955 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
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
Cdk5 phosphorylation of huntingtin reduces its cleavage by caspases: implications for mutant huntingtin toxicity
Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded polyglutamine (polyQ) tract in the huntingtin (htt) protein. Mutant htt toxicity is exposed after htt cleavage by caspases and other proteases release NH2-terminal fragments containing the polyQ expansion. Here, we show htt interacts and colocalizes with cdk5 in cellular membrane fractions. Cdk5 phosphorylates htt at Ser434, and this phosphorylation reduces caspase-mediated htt cleavage at residue 513. Reduced mutant htt cleavage resulting from cdk5 phosphorylation attenuated aggregate formation and toxicity in cells expressing the NH2-terminal 588 amino acids (htt588) of mutant htt. Cdk5 activity is reduced in the brains of HD transgenic mice compared with controls. This result can be accounted for by the polyQ-expanded htt fragments reducing the interaction between cdk5 and its activator p35. These data predict that the ability of cdk5 phosphorylation to protect against htt cleavage, aggregation, and toxicity is compromised in cells expressing toxic fragments of htt
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
Influence of thermal history on the structure and properties of silicate glasses
We studied a set of float glass samples prepared with different fictive
temperature by previous annealing around the glass transition temperature. We
compared the results to previous measurements on a series of amorphous silica
samples, also prepared with different fictive temperature. We showed that the
modifications on the structure at a local scale are very small, the changes of
physical properties are moderate but the changes on density fluctuations at a
nanometer scale are rather large: 12 and 20% in float glass and silica, for
relative changes of fictive temperature equal to 13 and 25% respectively. Local
order and mechanical properties of silica vary in the opposite way compared to
float glass (anomalous behavior) but the density fluctuations in both glasses
increase with temperature and fictive temperature
News from the Swampland -- Constraining string theory with astrophysics and cosmology
Our current best guess for a unified theory of gravitation and quantum field
theory (string theory) generically predicts a set of requirements for a
consistently quantized theory, the Swampland criteria. Refined versions of
these criteria have recently been shown to be in mild tension with cosmological
observations. We summarize the status of the current impact of and constraints
on the Swampland conjectures from cosmology, and subject a variety of dark
energy quintessence models to recently released cosmological datasets. We find
that instead of tightening the tension, the new data allows for slightly more
freedom in the Swampland criteria. We further demonstrate that if there is no
theoretical argument made to prevent interactions of the moduli fields with the
electromagnetic sector, a novel fine-tuning argument arises from the extremely
tight current constraints on such interactions. Finally, we conclude with a
cautionary tale on model-independent reconstructions of the Swampland criteria
from expansion rate data.Comment: 35 pages, 20 figures, 4 tables. All comments are welcome! [v2: Added
citations, corrected eq. 2.6
Runaway dilaton models: improved constraints from the full cosmological evolution
One of the few firm predictions of string theory is the existence of a
massless scalar field coupled to gravity, the dilaton. In its presence, the
value of the fundamental constants of the universe, such as the fine-structure
constant, will vary with the time-dependent vacuum expectation value of this
field, in direct violation of the Einstein Equivalence Principle. The
\emph{runaway dilaton} proposed by Damour, Piazza, and Veneziano provides a
physically motivated cosmological scenario which reconciles the existence of a
massless dilaton with observations, while still providing non-standard and
testable predictions. Furthermore, the field can provide a natural candidate
for dynamical dark energy. While this model has been previously constrained
from local laboratory experiments and low-redshift observations, we provide
here the first full self-consistent constraints, also including high redshift
data, in particular from the cosmic microwave background. We consider various
possible scenarios in which the field could act as quintessence. Despite the
wider parameter space, we make use of recent observational progress to
significantly improve constraints on the model's coupling parameters, showing
that order unity couplings (which would be natural in string theory) are ruled
out.Comment: Published in Phys. Rev.
Fluctuating Bond Aggregation: a Model for Chemical Gel Formation
The Diffusion-Limited Cluster-Cluster Aggregation (DLCA) model is modified by
including cluster deformations using the {\it bond fluctuation} algorithm. From
3 computer simulations, it is shown that, below a given threshold value
of the volumic fraction , the realization of all intra-aggregate
bonding possibilities prevents the formation of a gelling network. For ,
the sol-gel transition occurs at a time which, in contrast to DLCA,
doesnot diverge with the box size. Several results are reported including small
angle scattering curves and possible applications are discussed.Comment: RevTex, 9 pages + 3 postscript figures appended using "uufiles". To
appear in Phys. Rev. Let
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