  First-principles density functional calculations are performed to investigate
the interplay between inplane strains and interface effects in 1by1
PbTiO3/SrTiO3 and BaTiO3/SrTiO3 superlattices of tetragonal symmetry. One
particular emphasis of this study is to conduct side-by-side comparisons on
various ferroelectric properties in short-period superlattices and in
constituent bulk materials, which turns out to be rather useful in terms of
obtaining valuable insight into the different physics when ferroelectric bulks
form superlattices. The various properties that are studied in this work
include the equilibrium structure, strain dependence of mixing energy,
microscopic ferroelectric off-center displacements, macroscopic polarization,
piezoelectric coeffcients, effective charges, and the recently formulated
k-dependent polarization dispersion structure. The details of our findings are
rather lengthy, and are summarized in Sec. IV.
