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 $\Lambda$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, $v-$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

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

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.

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

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$d$ computer simulations, it is shown that, below a given threshold value $c_g$ of the volumic fraction $c$, the realization of all intra-aggregate bonding possibilities prevents the formation of a gelling network. For $c>c_g$, the sol-gel transition occurs at a time $t_g$ 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