Interface effect to the nanoscale ferroelectricity

Recent experimental results demonstrate that in thin films ferroelectricity persists down to film thickness of a few unit cells. This finding opens an avenue for novel electronic devices based on ultrathin ferroelectrics, but also raises questions about factors controlling ferroelectricity and the nature of the ferroelectric state at the nanoscale. Here we report a first-principles study of KNbO$_3$ ferroelectric thin films placed between two metal electrodes, either SrRuO$_3$ or Pt. We show that the bonding at the ferroelectric-metal interface imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode in thin ferroelectric films. This does not, however, quench local polarization. The critical thickness for the net polarization of the KNbO$_3$ film we found are about 1 nm for Pt and 1.8 nm for SrRuO$_3$ electrodes.