with BaTiO$_3$ fraction larger than 40%. In contrast to their bulk paraelectric character, the SrTiO$_3$ layers are found to be {\it tetragonal and polar}, possessing nearly the same polarization as the BaTiO$_3$ layers. General electrostatic arguments elucidate the origin of the polarization in the SrTiO$_3$ layers, with important implications for other ferroelectric nanostructures.