Dielectric polarization and refractive indices of ultrathin barium titanate films on strontiumtitanate single crystals

Abstract

The electrical and optical properties of ultrathin films of tetragonal barium titanate(BaTiO3) on strontiumtitanate (SrTiO3) single-crystal substrates are theoretically investigated using a microscopic quantummechanical model based on the orbital approximation in correlation with thedipole–dipole interaction. First-, second-, and third-order electronic polarizabilitieshad to be considered in this calculation in order to obtain accurate results forboth the dielectric spontaneous polarization and the refractive indices of theBaTiO3 overlayer. The spontaneous polarization is drastically reduced in the film as itsthickness decreases. However, an electronic polarization appears within theSrTiO3 substratein the neighbourhood of the interface. This polarization, which vanishes far away from the interface into theSrTiO3 bulk, is induced bythe polarization of the BaTiO3 film. Furthermore, we find the refractive index for both theBaTiO3 filmand the SrTiO3 substrate to be deeply reduced for light polarized perpendicular to the surface.