Surface and Interface Effects on the Dielectric Polarization and Refractive Indices of BaTiO_3 Ultrathin Films

摘要

The surface effect on the dielectric polarization and refractive indices of BaTiO_3 ultrathin films on SrTiO_3 single crystal substrates is investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole-dipole interaction. The spontaneous polarization of the BaTiO_3/SrTiO_3 film is reduced 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_3films or for SrTiO_3 substrates to be strongly reduced for light polarized perpendicular to the surface.