960 research outputs found
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
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
Observation of the onset of strong scattering on high frequency acoustic phonons in densified silica glass
The linewidth of longitudinal acoustic waves in densified silica glass is
obtained by inelastic x-ray scattering. It increases with a high power alpha of
the frequency up to a crossover where the waves experience strong scattering.
We find that \alpha is at least 4, and probably larger. Resonance and
hybridization of acoustic waves with the boson-peak modes seems to be a more
likely explanation for these findings than Rayleigh scattering from disorder.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Letter
Universal behavior of internal friction in glasses below T : anharmonicity vs relaxation
Comparison of the internal friction at hypersonic frequencies between a few K
and the glass transition temperature Tg for various glasses brings out general
features. At low temperature, internal friction is only weakly dependent on the
material. At high temperature but still below Tg the internal friction for
strong glasses shows a T-independent plateau in a very wide domain of
temperature; in contrast, for fragile glass, a nearly linear variation of
internal friction with T is observed. Anharmonicity appears dominant over
thermally activated relaxational processes at high temperature.Comment: accepted in Physical Review
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
Eliminating the broadening by finite aperture in Brillouin spectroscopy
We present a new optical arrangement which allows to avoid the broadening by
finite aperture in Brillouin spectroscopy. In this system, all the rays
scattered at the same angle by the whole scattering volume are collected on a
single pixel of the area detector. This allows to use large collection angles,
increasing the luminosity without lowering the accuracy of the frequency-shift
and linewidth measurements. Several results of experimental checks are
provided, showing the efficiency of the device.Comment: 14 pages, 8 figure
Small Angle Scattering by Fractal Aggregates: A Numerical Investigation of the Crossover Between the Fractal Regime and the Porod Regime
Fractal aggregates are built on a computer using off-lattice cluster-cluster
aggregation models. The aggregates are made of spherical particles of different
sizes distributed according to a Gaussian-like distribution characterised by a
mean and a standard deviation . The wave vector dependent
scattered intensity is computed in order to study the influence of the
particle polydispersity on the crossover between the fractal regime and the
Porod regime. It is shown that, given , the location of the
crossover decreases as increases. The dependence of on
can be understood from the evolution of the shape of the center-to-center
interparticle-distance distribution function.Comment: RevTex, 4 pages + 6 postscript figures, compressed using "uufiles",
published in Phys. Rev. B 50, 1305 (1994
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
The Boson Peak and its Relation with Acoustic Attenuation in Glasses
Experimental results on the density of states and on the acoustic modes of
glasses in the THz region are compared to the predictions of two categories of
models. A recent one, solely based on an elastic instability, does not account
for most observations. Good agreement without adjustable parameters is obtained
with models including the existence of non-acoustic vibrational modes at THz
frequency, providing in many cases a comprehensive picture for a range of glass
anomalies.Comment: 4 pages, 3 figures, Physical Review Letters in pres
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
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