Dispersion of the softE-mode polariton in BaTiO3

Abstract

In the Raman spectra of BaTi${\mathrm{O}}_3$, the soft $E$-symmetry phonon appears as an overdamped mode with its peak at zero frequency. Numerical fitting of this peak shape to the overdamped harmonic-oscillator model gives the quasiharmonic frequency of 33 ${\mathrm{cm}}^{-1\mathrm{}}$ for the right-angle scattering geometry. We have investigated the polariton behavior of this mode by means of near-forward Raman scattering. We found that for small wave vectors the quasiharmonic frequency of the $E$-mode polariton follows a dispersion curve similar to that for under-damped polaritons in other crystals. From the experimentally obtained dispersion curve of the quasiharmonic frequency, we obtained the value of the static dielectric constant ${\epsilon }_{\perp }\mathrm{}\left(0\right)\mathrm{}$ perpendicular to the $c$ axis to be 1250 ± 350. Our present study shows that the soft $E$ mode has the dispersive character appropriate for phonons.