Quasielastic Light Scattering and Second Sound in SrTiO3

We study the light-scattering spectrum in SrTiO 3 in the low frequency range (< 1000 GHz). Two components of quasielastic peaks were observed at relatively high temperatures. We found that the collective phonon-excitation in a crystal accounts for both quasielastic components. The anomalous broad Brillouin component in SrTiO 3 , which was reported previously, can also be accounted for by our model, and it has been strongly indicated that the origin of the broad doublet is “second sound”. The broader quasielastic component arises from the interactions between two second sounds, and it can be understood as the “Mountain mode” in crystals.