350 research outputs found
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,
LiO--2BO, 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,
BO. 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 symmetry
of BO triangles and BO boroxol rings. The results show that in
BO 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
VUV Brillouin scattering from superpolished vitreous silica
A previous inelastic UV scattering experiment on silica glass is reproduced
using a high grade superpolished sample. In the pristine sample condition,
surface scattering is not observable compared to Rayleigh scattering from the
bulk. However, exposure to a fluence of the order of 100 J/cm at photon
energies slightly below the electronic gap generates observable surface damage.
This occurs after a few hours illumination with the monochromatic spectrometer
beam. No anomaly in the Brillouin linewidth was found up to an excitation
energy of 7.8 eV.Comment: 4 pages, 4 figures, revised manuscrip
Hyper-Raman scattering from vitreous boron oxide: coherent enhancement of the boson peak
Hyper-Raman scattering spectra of vitreous BO 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
BO boroxol rings and BO 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 . Such variations cannot be explained solely with changes
in the Debye energy. New Brillouin scattering measurements are presented which
allow determining the -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 and to set the
energy scale for the soft potentials.Comment: Accepted for publication in Physical Review Letter
Acoustic damping in LiO-2BO 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 using high resolution inelastic
x-ray scattering. Much attention has been devoted to the low -range, below
the observed Ioffe-Regel crossover \qco{} 2.1 nm. We find that
below \qco{}, the linewidth of longitudinal acoustic waves increases with a
high power of either , or of the frequency , up to the crossover
frequency \OMco{} 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 -dependence of the acoustic damping
over the entire frequency range.Comment: 5 pages, 5 figures, submitted to the proceedings of IDMRCS 2005,
Lille, Franc
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