Quasielastic light scattering in rutile, ZnSe, silicon, and SrTiO3

Quasielastic light scattering (QELS) has been investigated in the crystals of TiO2 (rutile), ZnSe, silicon, and SrTiO3. The temperature dependence of the linewidth for the QELS has been measured in detail by backward light scattering interferometry and by impulsive stimulated thermal scattering technique in the temperature range from 5 to 670 K. The quasielastically scattered spectra observed consist of two components, which can be classified into two types, namely, types I and II, depending on the linewidth. The analyses have shown that the linewidth of the QELS I changes from the well-known q2 to a q1 dependence with either decreasing temperature or increasing q, where q is the wave-vector transfer in the scattering experiment. It has been found that the linewidth of the QELS I in arbitrary phonon regimes including "hydrodynamic," "collisionless," and "intermediate" can be roughly estimated solely in terms of average sound velocity and the "phonon Knudsen number" q l̄, where l̄ is the mean free path of thermal phonons. A broad doublet spectrum, which was first reported by Hehlen in Phys. Rev. Lett. 75, 2416 (1995), has been observed in SrTiO3 at low temperatures, and its origin has been also discussed in terms of phonon Knudsen number.