Vol. 51 No. 2 (2005): Revista Mexicana de Física.

Published: 2005-01-01

Articles

  • Mathematical properties of DNA sequences from coding and noncoding regions

    J.A. García, M.V. José
    122-0
    Abstract:
    Several nonlinear techniques have been applied to analyze DNA sequences. As a result, some mathematical properties that distinguish both coding and noncoding regions have emerged. We review and apply some of these techniques selecting some examples and comparing our results with previously published data. We also discuss the main controversies that have been raised in terms of the different taken approaches, particularly the presence or absence of long-range correlations in coding regions.
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  • Efecto de estructuras turbulentas no estacionarias sobre la térmica de flujos en conductos de sección cuadrada con un flujo de calor no simétrico

    M. Salinas Vázquez, W.V. Rodríguez
    131-0
    Abstract:
    From large eddy simulations of heated square ducts, we study the effect of non-stationary structures over the flow thermal behavior. Indeed the net decrease of the turbulent intensity over the heated wall, the velocity temperature correlation intensity is enhanced. This correlation which represents the turbulent heat transfer is fed by turbulent structures of high intensity, but with a low frequency. These turbulent structures create high amplitude temperature fluctuations. A conditional statistical study is performed for isolating these turbulent structures and knowing its local and global effects over the flow.
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  • Producción de películas delgadas de CdS$_{x}$ Te$_{1 - x }$ con estructura cúbica para $0 \le x \le 1$

    M. Zapata-Torres, F. Chale-Lara, R. Castro-Rodríguez, O. Calzadilla Ama, a., M. Meléndez-Lira, J.L. Peña
    138-0
    Abstract:
    The CdS/CdTe heterostructure has great potential for the production of photovoltaic systems with more efficiency. One of the limitations to increase the efficiency is the difference in the crystal structure of CdTe and CdS that are cubic and hexagonal. Additionally, the presence of a region with gradual composition in the interfaz CdS/CdTe limits the uniformity of the interfacial electric field necessary to have a efficient separation of charge. To have a better knowledge of CdSTe alloy and eventually apport results that increase the actual efficiency we have grown thin films of CdS$_{x}$Te$_{1 - x}$ on glass substrate using the close spaced vapor transport combined with free evaporation technique (CSVT-FE) using the coevaporation of CdTe and CdS. The incorporation of the S was controlled by means of the CdS source temperature. The samples were characterized using X- ray diffraction, energy dispersive X ray spectroscopy and optical transmission. It was found that the samples have a cubic structure, for $0 \le x \le 1$, with their lattice parameter following the approach of virtual crystal. The forbidden band gap has a similar behavior to the one reported in the literature without showing changes due to the cubic structure for the whole range of compositions.
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  • An entropy based theory for the viscosity of strong glasses

    L. Dagdug, L.S. García-Colín, P. Goldstein
    144-0
    Abstract:
    One of the main applications of the stochastic matrix method is the evaluation of the probability of finding boroxol rings and dendrites in a boron oxide glass. In this work, we calculate the configurational entropy of B$_{2}O_{3}$ using this probability and Shannon's definition of entropy. The isentropic temperature is evaluated obtaining a very good agreement with experimental data. Entropy is introduced into the Adam-Gibbs equation to calculate viscosity. The resulting expression fits the complete range of temperatures for the supercooled liquid.
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  • Propiedades térmicas de un cristal dieléctrico ternario a bajas temperaturas

    R. Rodríguez-Mijangos, G. Moro, oqui-Estrella., G. Vázquez-Polo, R. Pérez-Salas
    149-0
    Abstract:
    Using simple arguments supported by basic principles of thermal physics we studied the behavior of heat capacity and Helmholtz free energy of ternary mixed alkali halide crystals. For heat capacity is considered the possibility of F centers contribution in colored samples making a simple calculus, finding that it is negligible. We showed the behavior at low temperatures of the Debye temperature versus the components concentration of mixed ternary crystal, with this temperature is obtained the heat capacity at any concentration of a mixed sample. Finally we consider the partition function strongly related with an average binding energy function of the components concentration, values of this energy are shown and starting with this information we obtain the Helmholtz free energy of the studied material.
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  • Density and temperature maps of an aluminium plasma produced by laser ablation

    E. Pérez-Tijerina, J. Bohigas, R. Machorro
    153-0
    Abstract:
    A wide field spectrograph was used to calculate maps of the electron density and temperature of an aluminium plasma produced by laser ablation. Data were acquired from a single laser pulse. Line widths of singly and doubly ionized aluminium were used to calculate the density from the Stark effect. Temperature was determined using relative line intensities from successive ionization stages. This single pulse, two-dimensional diagnostic of the plasma may contribute to a better understanding of thin film synthesis by pulsed laser deposition.
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  • Cálculo del esfuerzo ideal de metales nobles mediante primeros principios en la dirección $\langle$100$\rangle$

    A. Bautista-Hernández, M. López-Fuentes, V. Pacheco-Espejel, J.F. Rivas-Silva
    157-0
    Abstract:
    We present calculations of the ideal strength on the $\langle$100$\rangle$ direction for noble metals (Cu, Ag and Au), by means of first principles calculations. First, we obtain the structural parameters (cell parameters, bulk modulus) for each studied metal. We deform on the $\langle$100$\rangle$ direction calculating the total energy and the stress tensor through the Hellman-Feynman theorem, by the relaxation of the unit cell in the perpendicular directions to the deformation one. The calculated cell constants differ 1.3% from experimental data. The maximum ideal strength are 29.6, 17 and 19 GPa for Cu, Ag and Au respectively. Meanwhile, the calculated elastic modulus are 106 (Cu), 71 (Ag), and 45 GPa (Au) and are in agreement with the experimental values for polycrystalline samples. The values of maximum strength are explained by the optimum volume values due to the atomic radius size for each element.
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  • Autodifracción vectorial de dos ondas degeneradas en medios con efecto Kerr óptico

    C. Torres-Torres, A.V. Khomenko
    162-0
    Abstract:
    Theoretical and experimental analyses of degenerated two-wave interaction in media possessing the optical Kerr effect are presented. A simple method for the investigation of physical mechanism of optical nonlinearity based on the two-wave self-diffraction is proposed. It has been demonstrated theoretically and experimentally that the interference of two waves with orthogonal polarizations in the nonlinear Kerr media results in a diffraction grating, which provokes the light self-diffraction.
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  • Electronic and optical properties of InAs(110)

    X. López-Lozano, C. Noguez, L. Meza-Montes
    168-0
    Abstract:
    The electronic and optical properties of the cleavage InAs(110) surface are studied using a semi-empirical tight-binding method which employs an extended, atomic-like, basis set. The surface electronic states are discussed in terms of their electronic character, and compared with other theoretical approaches, and experimental observations. The surface electronic band structure and the Reflectance Anisotropy Spectrum (RAS) are calculated and discussed in terms of the surface electronic states and the atomic structure of the surface.
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  • Time-domain flaw imaging system

    L. Medina
    176-0
    Abstract:
    Ultrasonic Non Destructive Evaluation of materials is a useful tool for flaw detection and characterization. A typical ultrasonic imaging system may consist of a single transducer or an array of sensors working in a B-scan mode. This mode operates by transmitting a pulse of train of pulses from several locations and detecting the echoes coming from in-homogeneities. The reflected energy can be represented as a map of ultrasonic reflectivity. A time-delay beamformer has been successfully used to reconstruct the image, and localize the in-homogeneities within the scanned medium, by time shifting the signals, and summing them up. This process enables to locate regions at which signals are added constructively. It is however, a time consuming process and requires $\lambda/2$ distance of motor steps or inter-element distance between array elements. An algorithm based on time-domain envelope beamformer is presented here. This algorithm is able to diminish the number of computational operations without losing relevant information about the location of in-homogeneities. A comparison between classical and envelope beam-formers is presented when applied to sets of simulated signals. Lateral and longitudinal resolutions are also computed when two targets are within the scanned medium.
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  • Estructura electrónica y origen de la ferroelectricidad en el Bi$_{4}$Ti$_{3}$O$_{12}$, por medio de cálculos de primeros principios

    R. Machado, M.G. Stachiotti, R.L. Migoni, A. Huanosta Tera
    186-0
    Abstract:
    Using the full-potential augmented plane-wave method (LAPW-method), theoretical results on the electronic structure and ferroelectric instabilities in the structure of Bi$_{4}$Ti$_{3}$O$_{12 }$are presented here. The equilibrium coordinates of all the atoms in the tetragonal phase (paraelectric) were calculated. Using the frozen phonon method, it was found that in the considered crystalline structure there are two unstable phonon modes with different displacement patterns, which explain at least partially the primary source for ferroelectricity in the studied compound. Besides, the band structure, the total and site-projected electronic densities of states for the valence and conduction bands of Bi$_{4}$Ti$_{3}$O$_{12 }$are reported, which allowed us to understand the electronic origin of the ferroelectric instabilities.
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  • EPR of borax-alanine mixtures irradiated with thermal neutrons

    S. Galindo, J. Klapp
    193-0
    Abstract:
    Electron Paramagnetic Resonance (EPR) of neutron irradiated crystallite mixtures of alanine and borax is reported. These experiments are aimed at establishing an EPR dosimetry technique for thermal neutrons based on the resonance signal enhancement of alanine due to the boron content in the mixtures via a neutron capture reaction. Signal response as a function of boron content and crystallite size of the mixtures is presented.
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  • Single crystal EPR study of trivalent gadolinium in Cs$_2$NaInCl$_6$ in the 6 -- 300 K interval

    J.L. Boldú, R.J. Gleason, E. Muñoz P.
    199-0
    Abstract:
    A cubic symmetry site is reported for Gd$^{3+}$ in Cs$_2$NaInCl$_6$ at room temperature. The temperature dependence of the cubic spin-Hamiltonian parameter has been measured in the temperature interval 6--300 K using electron paramagnetic resonance (EPR) spectrometry. A thorough EPR examination for a crystal sample clearly demonstrates that there are no new symmetry sites arising in the temperature interval under study (6-300 K) suggesting, therefore, that the site symmetry remains cubic. We fit an analytical expression for the cubic spin-Hamiltonian parameter $b_4^0$ as a function of temperature. The absolute sign of this spin Hamiltonian parameter is also determined.
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  • Production of $W^{\mp}$ with an anomalous magnetic moment via the collision of an ultrahigh-energy (anti)neutrino on a target nucleon

    R.M. García-Hidalgo, A. Rosado
    203-0
    Abstract:
    We discuss the production of $W^{\mp}$ bosons in deep inelastic processes $(\bar{\nu}_l)\nu_l + {\cal N} \rightarrow l^{\pm} + W^{\mp} + X$ (${\cal N}$: $P$ proton, $N$ neutron), in the context of an electroweak model, in which the vector boson self interactions may be different from those prescribed by the electroweak standard model. We present results which show the strong dependence of the cross section on the anomalous magnetic dipole moment $\kappa$ of the $W^{\pm}$. We show that even small deviations from the standard model value of $\kappa$ $(\kappa =1)$ could imply large deviations in the cross section rates of $W^{\mp}$ production through the collision of an ultrahigh energy (anti)neutrino on a target nucleon. However, the enhancement of the cross section rates is not large enough to be detectable.
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  • SAR inducido en un modelo bioplasmático quiral por radiación de teléfonos celulares

    M. Zamorano, H. Torres-Silva
    209-0
    Abstract:
    An electrodynamics model formed by chiral bioplasma, which represents the human head inner structure and makes possible to analyze its behavior when it is radiated by a microwave electromagnetic field from cellular phones is presented. The finite difference time domain, FDTD, numeric technique is used, which allows simulation of the electromagnetic fields, deduced with Maxwell equations and to simulate the specific absorption rate, SAR. The results show the SAR behavior in function of input power, antenna impedance and the chirality factor. More important conclusion of our work is that, in considering the chiral factor of propagating media in the proposed model, the human head absorption of the electromagnetic fields from cellular phones is stronger, so the SAR coefficient may be 68% higher than classical model, for the standard values of chiral factor.
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