Isotope effects on the lattice parameter of cubic SiC

Path-integral molecular dynamics simulations in the isothermal-isobaric (NPT) ensemble have been carried out to study the dependence of the lattice parameter of 3C-SiC upon isotope mass. This computational method allows a quantitative and nonperturbative study of such anharmonic effect. Atomic nuclei were treated as quantum particles interacting via a tight-binding-type potential. At 300 K, the difference Delta a between lattice parameters of 3C-SiC crystals with 12C and 13C amounts to 2.1 x 10^{-4} A. The effect due to Si isotopes is smaller, and amounts to 3.5 x 10^{-5} A when replacing 28Si by 29Si. Results of the PIMD simulations are interpreted in terms of a quasiharmonic approximation for the lattice vibrations.Comment: 4 pages, 3 figure

HST and Spitzer point source detection and optical extinction in powerful narrow-line radio galaxies

We present the analysis of infrared HST and Spitzer data for a sample of 13 FRII radio galaxies at 0.03<z<0.11 that are classified as narrow-line radio galaxies (NLRG). In the context of the unified schemes for active galactic nuclei (AGN), our direct view of the AGN in NLRG is impeded by a parsec-scale dusty torus structure. Our high resolution infrared observations provide new information about the degree of extinction induced by the torus, and the incidence of obscured AGN in NLRG. We find that the point-like nucleus detection rate increases from 25 per cent at 1.025$\mu$m, to 80 per cent at 2.05$\mu$m, and to 100 per cent at 8.0$\mu$m. This supports the idea that most NLRG host an obscured AGN in their centre. We estimate the extinction from the obscuring structures using X-ray, near-IR and mid-IR data. We find that the optical extinction derived from the 9.7$\mu$m silicate absorption feature is consistently lower than the extinction derived using other techniques. This discrepancy challenges the assumption that all the mid-infrared emission of NLRG is extinguished by a simple screen of dust at larger radii. This disagreement can be explained in terms of either weakening of the silicate absorption feature by (i) thermal mid-IR emission from the narrow-line region, (ii) non-thermal emission from the base of the radio jets, or (iii) by direct warm dust emission that leaks through a clumpy torus without suffering major attenuation.Comment: 18 pages, 7 figures, 8 tables, accepted for publication in MNRA

Manual practico para la deteccion electroforetica de isoenzimas y otras proteinas

Procedures for the electrophoretic detection of isoenzymes and other proteins contained in cassava, beans, and legumes are described. Diagrams illustrating the construction and assembly of equipment required to conduct starch gel and polyacrylamide gel electrophoresis and protein electrophoresis are included. The preparation of the starch and polyacrylamide gels, buffer solutions, and sample solutions is described in detail. Procedures for sample cutting, voltage application time and values, and staining of the main enzymes under study (acid phosphatase, catalase, esterase, fumarase, and peroxidase) are specified. Protein electrophoresis is a much simpler procedure, since protein bands are not influenced by the equipment used in extraction procedures or by the physiological state of the plant material. Total proteins, water-soluble proteins (albumins), salt-soluble proteins (globulins), alcohol-soluble proteins (gliadins), acetic acid-soluble proteins (glutenins), and insoluble or residual proteins can be extracted by these methods. The general procedure for protein extraction consists in mixing 0.4 N sodium chloride and centrifuging until the desired protein is obtained; electrophoresis using basic and acid polyacrylamide gels is described. An analysis is presented of electrophoretic patterns, which can be made by using samples of known identity or through visual intensity measurements. (CIAT)Se describen los procedimientos para la deteccion electroforetica de isoenzimas y proteinas presentes en materiales de yuca, frijol y leguminosas. Se detallan por medio de graficas la construccion y ensamble del equipo necesario para llevar a cabo electroforesis con gel de almidon y con gel de poliacrilamida y la electroforesis de proteinas. Se describen en detalle la preparacion de la gel de almidon y de poliacrilamida, las soluciones tampon, las soluciones para la preparacion de las muestras. Se especifican procedimientos de corte, tiempos de aplicacion de voltajes, voltajes y procedimientos para coloracion de las principales enzimas evaluadas (fosfatasa acida, catalasa, esterasa, fumarasa y peroxidasa). La electroforesis de proteinas es un procedimiento mas sencillo, dado que las bandas proteinicas no estan influenciadas por los artefactos utilizados en los procedimientos de extraccion o el estado fisiologico del material vegetal. Se pueden extraer por medio de estos metodos las proteinas totales, proteinas solubles en agua (albuminas), proteinas solubles en sal (globulinas), proteinas solubles en alcohol (gliadinas), proteinas solubles en acido acetico (gluteninas), proteinas insolubles o residuales. El procedimiento en general para la extraccion de las proteinas consiste en hacer mezclas de cloruro de sodio 0.4 N y centrifugar hasta conseguir la proteina deseada; se describe la electroforesis en geles de poliacrilamida basica y la acida. Se presenta un analisis de evaluacion de los patrones electroforeticos, el cual puede ser mediante la comparacion con muestras de identidad conocida o medicion de la intensidad visual. (CIAT

Metallicity of Red Giants in the Galactic Bulge from Near-Infrared Spectroscopy

We present K-band spectra of more than 110 M giants in Galactic bulge fields interior to -4 degrees and as close as 0.2 degrees of the Galactic Center. From the equivalent widths of three features in these spectra, EW(Na),EW(Ca), and EW(CO) we calculate [Fe/H] for the stars with a calibration derived from globular clusters Stephens et al (2000). The mean [Fe/H] for each field is in good agreement with the results from Frogel et al. (1999) based on the slope of the giant branch method. We find no evidence for a metallicity gradient along the minor or major axes of the inner bulge (R < 0.6 kpc). A metallicity gradient along the minor axis, found earlier, arises when fields located at larger galactic radius are included. However, these more distant fields are located outside of the infrared bulge defined by the COBE/DIRBE observations. We compute the [Fe/H] distribution for the inner bulge and find a mean value of -0.21 dex with a full width dispersion of 0.30 dex, close to the values found for Baade's Window (BW) by Sadler et al. (1996) and to a theoretical prediction for a bulge formed by dissipative collapse Molla et al (2000).Comment: 32 pages, 10 figures, AJ submitte

Strong covalent bonding between two graphene layers

We show that two graphene layers stacked directly on top of each other (AA stacking) form strong chemical bonds when the distance between planes is 0.156 nm. Simultaneously, C-C in-plane bonds are considerably weakened from partial double-bond (0.141 nm) to single bond (0.154 nm). This polymorphic form of graphene bilayer is meta-stable w.r.t. the one bound by van der Waals forces at a larger separation (0.335 nm) with an activation energy of 0.16 eV/cell. Similarly to the structure found in hexaprismane, C forms four single bonds in a geometry mixing 90^{0} and 120^{0} angles. Intermediate separations between layers can be stabilized under external anisotropic stresses showing a rich electronic structure changing from semimetal at van der Waals distance, to metal when compressed, to wide gap semiconductor at the meta-stable minimum.Comment: tar gzip latex 4 pages 4 figure

Scaling properties of granular materials

Given an assembly of viscoelastic spheres with certain material properties, we raise the question how the macroscopic properties of the assembly will change if all lengths of the system, i.e. radii, container size etc., are scaled by a constant. The result leads to a method to scale down experiments to lab-size.Comment: 4 pages, 2 figure