Vol. 57 No. 5 (2011): Revista Mexicana de Física.

Published: 2011-01-01

Articles

  • Resistencia mecánica ideal de C, Si y Ge con estructura cúbica: un estudio de primeros principios

    A. Bautista Hernández, M. Salazar Villanueva, F.L. Pérez Sánchez, O. Vázquez Cuchillo, J.F. Sánchez Ramírez
    388-0
    Abstract:
    We present a study of the compressive ideal strength of Carbon (C), Silicon (Si) and Germanium (Ge) with cubic structure (diamond) by means of first principles calculations. Lattice parameters, bulk modulus, shear and Young modulus and elastic constants are obtained as a function of applied stress. The values obtained about lattice parameters and elasticity constants without stress are in according with previous experimental and theoretical reports. Based on the Born-Wang and phonon criteria we have studied the ideal strength of each element. The maximum stresses values (773, 19.5 and 21.7 GPa for C, Si and Ge, respectively) are explained in terms of the band structure, charge density and atomic populations.
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  • Electronic heat transport for a multiband superconducting gap in Sr$_2$RuO$_4$

    P.L. Contreras
    395-0
    Abstract:
    This paper gives a detailed numerical study of the superconducting electronic heat transport in the unconventional multiband superconductor Strontium Ruthenate \sr. The study demostrates that a model with different nodal structures on different sheets of the Fermi surface is able to describe quantitatively experimental heat transport data. The contribution of the density of states DOS is given for each sheet of the Fermi surface and the total contribution is also calculated. Finally, a discussion of the universal character of the electronic heat transport in unconventional superconductors and its relation to the DOS based on the type of nodal structure of the superconducting gap in \sr is given.
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  • Oriented matroid theory and loop quantum gravity in (2+2) and eight dimensions

    J.A. Nieto
    400-0
    Abstract:
    We establish a connection between oriented matroid theory and loop quantum gravity in (2+2) (two time and two space dimensions) and 8-dimensions. We start by observing that supersymmetry implies that the structure constants of the real numbers, complex numbers, quaternions and octonions can be identified with the chirotope concept. This means, among other things, that normed divisions algebras, which are only possible in 1,2, 4 or 8-dimensions, are linked to oriented matroid theory. Therefore, we argue that the possibility for developing loop quantum gravity in 8-dimensions must be taken as important alternative. Moreover, we show that in 4-dimensions, loop quantum gravity theories in the (1+3) or (0+4) signatures are not the only possibilities. In fact, we show that loop quantum gravity associated with the (2+2)-signature may also be an interesting physical structure.
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  • The Jones vector as a spinor and its representation on the Poincaré sphere

    G.F. Torres del Castillo, I. Rubalcava García
    406-0
    Abstract:
    It is shown that the two complex Cartesian components of the electric field of a monochromatic electromagnetic plane wave, with a temporal and spatial dependence of the form ${\rm e}^{{\rm i} (kz - \omega t)}$, form a SU(2) spinor that corresponds to a tangent vector to the Poincaré sphere representing the state of polarization and phase of the wave. The geometrical representation on the Poincaré sphere of the effect of some optical filters is reviewed. It is also shown that in the case of a partially polarized beam, the coherency matrix defines two diametrically opposite points of the Poincaré sphere.
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  • Identificador con comparación entre dos estimadores

    J.J. Medel Juárez, J.C. García Infante, R. Urbieta Parrazales
    414-0
    Abstract:
    This paper describes the identification process as an adaptive digital filter using the estimated transition matrix, identification error and gain functions. One problem that the filter has is the time process, affecting the quality response and agreeing with the estimated transition function, which is a condition to be accomplished with respect to the reference time evolution system. Thus, the time process required must be met in all operations within the same time interval. Generally, the identifier filter answer is affected by the parameter used in the estimated transition function indirectly used in the other filter operations. In this case, seeking a better time estimation response two estimators were considered: the first expressed in a recursive form and the second, selected within the knowledge base gain used in accordance to fuzzy logic. The results show the convergence observed in the error differences and their approximations to the stochastic time model conditions with $k$ samples, using, MatLab$^{\mbox{\textregistered }}$ as a simulation software.
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  • Soluciones exactas a la ecuación de Schrodinger para una fibra óptica quiral

    A.H. Salas, J.E. Castillo
    421-0
    Abstract:
    This paper presents exact solutions to Maxwell's equations in a dispersive and nonlinear chiral singlemode optical fiber. The chirality is characterized through the Born-Fedorov formalism. This work allows us to analize the effect of chirality over the propagation equation of optical pulses. The results we present in this paper may help in the strudy of optical fiber.
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  • Viscosity enhancement in dilute magnetorheological fluids through magnetic perturbations

    F. Donado, U. S, oval., J.L. Carrillo
    426-0
    Abstract:
    The influence of a sinusoidal magnetic field on the effective viscosity of a magnetorheological dispersion in the low particle concentration regime is studied experimentally. When a sinusoidal magnetic field of low amplitude, conceived as perturbation, is applied transversally to the static field, a significant enhancement occurs in the measured effective viscosity. The magnitude of changes depends on a number of factors such as the amplitude and frequency of the perturbation, the particle concentration, the application time of the fields, and the shear rate. It has also been found that the behavior of the effective viscosity as a function of frequency presents a critical behavior. Therefore, an average Mason number is proposed, whose behavior as a function of frequency is similar to that shown by the effective viscosity.
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  • Study of bone cells by quantitative phase microscopy using a Mirau interferometer

    J. González-Laprea, A. Márquez, K. Noris-Suárez, R. Escalona
    435-0
    Abstract:
    The article presents the use of an interference microscope, using a Mirau objective for the study of the early adhesion process of osteoblast-like bone cells, using the phase shifting technique. The process is carried out on surgical stainless steel surfaces of interest for the development of bone implants. Experimental phase maps are directly related to cell profiles. These phase maps are obtained for several adhesion times, which indicate morphological changes in cells. Mainly the change in height profiles through time and the interaction with other surrounding cells are observed. The experimental system used is appropriate for the time scales observed, in the order of hours, showing strength and precision in the calculation of the optical phase.
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  • Cu(In,Ga)Se$_{2}$ thin films processed by co-evaporation and their application into solar cells

    J. Sastré-Hernández, M.E. Calixto, M. Tufiño-Velázquez, G. Contreras-Puente, A. Morales-Acevedo, G. Casados-Cruz, M.A. Hernández-Pérez, M.L. Albor-Aguilera, R. Mendoza-Pérez
    441-0
    Abstract:
    Polycrystalline Cu(In,Ga)Se$_{2}$ (CIGS) solar cells are attractive because low cost techniques can be used to obtain high efficiency thin film photovoltaic devices. Several research groups around the world have developed CIGS/CdS solar cells with efficiencies larger than 15%~[1] using evaporation, making it an attractive and reliable technique for thin film deposition. Our PVD system is provided with MBE-type Knudsen cells to deposit CIGS thin films on glass/Molibdenum (Mo) substrates. The deposition conditions for each metal source have been established by doing a deposition profile of temperature data vs. growth rate by co-evaporation to obtain CIGS thin film for solar cells. Characterization of the co-evaporated CIGS thin films was performed by X-ray diffraction (X-RD), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) techniques. Good quality polycrystalline films were obtained as shown by X-RD patterns. SEM micrographs show films having a very uniform appearance with large grain sizes ($\sim $1 $\mu $m). Photoluminescence (PL) studies on CIGS samples with different Ga and Cu concentrations (Ga/Ga+In) = 0.25 and 0.34 and (Cu/In+Ga) = 0.83, 0.88 and 0.94) have been performed. The EDS results have shown that is possible to control very precisely the CIGS thin film composition using these Knudsen cells. Film thicknesses of $ \sim $3-4 $\mu $m, were measured with an Ambios profilemeter XP 100 stylus type. A conversion efficiency of 10.9 % has been achieved for solar cells made from the co-evaporated absorbers.
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  • Dos configuraciones diferentes para la descripción temporal de actividad de Escherichia coli mediante speckle dinámico

    E.E. Ramírez Miquet, L. Martí López, O.R. Contreras Alarcón
    446-0
    Abstract:
    We present the experimental results of applying the laser dynamic speckle technique to describe bacterial cells activity corresponding to the species Escherichia coli (ATCC 25922) cultivated in two-compartment Petri dishes, using one of them as a non-inoculated control blank. In the experimental series we registered speckled images and after processing them, we obtained optical signals of the bacterial activity corresponding to a single cell concentration and two different laser beam impact configurations. The results for both configurations are discussed. We conclude that the method, the designed and mounted set-up and the associated image processing allowed the detection and description of the bacterial activity.
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  • Photoacoustic technique in the transmission configuration for quantitative analysis of liquids

    J.A. Balderas-López, J. Díaz-Re, es.
    452-0
    Abstract:
    A photoacoustic methodology for the measurement of optical absorption coefficient for pigments in liquid solution is introduced. The mathematical model, involving the Beer-Lambert model for light absorption, is a generalization of a previously reported photoacoustic methodology, involving a surface absorption model, used for thermal diffusivity measurements of liquids. The optical absorption coefficient for aqueous solutions of copper sulfate and methylene blue at various concentrations where measured at a wave-length of 658 nm, to show the range of optical absorption coefficients where the Beer-Lambert model for light absorption applies. It was experimentally shown that at high pigment's concentrations the surface absorption limit is more adequate and, consequently, the sample's thermal diffusivity is obtained instead of the optical absorption coefficient. Optical and thermal properties obtained by means of this photoacoustic methodology resulted in close agreement with the corresponding ones obtained by means of conventional optical spectroscopy and the previously reported photoacoustic methodology for thermal diffusivity measurements, respectively. Results show that this technique has promise applications in the field of quantitative analysis.
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  • Constantes fundamentales: la última frontera para el Sistema Internacional de Unidades

    J.M. López Romero, R.J. Lazos Martínez
    460-0
    Abstract:
    The phrase ``one measure for all men and all times'' summarizes the ideal that would lead to the development of the International System of Units, SI. The evolution of the definitions of the SI units of measurement may be somehow considered as a series of approximations to such an ideal in which the units of measurement are invariant in time and space, immutable, and susceptible to equivalent realizations, reproducible, and accessible to be useful in measuring processes of practical, industrial, technological and scientific purposes. In the current state of the science and technology the fundamental constants appear as the last frontier to define the SI units of measurement. The abandonment of artifacts to support the definitions of SI units began in 1960 when the wavelength of a krypton 86 radiation was used to redefine the unit of length, the meter. Moreover, the time unit, defined in 1967 in terms of the separation of the ground state hyperfine levels of the Cesium-133 atom, was another important step towards the incorporation of the fundamental constants in the SI units. In 1982, the unit of length was established in terms of the speed of light in vacuum and time measurements. This clearly illustrates how the SI evolves towards the incorporation of fundamental constants, or combinations of them, to support the units of measurement. In some way or another, all units of measurement have gone through a process of redefinition with the notable exception of the mass unit, the kilogram. Currently, among the SI seven base units, the kilogram is the only one based on the properties of a specific artifact, namely a cylinder of platinum. However, this may change in the near future by setting a value without uncertainty for Planck's constant and by defining the kilogram in terms of electromagnetic forces generated in a balance, usually called ``watt balance''. Thus, the definition of the kilogram would be framed in the context of the quantum mechanics, since it would rely experimentally on the time unit, and therefore on the hyperfine separation of the ground state of Cesium-133 atom, on the realizations of the volt in terms of the Josephson effect and on the ohm in terms of the quantum Hall effect. Definitions for the kelvin in terms of the Boltzmann constant, the mole in terms of Avogadro's number and the candela in terms of luminous efficacy of monochromatic radiation will allow the fundamental constants to be the support of the SI in order to provide measurement systems with a very high accuracy to pursuit the scientific and technological advances to come.
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