871 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
Photoionization time delays
International audienceThe material presented in this chapter is based on important advances realized in " attophysics " which make feasible to follow the motion of electrons in atoms and molecules with attosecond-level time resolution. In this context, time-delays have been recently determined in the process of photoionization by extreme-ultraviolet (xuv) pulses and the question of the significance of these measured delays arises. As we shall outline here, numerical experiments show that they are intimately related to the structure of the ionized species' continuous spectrum. Another point addressed here is that, in experiments, the measurements have the common characteristic to be performed in the presence of an auxiliary infra-red (IR) field, used to " clock " the timing of the process. This implies to adapt the theory treatment to handle such " two-color " photoionization processes. We review a systematic analysis of these features that are characteristic of this class of electronic transitions, when viewed in the time domain
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
Assessment of digital image correlation measurement errors: methodology and results
Optical full-field measurement methods such as Digital Image Correlation (DIC) are increasingly used in the field of experimental mechanics, but they still suffer from a lack of information about their metrological performances. To assess the performance of DIC techniques and give some practical rules for users, a collaborative work has been carried out by the Workgroup “Metrology” of the French CNRS research network 2519 “MCIMS (Mesures de Champs et Identification en Mécanique des Solides / Full-field measurement and identification in solid mechanics, http://www.ifma.fr/lami/gdr2519)”. A methodology is proposed to assess the metrological performances of the image processing algorithms that constitute their main component, the knowledge of which being required for a global assessment of the whole measurement system. The study is based on displacement error assessment from synthetic speckle images. Series of synthetic reference and deformed images with random patterns have been generated, assuming a sinusoidal displacement field with various frequencies and amplitudes. Displacements are evaluated by several DIC packages based on various formulations and used in the French community. Evaluated displacements are compared with the exact imposed values and errors are statistically analyzed. Results show general trends rather independent of the implementations but strongly correlated with the assumptions of the underlying algorithms. Various error regimes are identified, for which the dependence of the uncertainty with the parameters of the algorithms, such as subset size, gray level interpolation or shape functions, is discussed
Cancer and renal insufficiency results of the BIRMA study
Background: Half of anticancer drugs are predominantly excreted in urine. Dosage adjustment in renal insufficiency (RI) is, therefore, a crucial issue. Moreover, patients with abnormal renal function are at high risk for drug-induced nephrotoxicity. The Belgian Renal Insufficiency and Anticancer Medications (BIRMA) study investigated the prevalence of RI in cancer patients, and the profile/dosing of anticancer drugs prescribed. Methods:Primary end point: to estimate the prevalence of abnormal glomerular filtration rate (GFR; estimated with the abbreviated Modification of Diet in Renal Disease formula) and RI in cancer patient. Secondary end point: to describe the profile of anticancer drugs prescribed (dose reduction/nephrotoxicity). Data were collected for patients presenting at one of the seven Belgian BIRMA centres in March 2006. Results: A total of 1218 patients were included. The prevalence of elevated SCR (1.2 mg per 100 ml) was 14.9%, but 64.0% had a GFR90 ml min 1 per 1.73 m 2. In all, 78.6% of treated patients (n1087) were receiving at least one drug needing dosage adjustment and 78.1% received at least one nephrotoxic drug. In all, 56.5% of RI patients receiving chemotherapy requiring dose reduction in case of RI did not receive dose adjustment. Conclusions: The RI is highly frequent in cancer patients. In all, 80% of the patients receive potentially nephrotoxic drugs and/or for which dosage must be adjusted in RI. Oncologists should check the appropriate dose of chemotherapeutic drugs in relation to renal function before prescribing. © 2010 Cancer Research UK.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
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
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
Band Formation during Gaseous Diffusion in Aerogels
We study experimentally how gaseous HCl and NH_3 diffuse from opposite sides
of and react in silica aerogel rods with porosity of 92 % and average pore size
of about 50 nm. The reaction leads to solid NH_4Cl, which is deposited in thin
sheet-like structures. We present a numerical study of the phenomenon. Due to
the difference in boundary conditions between this system and those usually
studied, we find the sheet-like structures in the aerogel to differ
significantly from older studies. The influence of random nucleation centers
and inhomogeneities in the aerogel is studied numerically.Comment: 7 pages RevTex and 8 figures. Figs. 4-8 in Postscript, Figs. 1-3 on
request from author
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