Surface and interface effects on the dielectric polarization and refractive indices of BaTiO3 single crystals and ultrathin films

The surface and interface effects on the dielectric polarization and refractive indices of BaTiO
₃
single crystals and BaTiO
₃
ultrathin films on SrTiO
₃
single crystal substrates are investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole-dipole interaction. The spontaneous polarization of BaTiO
₃
single crystals is drastically reduced near the c-surface. For BaTiO
₃
/SrTiO
₃
films, the spontaneous polarization is reduced in the film as its thickness decreases. However, an electronic polarization appears within the SrTiO
₃
substrate in the neighborhood of the interface. This polarization, which vanishes far away from the interface into the SrTiO
₃
bulk, is induced by the polarization of the BaTiO
₃
film. Furthermore, we find the refractive index either for BaTiO
₃
single crystals or for BaTiO
₃
films and SrTiO
₃
substrates to be strongly reduced for light polarized perpendicular to the surface.