The Geometry of the Cholesteric Phase

We propose a construction of a cholesteric pitch axis for an arbitrary nematic director field as an eigenvalue problem. Our definition leads to a Frenet-Serret description of an orthonormal triad determined by this axis, the director, and the mutually perpendicular direction. With this tool we are able to compare defect structures in cholesterics, biaxial nematics, and smectics. Though they all have similar ground state manifolds, the defect structures are different and cannot be, in general, translated from one phase to the other.Comment: 5 pages, the full catastroph

The uptake, tissue distribution and metabolism of the mothproofing agent Eulan WA new in species of freshwater fish

The active ingredients of the mothproofing agent Eulan WA New are polychloro-2-(chloromethyl sulphonamido) diphenyl ether (PCSD) components, differing in their degree of aromatic ring chlorination. It has been shown that direct release of textile effluents has resulted in the contamination of the aquatic environment by Eulan WA New and the exposure of aquatic organisms to this complex xenobiotic. Dosing studies of this work showed that the accumulation of the major active ingredient of Eulan WA New, 6—PCSD (hexachloro-2- [chloromethyl sulphonamido] diphenyl ether) in freshwater fish tissue occurred mainly by gill and cutaneous routes. It accumulates to high concentrations, especially in the bile of pike E. lucius for example, after short periods of dosing. Analysis of pike and goldfish C. auratus tissue showed generally high ratios of 5-PAD:6-PCSD providing evidence for the in vivo cleavage of the sulphonamido bond of 6-PCSD with the formation of 5-PAD metabolite. The primary locations of the 5-PAD forming activity in these fish were identified as the liver and digestive tissue. In vitro studies showed the presence of an activity in goldfish, carp C. carpio and pike liver (homogenates and subcellular fractions) which catalysed the formation of 5-PAD from 6-PCSD. The maximum rate of 5-PAD formation was observed when liver homogenate was prepared in phosphate buffer (40 mM) of neutral to alkaline pH, containing dithiothreitol, NADPH and/or NADH, and (from a preliminary study) showing a requirement for oxygen. The activity is characteristic of a microsomal monooxygenase type (cyt.P-450 dependent and/or cyt.P-450 independent). Complementary data of contrasting differences in the ability of species of freshwater fish to metabolise 6-PCSD to 5-PAD was obtained from in vivo and in vitro studies. 5-PAD formation occurred in goldfish, carp and pike liver tissue with an apparent absence (or greatly reduced activity) in tissue of rainbow trout S. gairdneri, perch P. fluviatilis and eel A. anguilla. Such apparent qualitative differences in the metabolism of Eulan WA New by species of freshwater fish highlight the importance of a careful choice of fish species as model systems to assess environmental contamination/metabolism of this xenobiotic

Geometry of the cholesteric phase

We propose a construction of a cholesteric pitch axis for an arbitrary nematic director field as an eigenvalue problem. Our definition leads to a Frenet-Serret description of an orthonormal triad determined by this axis, the director, and the mutually perpendicular direction. With this tool, we are able to compare defect structures in cholesterics, biaxial nematics, and smectics. Though they all have similar ground state manifolds, the defect structures are different and cannot, in general, be translated from one phase to the other

High pressure x-ray diffraction study of the volume collapse in Ba24Si100 clathrate

International audienceThe high pressure stability of the silicon type-III clathrate Ba24Si100 has been studied by x-ray diffraction (XRD) up to a maximum pressure of 37.4 GPa. The high pressure behavior of this Si type-III clathrate appears to be analogous to the structural type-I parent Ba8Si46. An isostructural volume collapse is observed at ~23 GPa, a value higher than for Ba8Si46 (13-15 GPa). The crystallinity of the structure is preserved up to the maximum attained pressure without amorphization, which appears to be in contradiction with the interpretation given in a Raman spectroscopy study [Shimizu et al., Phys. Rev. B 71, 094108 (2005)]. Nevertheless, the XRD analysis shows the appearance of a type-III disordered nanocaged-based crystalline structure after the volume collapse. Moreover, we find that the volume collapse transformation is (quasi)reversible after pressure release. Additionally, a low pressure transition first evidenced by Raman spectroscopy is also observed in our XRD study at 5 GPa: The variation of the isotropic thermal factors of Ba atoms shows a clear discontinuity at this pressure while the average positions of Ba atoms remain identical

High-pressure phase transformations, pressure-induced amorphization, and polyamorphic transition of the clathrate Rb6.15Si46

International audienceThe type-I clathrate Rb6.15Si46 with partly empty cage sites has been studied up to 36 GPa using Raman spectroscopy, synchrotron x-ray diffraction in diamond-anvil cells, and ab initio total-energy and lattice-dynamics calculations. A first phase transition is observed at 13±1 GPa and a "volume collapse" transition within the clathrate structure is then observed at 24±1 GPa. Pressure-induced amorphization into a high-density amorphous (HDA) state occurs above P=33±1 GPa. The HDA form transforms into a low-density amorphous polymorph during decompression. During the compression study using angle dispersive synchrotron x-ray diffraction techniques we measured bulk modulus parameters for rocksalt-structured TaO, included adventitiously in the clathrate sample [K0=293(3) GPa and K′0=5.4(3)]

Monte Carlo study of coaxially gated CNTFETs: capacitive effects and dynamic performance

Carbon Nanotube (CNT) appears as a promising candidate to shrink field-effect transistors (FET) to the nanometer scale. Extensive experimental works have been performed recently to develop the appropriate technology and to explore DC characteristics of carbon nanotube field effect transistor (CNTFET). In this work, we present results of Monte Carlo simulation of a coaxially gated CNTFET including electron-phonon scattering. Our purpose is to present the intrinsic transport properties of such material through the evaluation of electron mean-free-path. To highlight the potential of high performance level of CNTFET, we then perform a study of DC characteristics and of the impact of capacitive effects. Finally, we compare the performance of CNTFET with that of Si nanowire MOSFET.Comment: 15 pages, 14 figures, final version to be published in C. R. Acad. Sci. Pari

Probing high pressure properties of single wall carbon nanotubes through fullerene encapsulation

The high pressure behavior of bundled 1.35±0.1nm diameter single wall carbon nanotubes (SWNT) filled with C70 fullerenes (usually called peapods) has been investigated by Raman spectroscopy and compared with the corresponding behavior of the nonfilled SWNT. We show experimentally that two reversible pressure-induced transitions take place in the compressed bundle SWNT. The first transition, in the 2–2.5GPa range, is in good correspondence with predictions of the thermodynamic instability of the nanotube circular cross section for the studied tube diameter. An interaction between the fullerenes and the tube walls is then observed at about 3.5GPa, which evidences a progressive deformation of the tube cross section. The second transition takes place at pressures between 10 and 30GPa, and is evidenced by two effects by a strong frequency downshift of the Raman transverse modes and the concomitant disappearance of the fullerenes Raman modes in peapods. The pressure at which the second transition takes place is strongly dependent on the nature of the pressure transmitting medium. We also report irreversible effects at high pressure as the shortening of the tubes, the formation of nanostructures and the disappearance of the C70 Raman signal in some cases. Transmission electron microscopy studies are also reported supporting these transformations

High-pressure behavior of polyiodides confined into single-walled carbon nanotubes: A Raman study

International audienceThe high-pressure behavior of polyiodides confined into the hollow core of single-walled carbon nanotubes organized into bundles has been studied by means of Raman spectroscopy. Several regimes of the structural properties are observed for the nanotubes and the polyiodides under pressure. Raman responses of both compounds exhibit correlations over the whole pressure range (0–17 GPa). Modifications, in particular, take place, respectively, between 1 and 2.3 GPa for polyiodides and between 7 and 9 GPa for nanotubes, depending on the experiment. Differences between one experiment to another are discussed in terms of nanotube filling homogeneity. These transitions can be presumably assigned to the tube ovalization pressure and to the tube collapse pressure. A nonreversibility of several polyiodide mode modifications is evidenced and interpreted in terms of a progressive linearization of the iodine polyanions and a reduction in the charged species on pressure release. Furthermore, the significant change in the mode intensities could be associated to an enhancement of lattice modes, suggesting the formation of a new structure inside the nanotube. Changes in the nanotube mode positions after pressure release point out a decrease in the charge transfer in the hybrid system consistent with the observed evolution of the charged species

Do centenarians die healthier than younger elders? A comparative epidemiological study in Spain

This study aims to describe the clinical course, drug use, and health services use characteristics during the last year of life of elders who die being centenarians and to identify key aspects differentiating them from elders who die at an earlier age, with a particular focus on sex differences. We conducted an observational, population-based study in the EpiChron Cohort (Aragon, Spain). The population was stratified by sex and into three age sub-populations (80-89, 90-99, and >= 100 years), and their characteristics were described and compared. Multimorbidity was the rule in our elders, affecting up to 3 in 4 centenarians and 9 in 10 octogenarians and nonagenarians. Polypharmacy was also observed in half of the centenarian population and in most of the younger elders. Risk factors for cardiovascular disease (i.e., hypertension, dyslipidaemia, diabetes), cerebrovascular disease and dementia were amongst the most common chronic conditions in all age groups, whereas the gastroprotective drugs and antithrombotic agents were the most dispensed drugs. Centenarians presented in general lower morbidity and treatment burden and lower use of both primary and hospital healthcare services than octogenarians and nonagenarians, suggesting a better health status. Sex-differences in their clinical characteristics were more striking in octogenarians and tended to decrease with age