Soft-mode spectroscopy of epitaxial BaTiO3/SrTiO3 superlattices

It is demonstrated that soft modes in 240 nm think BaTiO3/SrTiO3 ferroelectric superlattices can be probed by infrared reflectivity measurements, which allows us to estimate the intrinsic in-plane dielectric constants of such thin superlattice slabs. The infrared reflectance spectrum of the (BaTiO3)(8)/(SrTiO3)(4) superlattice epitaxially grown a DyScO3 single-crystal substrate reveals a strong polar mode with a transverse optic frequency of about 70 cm(-1) and moderate damping (about 30 cm(-1)). Our analysis of the experimental results is combined with the first-principles based theoretical evaluation of optical polar modes in a model system representing the 60-atom periodic superlattice unit cell. This hybrid approach suggests that the observed soft mode is associated with a Slater-type polar mode confined mostly in the SrTiO3 component layer.