Comparison of structural transformations and superconductivity in compressed Sulfur and Selenium

Density-functional calculations are presented for high-pressure structural phases of S and Se. The structural phase diagrams, phonon spectra, electron-phonon coupling, and superconducting properties of the isovalent elements are compared. We find that with increasing pressure, Se adopts a sequence of ever more closely packed structures (beta-Po, bcc, fcc), while S favors more open structures (beta-Po, simple cubic, bcc). These differences are shown to be attributable to differences in the S and Se core states. All the compressed phases of S and Se considered are calculated to have weak to moderate electron-phonon coupling strengths consistent with superconducting transition temperatures in the range of 1 to 20 K. Our results compare well with experimental data on the beta-Po --> bcc transition pressure in Se and on the superconducting transition temperature in beta-Po S. Further experiments are suggested to search for the other structural phases predicted at higher pressures and to test theoretical results on the electron-phonon interaction and superconducting properties

Optical properties of pyrochlore oxide Pb2Ru2O7−δPb_{2}Ru_{2}O_{7-{\delta}}

We present optical conductivity spectra for $Pb_{2}Ru_{2}O_{7-{\delta}}$ single crystal at different temperatures. Among reported pyrochlore ruthenates, this compound exhibits metallic behavior in a wide temperature range and has the least resistivity. At low frequencies, the optical spectra show typical Drude responses, but with a knee feature around 1000 \cm. Above 20000 \cm, a broad absorption feature is observed. Our analysis suggests that the low frequency responses can be understood from two Drude components arising from the partially filled Ru $t_{2g}$ bands with different plasma frequencies and scattering rates. The high frequency broad absorption may be contributed by two interband transitions: from occupied Ru $t_{2g}$ states to empty $e_{g}$ bands and from the fully filled O 2p bands to unoccupied Ru $t_{2g}$ states.Comment: 4 pages, 6 figure

The Alignment Between 3-D Data and Articulated Shapes with Bending Surfaces

International audienceIn this paper we address the problem of aligning 3-D data with articulated shapes. This problem resides at the core of many motion tracking methods with applications in human motion capture, action recognition, medical-image analysis, etc. We describe an articulated and bending surface representation well suited for this task as well as a method which aligns (or registers) such a surface to 3-D data. Articulated objects, e.g., humans and animals, are covered with clothes and skin which may be seen as textured surfaces. These surfaces are both articulated and deformable and one realistic way to model them is to assume that they bend in the neighborhood of the shape's joints. We will introduce a surface-bending model as a function of the articulated-motion parameters. This combined articulated-motion and surface-bending model better predicts the observed phenomena in the data and therefore is well suited for surface registration. Given a set of sparse 3-D data (gathered with a stereo camera pair) and a textured, articulated, and bending surface, we describe a register-and-fit method that proceeds as follows. First, the data-to-surface registration problem is formalized as a classifier and is carried out using an EM algorithm. Second, the data-to-surface fitting problem is carried out by minimizing the distance from the registered data points to the surface over the joint variables. In order to illustrate the method we applied it to the problem of hand tracking. A hand model with 27 degrees of freedom is successfully registered and fitted to a sequence of 3-D data points gathered with a stereo camera pair

Numerical solution of the eXtended Pom-Pom model for viscoelastic free surface flows

In this paper we present a finite difference method for solving two-dimensional viscoelastic unsteady free surface flows governed by the single equation version of the eXtended Pom-Pom (XPP) model. The momentum equations are solved by a projection method which uncouples the velocity and pressure fields. We are interested in low Reynolds number flows and, to enhance the stability of the numerical method, an implicit technique for computing the pressure condition on the free surface is employed. This strategy is invoked to solve the governing equations within a Marker-and-Cell type approach while simultaneously calculating the correct normal stress condition on the free surface. The numerical code is validated by performing mesh refinement on a two-dimensional channel flow. Numerical results include an investigation of the influence of the parameters of the XPP equation on the extrudate swelling ratio and the simulation of the Barus effect for XPP fluids

On Material Removal Regimes for the Shaping of Glass Edges: Force Analysis, Surface Topography and Damage Mechanisms

Glass shaping, which corresponds to the removal of the edges of a specimen, is the last finishing operation in glass manufacturing. This process has several functions on the final shaped glass including removing sharp edges, improving mechanical resistance, decreasing surface damage and giving it an aesthetical aspect. This article addresses the effects of working parameters, including grinding forces and consumed power, on surface edge finishing and damage mechanism induced during glass grinding. Microscopic observations and multi-scale analysis have also been conducted to investigate the surface edge characteristics. Experimental results show three damage regimes. The first (regime I) is a partial ductile regime with cutting action accompanied by chip formation. The second (regime II) is a crushing (or fragmentation) regime. The last (regime III) is also a partial ductile regime but by ploughing action with displaced material. The shaped surface obtained in the regime II has a better roughness than that obtained in regime I and III. However, regimes I and III include streaks and form defects which are not present in regime II. Similar to metallic materials, the evolution of force components show a linear relationship between normal and tangential forces. This implicates a constant average contact pressure and friction coefficient (l) between the flat grains and the workpiece

Bioinspired and bioinstructive surfaces to control mesenchymal stem cells

Life is a dynamic event, and cells represent its smallest structural and functional units that can exist on their own. Cells and the surrounding extra cellular matrix are in a constant flux of change imposed by each other. The aim of this chapter is to look into the complex interplay of physical and chemical factors imposed on cells, in particular mesenchymal stem cells (MSCs), via their microenvironment and how such factors influence MSCs response. Some of the defined and independently varied factors such as surface chemistry, modulus and topography are reviewed in this chapter, with a focus on cell physicochemical memory formation. Acknowledging the importance of mechanical and chemical stimuli experienced by cells and their polarity, while investigating the interdependent relationship between them, will not only help develop more fitting culture systems, for instance to maintain stem cell multipotency, but advance the understanding of how such extracellular stimuli are understood and acted upon by cells during their lifetime