550 research outputs found
Non-dynamic origin of the acoustic attenuation at high frequency in glasses
The sound attenuation in the THz region is studied down to T=16 K in glassy
glycerol by inelastic x-ray scattering. At striking variance with the decrease
found below 100 K in the GHz data, the attenuation in the THz range does not
show any T dependence. This result i) indicates the presence of two different
attenuation mechanisms, active respectively in the high and low frequency
limits; ii) demonstrates the non-dynamic origin of the attenuation of THz sound
waves, and confirms a similar conclusion obtained in SiO2 glass by molecular
dynamics; and iii) supports the low frequency attenuation mechanism proposed by
Fabian and Allen (Phys.Rev.Lett. 82, 1478 (1999)).Comment: 3 pages, 5 Figures, To be published in PR
Elastic consequences of a single plastic event : a step towards the microscopic modeling of the flow of yield stress fluids
With the eventual aim of describing flowing elasto-plastic materials, we
focus on the elementary brick of such a flow, a plastic event, and compute the
long-range perturbation it elastically induces in a medium submitted to a
global shear strain. We characterize the effect of a nearby wall on this
perturbation, and quantify the importance of finite size effects. Although for
the sake of simplicity most of our explicit formulae deal with a 2D situation,
our statements hold for 3D situations as well.Comment: submitted to EPJ
A study of the static yield stress in a binary Lennard-Jones glass
The stress-strain relations and the yield behavior of model glass (a 80:20
binary Lennard-Jones mixture) is studied by means of MD simulations. First, a
thorough analysis of the static yield stress is presented via simulations under
imposed stress. Furthermore, using steady shear simulations, the effect of
physical aging, shear rate and temperature on the stress-strain relation is
investigated. In particular, we find that the stress at the yield point (the
``peak''-value of the stress-strain curve) exhibits a logarithmic dependence
both on the imposed shear rate and on the ``age'' of the system in qualitative
agreement with experiments on amorphous polymers and on metallic glasses. In
addition to the very observation of the yield stress which is an important
feature seen in experiments on complex systems like pastes, dense colloidal
suspensions and foams, further links between our model and soft glassy
materials are found. An example are hysteresis loops in the system response to
a varying imposed stress. Finally, we measure the static yield stress for our
model and study its dependence on temperature. We find that for temperatures
far below the mode coupling critical temperature of the model (),
\sigmay decreases slowly upon heating followed by a stronger decrease as
\Tc is approached. We discuss the reliability of results on the static yield
stress and give a criterion for its validity in terms of the time scales
relevant to the problem.Comment: 14 pages, 18 figure
Anomalous acoustic reflection on a sliding interface or a shear band
We study the reflection of an acoustic plane wave from a steadily sliding
planar interface with velocity strengthening friction or a shear band in a
confined granular medium. The corresponding acoustic impedance is utterly
different from that of the static interface. In particular, the system being
open, the energy of an in-plane polarized wave is no longer conserved, the work
of the external pulling force being partitioned between frictional dissipation
and gain (of either sign) of coherent acoustic energy. Large values of the
friction coefficient favor energy gain, while velocity strengthening tends to
suppress it. An interface with infinite elastic contrast (one rigid medium) and
V-independent (Coulomb) friction exhibits spontaneous acoustic emission, as
already shown by M. Nosonovsky and G.G. Adams (Int. J. Ing. Sci., {\bf 39},
1257 (2001)). But this pathology is cured by any finite elastic contrast, or by
a moderately large V-strengthening of friction.
We show that (i) positive gain should be observable for rough-on-flat
multicontact interfaces (ii) a sliding shear band in a granular medium should
give rise to sizeable reflection, which opens a promising possibility for the
detection of shear localization.Comment: 13 pages, 10 figure
Spatial distribution of psychotic disorders in an urban area of France: an ecological study
Previous analyses of neighbourhood variations of non-affective psychotic disorders (NAPD) have focused mainly on incidence. However, prevalence studies provide important insights on factors associated with disease evolution as well as for healthcare resource allocation. This study aimed to investigate the distribution of prevalent NAPD cases in an urban area in France. The number of cases in each neighbourhood was modelled as a function of potential confounders and ecological variables, namely: migrant density, economic deprivation and social fragmentation. This was modelled using statistical models of increasing complexity: frequentist models (using Poisson and negative binomial regressions), and several Bayesian models. For each model, assumptions validity were checked and compared as to how this fitted to the data, in order to test for possible spatial variation in prevalence. Data showed significant overdispersion (invalidating the Poisson regression model) and residual autocorrelation (suggesting the need to use Bayesian models). The best Bayesian model was Leroux's model (i.e. a model with both strong correlation between neighbouring areas and weaker correlation between areas further apart), with economic deprivation as an explanatory variable (OR = 1.13, 95% CI [1.02-1.25]). In comparison with frequentist methods, the Bayesian model showed a better fit. The number of cases showed non-random spatial distribution and was linked to economic deprivation
Prognostic value of clinicopathological parameters in head and neck squamous cell carcinoma: a prospective analysis.
The prognostic weight of histological and biological factors was compared with that of known clinical prognostic factors in a population of 108 consecutive previously untreated patients with head and neck squamous cell carcinoma. Parameters studied were: tumour vascularisation, mitotic index, histological differentiation, nuclear grade, keratinisation, desmoplasia, growth pattern, inflammation, tumour emboli in peripheral vessels, keratins 6, 13, 19 immunohistochemical expression, cytofluorometric ploidy and S-phase. In multivariate analysis (Cox), only age and nodal status had a significant impact on the overall survival, whereas T stage was the only significant factor associated with locoregional failure. The cumulative incidence of metastases was correlated not only with age, T and N stage, but also with histological differentiation. All the other histological and biological factors studied failed to provide further prognostic information. These findings may help to select patients with high metastatic risk
Aging dynamics in a colloidal glass of Laponite
The aging dynamics of colloidal suspensions of Laponite, a synthetic clay, is
investigated using dynamic light stattering (DLS) and viscometry after a quench
into the glassy phase. DLS allows to follow the diffusion of Laponite particles
and reveals that there are two modes of relaxation. The fast mode corresponds
to a rapid diffusion of particles within "cages" formed by the neighboring
particles. The slow mode corresponds to escape from the cages: its average
relaxation time increases exponentially fast with the age of the glass. In
addition, the slow mode has a broad distribution of relaxation times, its
distribution becoming larger as the system ages. Measuring the concomitant
increase of viscosity as the system ages, we can relate the slowing down of the
particle dynamics to the viscosity.Comment: 9 pages, 8 Postscript figures, submitted to Phys. Rev.
In-plane and Out-of-plane Plasma Resonances in Optimally Doped La1.84Sr0.16CuO4
We addressed the inconsistency between the electron mass anisotropy ratios
determined by the far-infrared experiments and DC conductivity measurements. By
eliminating possible sources of error and increasing the sensitivity and
resolution in the far-infrared reflectivity measurement on the single
crystalline and on the polycrystalline La1.84Sr0.16CuO4, we have unambiguously
identified that the source of the mass anisotropy problem is in the estimation
of the free electron density involved in the charge transport and
superconductivity. In this study we found that only 2.8 % of the total
doping-induced charge density is itinerant at optimal doping. Our result not
only resolves the mass anisotropy puzzle but also points to a novel electronic
structure formed by the rest of the electrons that sets the stage for the high
temperature superconductivity
Yield stress, heterogeneities and activated processes in soft glassy materials
The rheological behavior of soft glassy materials basically results from the
interplay between shearing forces and an intrinsic slow dynamics. This
competition can be described by a microscopic theory, which can be viewed as a
nonequilibrium schematic mode-coupling theory. This statistical mechanics
approach to rheology results in a series of detailed theoretical predictions,
some of which still awaiting for their experimental verification. We present
new, preliminary, results about the description of yield stress, flow
heterogeneities and activated processes within this theoretical framework.Comment: Paper presented at "III Workshop on Non Equilibrium Phenomena...",
Pisa 22-27 Sep. 200
Shear-banding in a lyotropic lamellar phase, Part 2: Temporal fluctuations
We analyze the temporal fluctuations of the flow field associated to a
shear-induced transition in a lyotropic lamellar phase: the layering transition
of the onion texture. In the first part of this work [Salmon et al., submitted
to Phys. Rev. E], we have evidenced banded flows at the onset of this
shear-induced transition which are well accounted for by the classical picture
of shear-banding. In the present paper, we focus on the temporal fluctuations
of the flow field recorded in the coexistence domain. These striking dynamics
are very slow (100--1000s) and cannot be due to external mechanical noise.
Using velocimetry coupled to structural measurements, we show that these
fluctuations are due to a motion of the interface separating the two
differently sheared bands. Such a motion seems to be governed by the
fluctuations of , the local stress at the interface between the
two bands. Our results thus provide more evidence for the relevance of the
classical mechanical approach of shear-banding even if the mechanism leading to
the fluctuations of remains unclear
- …