Theory of polarization enhancement in epitaxial BaTiO${}_3$/SrTiO${}_3$ superlattices

Abstract

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.

4 pages, 2 Figures, 1 Table