427 research outputs found
Solid molecular hydrogen: The Broken Symmetry Phase
By performing constant-pressure variable-cell ab initio molecular dynamics
simulations we find a quadrupolar orthorhombic structure, of symmetry,
for the broken symmetry phase (phase II) of solid H2 at T=0 and P =110 - 150
GPa. We present results for the equation of state, lattice parameters and
vibronic frequencies, in very good agreement with experimental observations.
Anharmonic quantum corrections to the vibrational frequencies are estimated
using available data on H2 and D2. We assign the observed modes to specific
symmetry representations.Comment: 5 pages (twocolumn), 4 Postscript figures. To appear in Phys. Rev.
Let
X-ray standing wave and reflectometric characterization of multilayer structures
Microstructural characterization of synthetic periodic multilayers by x-ray
standing waves have been presented. It has been shown that the analysis of
multilayers by combined x-ray reflectometry (XRR) and x-ray standing wave (XSW)
techniques can overcome the deficiencies of the individual techniques in
microstructural analysis. While interface roughnesses are more accurately
determined by the XRR technique, layer composition is more accurately
determined by the XSW technique where an element is directly identified by its
characteristic emission. These aspects have been explained with an example of a
20 period Pt/C multilayer. The composition of the C-layers due to Pt
dissolution in the C-layers, PtC, has been determined by the XSW
technique. In the XSW analysis when the whole amount of Pt present in the
C-layers is assumed to be within the broadened interface, it l eads to larger
interface roughness values, inconsistent with those determined by the XRR
technique. Constraining the interface roughness values to those determined by
the XRR technique, requires an additional amount of dissolved Pt in the
C-layers to expl ain the Pt fluorescence yield excited by the standing wave
field. This analysis provides the average composition PtC of the
C-layers .Comment: 12 pages RevTex, 10 eps figures embedde
High-Pressure Amorphous Nitrogen
The phase diagram and stability limits of diatomic solid nitrogen have been
explored in a wide pressure--temperature range by several optical spectroscopic
techniques. A newly characterized narrow-gap semiconducting phase has
been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational
and optical properties of the phase produced under these conditions
indicate that it is largely amorphous and back transforms to a new molecular
phase. The band gap of the phase is found to decrease with pressure
indicating possible metallization by band overlap above 280 GPa.Comment: 5 pages, 4 figure
Power laws in microrheology experiments on living cells: comparative analysis and modelling
We compare and synthesize the results of two microrheological experiments on
the cytoskeleton of single cells. In the first one, the creep function J(t) of
a cell stretched between two glass plates is measured after applying a constant
force step. In the second one, a micrometric bead specifically bound to
transmembrane receptors is driven by an oscillating optical trap, and the
viscoelastic coefficient is retrieved. Both and
exhibit power law behavior: and , with the same exponent
. This power law behavior is very robust ; is
distributed over a narrow range, and shows almost no dependance on the cell
type, on the nature of the protein complex which transmits the mechanical
stress, nor on the typical length scale of the experiment. On the contrary, the
prefactors and appear very sensitive to these parameters. Whereas
the exponents are normally distributed over the cell population, the
prefactors and follow a log-normal repartition. These results are
compared with other data published in the litterature. We propose a global
interpretation, based on a semi-phenomenological model, which involves a broad
distribution of relaxation times in the system. The model predicts the power
law behavior and the statistical repartition of the mechanical parameters, as
experimentally observed for the cells. Moreover, it leads to an estimate of the
largest response time in the cytoskeletal network: s.Comment: 47 pages, 14 figures // v2: PDF file is now Acrobat Reader 4 (and up)
compatible // v3: Minor typos corrected - The presentation of the model have
been substantially rewritten (p. 17-18), in order to give more details -
Enhanced description of protocols // v4: Minor corrections in the text : the
immersion angles are estimated and not measured // v5: Minor typos corrected.
Two references were clarifie
The Mission Accessibility of Near-Earth Asteroids
The population of near-Earth asteroids (NEAs) that may be accessible for human space flight missions is defined by the Near-Earth Object Human Space Flight Accessible Targets Study (NHATS). The NHATS is an automated system designed to monitor the accessibility of, and particular mission opportunities offered by, the NEA population. This is analogous to systems that automatically monitor the impact risk posed to Earth by the NEA population. The NHATS system identifies NEAs that are potentially accessible for future round-trip human space flight missions and provides rapid notification to asteroid observers so that crucial follow-up observations can be obtained following discovery of accessible NEAs. The NHATS was developed in 2010 and was automated by early 2012. NHATS data are provided via an interactive web-site, and daily NHATS notification emails are transmitted to a mailing list; both resources are available to the public
Structural Phase Transition at High Temperatures in Solid Molecular Hydrogen and Deuterium
We study the effect of temperature up to 1000K on the structure of dense
molecular para-hydrogen and ortho-deuterium, using the path-integral Monte
Carlo method. We find a structural phase transition from orientationally
disordered hexagonal close packed (hcp) to an orthorhombic structure of Cmca
symmetry before melting. The transition is basically induced by thermal
fluctuations, but quantum fluctuations of protons (deuterons) are important in
determining the transition temperature through effectively hardening the
intermolecular interaction. We estimate the phase line between hcp and Cmca
phases as well as the melting line of the Cmca solid.Comment: 8 pages, 7 figures; accepted in Phys. Rev.
Observation of an Isotope Shift in the Superconducting Transition Temperature of La\u3csub\u3e1.85\u3c/sub\u3eSr\u3csub\u3e0.15\u3c/sub\u3eCuO\u3csub\u3e4\u3c/sub\u3e
An oxygen isotope shift is observed in superconducting La1.85Sr0.15CuO4 when 18O is substituted partially for 16O; the superconducting transition temperature Tc is lowered by 0.3 to 1.0 K in different samples. We examine these results using conventioanl phonon-mediated BCS theory and conclude that, for La1.85Sr0.15CuO4, phonons play an important role in the pairing mechanism
Theory and Applications of X-ray Standing Waves in Real Crystals
Theoretical aspects of x-ray standing wave method for investigation of the
real structure of crystals are considered in this review paper. Starting from
the general approach of the secondary radiation yield from deformed crystals
this theory is applied to different concreat cases. Various models of deformed
crystals like: bicrystal model, multilayer model, crystals with extended
deformation field are considered in detailes. Peculiarities of x-ray standing
wave behavior in different scattering geometries (Bragg, Laue) are analysed in
detailes. New possibilities to solve the phase problem with x-ray standing wave
method are discussed in the review. General theoretical approaches are
illustrated with a big number of experimental results.Comment: 101 pages, 43 figures, 3 table
Modulation of ATP/ADP Concentration at the Endothelial Cell Surface by Flow: Effect of Cell Topography
Determining how flow affects the concentration of the adenine nucleotides ATP and ADP at the vascular endothelial cell (EC) surface is essential for understanding flow-induced mobilization of intracellular calcium. Previously, mathematical models were formulated to describe the ATP/ADP concentration at the EC surface; however, all previous models assumed the endothelium to be flat. In the present study we investigate the effect of surface undulations on ATP/ADP concentration at the EC surface. The results demonstrate that under certain geometric and flow conditions, the ATP + ADP concentration at the EC surface is considerably lower for a wavy cell surface than for a flat surface. Because ECs in regions of disturbed arterial flow are expected to have larger undulations than cells in non-disturbed flow zones, our findings suggest that ECs in regions of flow disturbance would exhibit lower ATP + ADP concentrations at their surfaces, which may lead to impaired calcium signaling. If validated experimentally, the present results may contribute to our understanding of endothelial cell dysfunction observed in regions of disturbed flow
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