It is demonstrated that soft modes in 240 nm think ${\text{BaTiO}}_3/{\text{SrTiO}}_3$ 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 ${\left({\text{BaTiO}}_3\right)}_8/{\left({\text{SrTiO}}_3\right)}_4$ superlattice epitaxially grown a ${\text{DyScO}}_3$ single-crystal substrate reveals a strong polar mode with a transverse optic frequency of about $70{\text{cm}}^{-1}$ and moderate damping (about $30{\text{cm}}^{-1}$). 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 ${\text{SrTiO}}_3$ component layer.

• Received 17 September 2010

DOI:https://doi.org/10.1103/PhysRevB.82.224102