Effect of the surface polarization in polar perovskites studied from   first principles
  The (001) surfaces of polar perovskites BaTiO$_3$ and PbTiO$_3$ have been studied from first principles at T=0 K. For both cases of polarization, the most stable TiO-terminated interfaces show intrinsic ferroelectricity. In the topmost layer, where the O atoms are $>$0.1 \AA above Ti, this leads to metallic instead of the insulating behavior of the electronic states that may have important implications for multiferroic tunneling junctions. 
M. Fechner, S. Ostanin and I. Mertig