Soft-mode spectroscopy in cubic BaTiO3by hyper-Raman scattering

Abstract

Hyper-Raman scattering from cubic BaTi${\mathrm{O}}_3$ is studied to clarify the controversies about the low-frequency dielectric response in this material. Applying the fluctuation-dissipation theorem, we obtain the imaginary part ${\epsilon }^{\prime \prime }\left(\Omega \right)$ of the dielectric function in the wave-number range from 3 to 150 ${\mathrm{cm}}^{-1\mathrm{}}$. ${\epsilon }^{\prime \prime }\left(\Omega \right)$ can be adequately described by a classical single-oscillator dispersion formula. In approaching the Curie temperature, we find a continuous decrease of the mode frequency ${\Omega }_0$. The relative damping constant $\frac{\gamma }{{\Omega }_0}$ exceeds 2, so that the mode may be referred to as intermediate between oscillator and relaxator. Because of the high damping ${\epsilon }^{\prime \prime }\left(\Omega \right)$ can be formally written as the sum of two overdamped oscillator contributions. This would lead to the concept of a soft-mode saturation and an extra dispersion mechanism as has been recently inferred from the far-infrared reflectivity spectrum. However, we do not find any evidence for this mode splitting and, so far, regard it as artificial.