No abstract

We study collective phonon excitations in SrTiO3 by low-frequency light scattering. We employ extended thermodynamics for phonon gas to construct a theoretical spectral function that is applicable regardless of local thermal equilibrium. Our analysis reveals the temperature dependence of tauN, the relaxation time for the momentum-conserving phonon collisions (normal processes), in SrTiO3. These results indicate that the previously reported anomalous soundlike spectrum originates from second sound, which is a wavelike propagation of heat.

We report the high-resolution and broadband light-scattering spectroscopy of a single crystal of a prototypical relaxor ferroelectric, Pb(Mg(1/3)Nb(2/3))O(3). A self-similar broad central peak, whose intensity is expressed as I(ω) [Symbol: see text] ω(α) has been observed, indicating the presence of a fractal in the crystal. A strong correspondence exists between the temperature dependence of the exponent α and that of the reported behaviors of polar nanoregions. The estimated fractal dimension (d(f) ≈ 2.6) at low temperatures clearly indicates a percolation transition of the polar nanoregions at around 240 K.

Quasielastic light scattering consisting of two components has been observed in single crystals of rutile (TiO 2). The broad component with a linewidth of 330 GHz at 297 K becomes narrower with decreasing temperature. In contrast, the narrow component, which has a linewidth of 1.1 GHz at 297 K, broadens as the temperature decreases. We present a unified explanation for both components, which is based on two-phonon difference Raman scattering. For the narrow component, it will be shown that two-phonon difference processes from a single acoustic phonon branch explain the temperature dependence of the intensity and the wave vector dependence of the linewidth at low temperatures. The conventional explanation in terms of entropy-fluctuation scattering is also attempted for temperatures above 200 K where phonon collisions occur more frequently than at lower temperatures and the phonon system may be considered hydrodynamically. For the broad component, two-phonon difference scattering from different phonon branches will be shown to provide a good explanation for the temperature dependent intensity. Furthermore, temperature dependence of the linewidth and its insensitivity to changes in scattering wave vector are also explained with this model. ͓S0163-1829͑99͒02642-9͔

Quasielastic light scattering ͑QELS͒ has been investigated in the crystals of TiO 2 ͑rutile͒, ZnSe, silicon, and SrTiO 3 . 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 q 2 to a q 1 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" ql, where l is the mean free path of thermal phonons. A broad doublet spectrum, which was first reported by Hehlen et al. in Phys. Rev. Lett. 75, 2416 ͑1995͒, has been observed in SrTiO 3 at low temperatures, and its origin has been also discussed in terms of phonon Knudsen number.