Interface effect on ferroelectricity at the nanoscale

Interfaces play a critical role in nanoscale ferroelectricity. We perform a first-principles study of ultrathin KNbO ₃ ferroelectric films placed between two metal electrodes, either SrRuO ₃ 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 KNbO ₃ film is predicted to be about 1 nm for Pt and 1.8 nm for SrRuO ₃ electrodes.