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 BaTiO₃ single crystals and BaTiO₃ ultrathin films on SrTiO₃ 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 BaTiO₃ single crystals is drastically reduced near the c-surface. For BaTiO₃/SrTiO₃ films, the spontaneous polarization is reduced in the film as its thickness decreases. However, an electronic polarization appears within the SrTiO₃ substrate in the neighborhood of the interface. This polarization, which vanishes far away from the interface into the SrTiO₃ bulk, is induced by the polarization of the BaTiO₃ film. Furthermore, we find the refractive index either for BaTiO₃ single crystals or for BaTiO₃ films and SrTiO₃ substrates to be strongly reduced for light polarized perpendicular to the surface.