  The effect of intermixing at the interface of short period
PbTiO$_3$/SrTiO$_3$ superlattices is studied using first-principles density
functional theory. The results indicate that interfacial intermixing
significantly enhances the polarization within the superlattice. This
enhancement is directly related to the off-centering of Pb and Sr cations and
can be explained through a discussion of interacting dipoles. This picture
should be general for a wide range of multicomponent superlattices and may have
important consequences for the design of ferroelectric devices.
