First-principles studies of ultrathin ferroelectric capacitors with Ru-based perovskite electrodes*

First-principles calculations are used to investigate the electrostatics and polarization screening effects in ultrathin PbTiO$_{3}$ films capped with SrRuO$_{3}$ or CaRuO$_{3}$ electrodes under short-circuit boundary conditions. In accordance with previous results, we find that the SrRuO$_{3}$/PbTiO$_{3}$/SrRuO$_{3}$ system without antiferrodistortive octahedral rotations is ``non-pathological'' with respect to the metal/ferroelectric band alignment across the interface. Such rotations, however, have to be explicitly considered to correctly determine the band alignment and polarization screening in the SrRuO$_{3}$/PbTiO$_{3}$/SrRuO$_{3 }$nanocapacitor. (*) Present address: Vanderbilt University, Nashville, Tennessee 37235 and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831