Vol. 48 No. 6 (2002): Revista Mexicana de Física.

Published: 2002-01-01

Articles

  • Técnicas biomagnéticas y su comparación con los métodos bioeléctricos

    Abstract:
    Biological systems are able to generate magnetic fields. Despite these fields being so weak, now it is possible to detect them by means of sophisticated equipments, which opens a new research field. Recent studies on this phenomenon reveal that the measurement and analysis of the fields, generated in organs such as the brain, heart, lungs, liver, etc., can be useful in the detection and treatment of several sickness, among other applications. In this work a comparison between biomagnetic and standard bioelectric techniques is provided. The work emphasizes the importance of using both techniques as complementary methods in research and their usefulness in the diagnostic of pathologies and stand out the fact that both methods are based on the same physical principles.
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    M. Sosa, J. J, J. L, G. Gutiérrez Juárez, M. Vargas Luna, C. Caudillo-Cisneros, S. Márquez Gamiño, R. Huerta
    490-0

  • Effective magnetic moment of neutrinos in strong magnetic fields

    Abstract:
    In this paper we compute the effective magnetic moment of neutrinos propagating in dense high magnetized medium. Taking typical values of magnetic field and densities of astrophysical objects (such as the cores of supernovae and neutron stars) we obtain an effective type of dipole magnetic moment in agreement with astrophysical and cosmological bounds.
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    A. Pérez-Mart\'{\i }nez, Samina S. Masood, H. Perez Rojas, R. Gaitán, S. Rodriguez-Romo
    501-0

  • Variational anisotropic model of Wannier excitons compared with fractional-dimensional space approach

    Abstract:
    Binding energy of Wannier excitons in a quantum well of thickness $L$ is studied using two models: a two-parameter trial wave function and a fractional-dimensional space with dimension $2\leqslant \alpha \leqslant 3$. Since both models provide quantitative measures of the exciton spatial anisotropy as $L$ changes, we give physical arguments for a plausible definition of $\alpha =\alpha (L)$.
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    M. del Castillo-Mussot, G.J. Vázquez, J.A. Re, es.
    504-0

  • Theoretical studies of energy photoemission spectra (XPS) of S and SO\pmb{$ _{2}$} adsorbed on Ni clusters by Hartree-Fock method.

    Abstract:
    Theoretical results of photoemission energy spectral of the atomic sulfur and of the SO$_{2} $ molecule, adsorbed over surfaces of Ni(110) and~Ni(111)~clusters, are reported in this work. Clusters with 11, 13, 15 and 17 atoms of Ni were used for the model. The calculations were done by Hartree-Fock method, and basis sets of type STO-NG and p-q1G (p=3,6; q=2,3; N=3,6) were used. The ionization potentials (IP) were interpreted within the Koopmans Theorem. The results obtained for the IP of 1s, 2s and 2p orbitals are 2472.03 eV, 238.14 eV and 173.55 eV, respectively; while for the same orbitals of the sulfur in SO$_{2} $ these values are 2481.30 eV, 246.61 eV and 182.17 eV. The theoretical results were compared with experimental results reported in the references, and the error ranges are between 5 eV and 30 eV, in agreement with the stardard for the Hartree-Fock method.
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    E. Martínez, A. Rodríguez, L. Rincón
    508-0

  • The laser-backscattering equations and their application to the study of the atmospheric structure

    Abstract:
    In this work a method for interpreting backscattering signals acquired by a lidar is described. The method is based on the elastic scattering of laser radiation due to gases and particles suspended in the atmosphere (bulk effects). We propose a space-time diagram which helps to evaluate the arguments of the equation that serves to calculate the lidar signal in terms of the backscattering coefficient. We describe how the system detects gradients on this coefficient, along the laser optical path. To illustrate the method, we present some typical lidar results obtained in the neighborhood of Mexico City.
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    R. Castrejón-García, J. R, J. R, A. Morales
    513-0

  • Magnetic properties of Fe$_{\pmb 1}$/Cr$_{\pmb{\text{N}}}$ nanoinclusions in Fe

    Abstract:
    The magnetic properties of Fe$_{1}$/Cr$_{N}$ inclusions in a Fe matrix are calculated as a function of the Cr number of atoms $N$ at zero temperature, and for $N \leq 168$. The electronic structure is determined by using a realistic spd-band Hamiltonian. The local magnetic moments $\mu(i)$ at the various cluster sites i are calculated selfconsistently in the unrestricted Hartree-Fock approximation. The results show clearly the importance of the local geometry and the magnetic interactions between neighbors. The matrix Fe atoms couple always antiferromagnetically to the Cr atoms, imposing for small $N$ spin arrangements that overcome the antiferromagnetic interactions of the Cr cluster. The antiferromagnetic arrangement becomes possible in the interior of the cluster as the number of Cr atoms becomes larger than 88.
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    P.G. Alvarado-Le, va., J.M. Montejano-Carrizales, J.L. Morán-López
    519-0

  • The atomic and electronic structure of amorphous silicon nitride

    Abstract:
    Using a novel approach to the ab initio generation of random networks we constructed two nearly stoichiometric samples of amorphous silicon nitride with the same content $x$=1.29. The two 64-atom periodically-continued cubic diamond-like cells contain 28 silicons and 36 nitrogens randomly substituted, and were amorphized with a 6 fs time step by heating them to just below their melting temperature with a Harris-functional based, molecular dynamics code in the LDA approximation. The averaged total radial distribution function (RDF) obtained is compared with some existing Tersoff-like potential simulations and with experiment; ours agree with experiment. All the partial radial features are calculated and the composition of the second peak also agrees with experiment. The electronic structure is calculated and the optical gaps obtained using both a HOMO-LUMO approach and the Tauc-like procedure developed recently that gives reasonable gaps.
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    F. Alvarez, A. A
    528-0

  • Pattern formation in oscillatory granular flows

    Abstract:
    An experimental device has been used to examine the behavior of granular materials under vertical, horizontal and combined bi-axial vibrations. Containers with square, rectangular and circular cross sections geometries were used, with glass spheres, millet and lentil as granular media. The oscillating frequencies varied from 0 to 35 Hz, with amplitudes between 0 and 5.5 mm. Flow visualization was made possible with high-speed video cameras providing frontal and lateral views of the granular motion. Four different phenomena: heaping, arching, small amplitude surface waves and large amplitude surface waves were found. The onset of these phenomena depends basically on two dimensionless parameters; one involving the amplitude and frequency of oscillations coupled with gravity and the second one representing the amount of material in the vibrating container. In the case of vertical vibrations, results agreed well with those of Wassgren et al. (1996), although some discrepancies were found. For horizontal vibrations, convective cycles appeared rotating in opposite directions as well as lateral waves with one half the frequency of excitation. For the case of combined vibrations, a superposition of effects between those of horizontal and vertical vibrations was found and analyzed. Finally, comparison was made between experiments and numerical predictions based on molecular dynamics. The numerical solution agreed extremely well with experimental observations, particularly for the new results of combined vibrations.
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    J. Jaimes, G. G, E. Geffro, {\&} B. Mena., J. R
    534-0

  • Computerized DLTS system to characterize deep levels in semiconductors

    Abstract:
    A computerized system for deep level characterization in semiconductors has been set up. It is based on the well known DLTS (Deep Level Transient Spectroscopy) technique, but high versatility for data manipulation is achieved through an analog-to-digital conversion card (A/D) that digitizes capacitance transients. These transients are analyzed to provide information on the traps within the semiconductor. A PC-based program in Basic control acquisition, storage, analysis and presentation of results. The system is able of obtaining the desired parameters by only one temperature scan, which is an important advantage, taking into account the experimental time experimentally needed for the measurement. Experimental results for a silicon PIN power structure are shown, to illustrate its performance.
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    Alej., ro Avila García., Mario Alfredo Re., es Barranca.
    539-0

  • Fabricación de niobato de litio periódicamente polarizado para óptica no lineal

    Abstract:
    We describe how to produce periodically-poled lithium niobate (PPLN), which can be used to generate electromagnetic radiation of different frequencies through quasi-phase-matching. First we describe the basic principles of quasi-phase-matching and of ferroelectric crystals (which are used to obtain quasi-phase-matching), and then we give a detailed description of the fabrication process of PPLN. Using this technique we obtained PPLN samples over 5 cm long of high enough quality to obtain optical parametric generation through quasi-phase-matching.
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    R. S, L. A, M. J, F. Alonso, J. Fonseca, .
    548-0

  • Simple technique for root locus plotting

    Abstract:
    We present a simple technique for calculating and plotting the root locus of a linear control system. Although several programs are commercially available for this purpose, and remarks on this subject can be found in dedicated texts, a complete description of the program code is desirable to provide the designer with versatility and deeper insight. This report describes our technique and includes complete program codes. In this technique, the roots of the system's characteristic function are found by providing a single, unique guess value to be used in a first derivative search algorithm in order to calculate the unknown set of roots. Similarly to other techniques, the root locus is found by calculating the system's set of roots as the gain varies in the range of interest. The resolution obtained depends on the sampling rate of the gain parameter. The technique is suitable for any programming language and there is basically no limit on the root order of the system. To illustrate the technique a four-pole order system is analyzed. Then, to improve the response of the system a lead compensation network is added.. The usefulness of the technique is illustrated by comparing the time response of the compensated network over the uncompensated one.
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    M. C, wiak., M. Castro
    556-0

  • The Wigner function in paraxial optics I. Matrix methods in Fourier optics

    Abstract:
    The paraxial regime of scalar wave optics has the same structure as non-relativistic quantum mechanics, with wavelength taking the place of the Planck constant. The Wigner function is a central tool to explore the phase space of a system; in optics, moreover, it can be produced by purely optical means. In this first part, we present a matrix-based formalism for the study of paraxial optical systems, and apply it to the description of a setup that, as will be seen in the second part, produces the Wigner function.
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    Roberto Ortega-Martínez., Carlos J. Román-Moreno, Ana Leonor Rivera.
    565-0

  • Ciclo de Curzon y Ahlborn para un gas de van der Waals

    Abstract:
    For a Curzon and Ahlborn type heat engine with a van der Waals gas like a working substance, using maximization of power and maximization of ecological function, it is shown that the same expression of the efficiency for the ideal gas case as working substance, with the substitution of maximum volume and minimum volume spanned by the engine, $V_{max}\rightarrow V_{max}-b$ and $V_{min}\rightarrow V_{min}-b$, are obtained. The adiabatic branches in the cycle are consider non instantaneous.
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    D. Ladino-Luna
    575-0

  • Evaluation and measurement of forces between two conducting spheres

    Abstract:
    The forces between pairs of conducting spheres at different separations and with fixed potential differences are evaluated using bispherical coordinates on the basis of Ref. 1. The experimental arrangement, the method and the results of the measurement of such forces are reported. The analysis of the measurements is carried out by comparison with the forces evaluated for the corresponding values of the geometrical and electrical parameters, finding a good agreement.
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    A. Corona Cruz, E. Le, -Koo.
    579-0

  • Comportamiento asintótico para el periodo del péndulo simple

    Abstract:
    We investigate the asymptotic behaviors of the period for a simple pendulum with arbitrary initial conditions. The study of the vibrational and rotational solutions for the simple pendulum shows that in the asymptotic limit the behavior is of the same type for both motions, when the energy tends to $mgl$. Here we present a logical deduction for the behavior in both cases. We obtain that the asymptotic behavior of the period goes to infinity logarithmically for the two solutions of the pendulum.
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    F. Chaos Urdampilleta, L. Chaos Cador
    586-0