Optical mode crossings and the low temperature anomalies of SrTiO3

Optical mode crossing is not a plausible explanation for the new broad Brillouin doublet nor for the strong acoustic anomalies observed at low temperatures in SrTiO3. Data presented to support that explanation are also inconclusive.Comment: This is a comment to a paper from J.F. Scott (same ZFP volume

Glass-specific behavior in the damping of acoustic-like vibrations

High frequency sound is observed in lithium diborate glass, Li$_2$O--2B$_2$O$_3$, using Brillouin scattering of light and x-rays. The sound attenuation exhibits a non-trivial dependence on the wavevector, with a remarkably rapid increase towards a Ioffe-Regel crossover as the frequency approaches the boson peak from below. An analysis of literature results reveals the near coincidence of the boson-peak frequency with a Ioffe-Regel limit for sound in {\em all} sufficiently strong glasses. We conjecture that this behavior, specific to glassy materials, must be quite universal among them.Comment: 4 pages, 4 figures, revised versio

Hyper-Raman scattering analysis of the vibrations in vitreous boron oxide

Hyper-Raman scattering has been measured on vitreous boron oxide, $v-$B$_2$O$_3$. This spectroscopy, complemented with Raman scattering and infrared absorption, reveals the full set of vibrations that can be observed with light. A mode analysis is performed based on the local D$_{3h}$ symmetry of BO$_3$ triangles and B$_3$O$_3$ boroxol rings. The results show that in $v-$B$_2$O$_3$ the main spectral components can be succesfully assigned using this relatively simple model. In particular, it can be shown that the hyper-Raman boson peak arises from external modes that correspond mainly to librational motions of rigid boroxol rings.Comment: 13 pages, 11 figures, 2 table

Hyper-Raman scattering from vitreous boron oxide: coherent enhancement of the boson peak

Hyper-Raman scattering spectra of vitreous B$_2$O$_3$ are reported and compared to Raman scattering results. The main features are indexed in terms of vibrations of structural units. Particular attention is given to the low frequency boson peak which is shown to relate to out-of-plane librations of B$_3$O$_3$ boroxol rings and BO$_3$ triangles. Its hyper-Raman strength is comparable to that of cooperative polar modes. It points to a sizeable coherent enhancement of the hyper-Raman signal compared to the Raman one. This is explained by the symmetry of the structural units.Comment: 4 pages, 3 figure

The broad Brillouin doublets and central peak of KTaO_3

The incipient ferroelectric KTaO3 presents low-T Brillouin spectra anomalies,e.g. a broad central peak (CP), and some additional Brillouin doublets (BD), whose origin is interpreted in terms of phonon-density fluctuation processes. A parameterisation from new extensive high-resolution neutron-scattering measurements is used to show that hydrodynamic second sound from high damping (compared to BD frequency) TA phonons may exist in the crystal. Furthermore, low damping thermal phonons may scatter light through two-phonon difference processes and appear on the Brillouin spectra either as a sharp or a broader BD, depending on the phonon damping and group velocity . The comparison between computed anisotropies and experimental measurements favours the second process.Comment: 3 pages, 1 figure, ECNS99 Proceedings. See http://www.ill.fr

Scaling the Temperature-dependent Boson Peak of Vitreous Silica with the high-frequency Bulk Modulus derived from Brillouin Scattering Data

The position and strength of the boson peak in silica glass vary considerably with temperature $T$. Such variations cannot be explained solely with changes in the Debye energy. New Brillouin scattering measurements are presented which allow determining the $T$-dependence of unrelaxed acoustic velocities. Using a velocity based on the bulk modulus, scaling exponents are found which agree with the soft-potential model. The unrelaxed bulk modulus thus appears to be a good measure for the structural evolution of silica with $T$ and to set the energy scale for the soft potentials.Comment: Accepted for publication in Physical Review Letter

Acoustic damping in Li2_2O-2B2_2O3_3 glass observed by inelastic x-ray and optical Brillouin scattering

The dynamic structure factor of lithium-diborate glass has been measured at several values of the momentum transfer $Q$ using high resolution inelastic x-ray scattering. Much attention has been devoted to the low $Q$-range, below the observed Ioffe-Regel crossover \qco{}$\simeq$ 2.1 nm$^{-1}$. We find that below \qco{}, the linewidth of longitudinal acoustic waves increases with a high power of either $Q$, or of the frequency $\Omega$, up to the crossover frequency \OMco{} $\simeq$ 9 meV that nearly coincides with the center of the boson peak. This new finding strongly supports the view that resonance and hybridization of acoustic waves with a distribution of rather local low frequency modes forming the boson peak is responsible for the end of acoustic branches in strong glasses. Further, we present high resolution Brillouin light-scattering data obtained at much lower frequencies on the same sample. These clearly rule out a simple $\Omega^2$-dependence of the acoustic damping over the entire frequency range.Comment: 5 pages, 5 figures, submitted to the proceedings of IDMRCS 2005, Lille, Franc