1,198 research outputs found
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
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
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
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
Astrophysical and local constraints on string theory: runaway dilaton models
One of the clear predictions of string theory is the presence of a dynamical
scalar partner of the spin-2 graviton, known as the dilaton. This will violate
the Einstein Equivalence Principle, leading to a plethora of possibly
observable consequences which is a cosmological context include dynamical dark
energy and spacetime variations of nature's fundamental constants. The runaway
dilaton scenario of Damour, Piazza and Veneziano is a particularly interesting
class of string theory inspired models which can in principle reconcile a
massless dilaton with experimental data. Here we use the latest background
cosmology observations, astrophysical and laboratory tests of the stability of
the fine-structure constant and local tests of the Weak Equivalence Principle
to provide updated constraints on this scenario, under various simplifying
assumptions. Overall we find consistency with the standard CDM
paradigm, and we improve the existing constraints on the coupling of the
dilaton to baryonic matter by a factor of six, and to the dark sector by a
factor of two. At the one sigma level the current data already excludes dark
sector couplings of order unity, which would be their natural value.Comment: 7 pages, 4 figures; Phys. Rev. D (in press
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
Charge migration engineered by localisation: electron-nuclear dynamics in polyenes and glycine
We demonstrate that charge migration can be âengineeredâ in arbitrary molecular systems if a single localised orbital â that diabatically follows nuclear displacements â is ionised. Specifically, we describe the use of natural bonding orbitals in Complete Active Space Configuration Interaction (CASCI) calculations to form cationic states with localised charge, providing consistently well-defined initial conditions across a zero point energy vibrational ensemble of molecular geometries. In Ehrenfest dynamics simulations following localised ionisation of -electrons in model polyenes (hexatriene and decapentaene) and -electrons in glycine, oscillatory charge migration can be observed for several femtoseconds before dephasing. Including nuclear motion leads to slower dephasing compared to fixed-geometry electron-only dynamics results. For future work, we discuss the possibility of designing laser pulses that would lead to charge migration that is experimentally observable, based on the proposed diabatic orbital approach
Electron and nuclear dynamics following ionisation of modified bismethylene-adamantane
We have simulated the coupled electron and nuclear dynamics using the Ehrenfest method upon valence ionisation of modified bismethylene-adamantane (BMA) molecules where there is an electron transfer between the two Ï bonds. We have shown that the nuclear motion significantly affects the electron dynamics after a few fs when the electronic states involved are close in energy. We have also demonstrated how the non-stationary electronic wave packet determines the nuclear motion, more precisely the asymmetric stretching of the two Ï bonds, illustrating âcharge-directed reactivityâ. Taking into account the nuclear wave packet width results in the dephasing of electron dynamics with a half-life of 8 fs; this eventually leads to the equal delocalisation of the hole density over the two methylene groups and thus symmetric bond lengths
Anharmonic vs. relaxational sound damping in glasses: II. Vitreous silica
The temperature dependence of the frequency dispersion in the sound velocity
and damping of vitreous silica is reanalyzed. Thermally activated relaxation
accounts for the sound attenuation observed above 10 K at sonic and ultrasonic
frequencies. Its extrapolation to the hypersonic regime reveals that the
anharmonic coupling to the thermal bath becomes important in
Brillouin-scattering measurements. At 35 GHz and room temperature, the damping
due to this anharmonicity is found to be nearly twice that produced by
thermally activated relaxation. The analysis also reveals a sizeable velocity
increase with temperature which is not related with sound dispersion. This
suggests that silica experiences a gradual structural change that already
starts well below room temperature.Comment: 13 pages with 8 figure
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