  Artificial PbTiO$_{3}$/SrTiO$_{3}$ superlattices were constructed using
off-axis RF magnetron sputtering. X-ray diffraction and piezoelectric atomic
force microscopy were used to study the evolution of the ferroelectric
polarization as the ratio of PbTiO$_{3}$ to SrTiO$_{3}$ was changed. For
PbTiO$_{3}$ layer thicknesses larger than the 3-unit cells SrTiO$_{3}$
thickness used in the structure, the polarization is found to be reduced as the
PbTiO$_{3}$ thickness is decreased. This observation confirms the primary role
of the depolarization field in the polarization reduction in thin films. For
the samples with ratios of PbTiO$_{3}$ to SrTiO$_{3}$ of less than one a
surprising recovery of ferroelectricity that cannot be explained by
electrostatic considerations was observed.
