Interface effect on ferroelectricity at the nanoscale

Interfaces play a critical role in nanoscale ferroelectricity. We perform a first-principles study of ultrathin KNbO3 ferroelectric films placed between two metal electrodes, either SrRuO3 or Pt. We show that bonding at the ferroelectric-metal interfaces imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode. If the interface bonding is sufficiently strong, the ground-state represents a ferroelectric domain with an interface domain wall, driven by the intrinsic oppositely oriented dipole moments at the two interfaces. The critical thickness for the net polarization of the KNbO3 film is predicted to be about 1 nm for Pt and 1.8 nm for SrRuO3 electrodes.