Collective phonon excitation in KTaO₃

The collective excitation of phonons, including thermal diffusion and second sound (temperature wave), in KTaO₃ has been studied by low-frequency light-scattering and by time-domain light-scattering techniques. The observed spectra are analyzed with the new spectral formula, which is applicable consistently from hydrodynamic to ballistic (collisionless) phonon regimes. Our analysis of the quasielastic scattering and the anomalous soundlike peak (broad doublet) determines the temperature dependences of the phonon relaxation times for both the normal and resistive processes of the phonon-phonon scattering. It is shown that the normal process is anomalously enhanced at low temperatures as theoretically predicted, and the “frequency window” for the propagating second sound is defined approximately below 30 K.