Vol. 48 No. 3 (2002): Revista Mexicana de Física.

Published: 2002-01-01

Articles

  • A simple procedure to design Glitch-less switched-current cells

    F. S, oval-Ibarra., J. Santana, J. A, G. Suárez-Lizárraga
    182-0
    Abstract:
    The description of non-idealities in Cascode Switched-Current (SI) cell as well as its application in analog signal processing is described. A test chip (CMOS, 1.5$\mu $m, N-well, 0-5 V) was designed to verify that the proposed method works successfully in the magnitude glitch reduction problem.
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  • Earthquake-Induced Helmholz Resonance in Manzanillo Lagoon, Mexico

    N. Carbajal, M. A
    192-0
    Abstract:
    On 9th October, 1995, at 9:36 local time, an earthquake of magnitude 8.0 occurred on the west coast of Mexico. The epicenter was located at 18$^{\circ}$ 51.5' N and 104$^{\circ}$ 8.4' W, at approximately 25 km of Manzanillo. This important commercial harbor is connected by a narrow channel to a lagoon. A tidal gauge recorded the forced oscillations occurred in the lagoon. A dominant period of 36 minutes was observed. Analytical solutions for ideal rectangular flat basins and numerical modeling for realistic geometries show that the lagoon and the channel produce a Helmholz resonance with a period similar to that observed in the earthquake.
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  • Dispersión de luz por la superficie rugosa de una película excitónica delgada sobre un substrato metálico

    N. Atenco-Analco, F. Pérez-Rodríguez, J. Madrigal-Melchor
    197-0
    Abstract:
    The effect of the spatial dispersion of a semiconductor film, having a rough free surface and overlying a metal, on the differential reflection coefficient is investigated theoretically. The investigation is carried out near exciton resonance where the nonlocality is strong and leads to the generation of additional polaritonic waves. Applying a first-order perturbation theory, we calculate the angular and frequency dependencies of the differential reflection coefficient for the case of one-dimensional roughness. The spectra of diffuse reflection for CuCl on a Al metal for incident light with $s$-polarization exhibit pronounced resonances (peaks) at frequencies satisfying Fabry-Perot conditions for the additional polaritonic modes. These conditions also correspond to the size quantization of the exciton's translational motion. Our results show the usefulness of the spectra of diffuse reflection for studying the excitation of quantized polaritonic modes in nonlocal films.
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  • Quantum cosmology for inflationary scenary

    J. Socorro, O. Obregón
    205-0
    Abstract:
    We study quantum and super quantum cosmology for a Friedman-Robertson Walker (FRW) flat universe in the presence of an inflation exponential potential with the corresponding kinetic term for the scalar field. We exhibit exact solutions for the corresponding Wheeler-DeWitt (WDW) equation and its square root. In both cases, solutions, as is known for the Bianchi models, of the form $e^{i\Phi}$ exist, where $\Phi$ is the Hamilton-Jacobi function. For a particular factor ordering of the standard Wheeler-DeWitt equation we show a ``wave packet" in which for large $x$ (with the radius of expansion of the universe $a \sim e^{-x}$), $\varphi$ the scalar field and $x$ ``compete" modulating the behaviour of the ``wave packet".
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  • Elastic scattering of electrons by Ne at intermediate energies using the Schwinger variational principle with plane wave as a trial basis set

    Jorge L.S. Lino
    215-0
    Abstract:
    We report a application of the Schwinger variational principle with plane waves as a trial basis set. Differential cross sections are obtained for $e^-$--Ne collisions from~$130$ to~$500 $eV. Our differential cross sections are found to be in reasonable agreement with existing measurements.
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  • Modelo hidrodinamico del termotransporte de electrones fuera de equilibrio en metales

    A. Rangel-Huerta, J.L. Carrillo
    219-0
    Abstract:
    We present a semiclassical kinetic model to describe a nonequilibrium metal gas electrons. By means of the Grad method of moments, we solve the modified Boltzmann transport equation in the scheme of the Lorentz-Sommerfeld theory. The non conserved balance equations for the electric current density and the heat flux are used to describe the main relaxation processes and the frequency dependent thermoelectric coefficients. A relevant characteristic of these coefficients are the presence of some kinetic relaxation times expressed through the Fermi-Dirac integrals.
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  • Otto and Diesel engine models with cyclic variability

    J. A, T. D, C. G, R. Páez-Hernández, F. Angulo-Brown
    228-0
    Abstract:
    Typically, in an internal combustion engine, thousands of cycles are performed in a minute. In this sequence of cycles many physical and chemical quantities change from cycle to cycle. For example, the combustion heat changes due to residual gases, imperfect combustion and other reasons. In this work, we present two finite-time thermodynamics models for both an Otto and a Diesel cycle, in which the cyclic variability is studied as occurring in the heat capacities of the working fluid. The fluctuations considered are of the uncorrelated type (uniform and gaussian) and one correlated case (logistic map distribution). We find that in the correlated fluctuations case, the power output and the efficiency of both cycles reach bigger fluctuations than in the uncorrelated cases. This result can provide insights over the performance of internal combustion engines.
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  • On the field enhanced carrier generation in MOS structures

    P. Pe, kov., T. Diaz., H. Juárez, R. Castanedo, G. Romero
    235-0
    Abstract:
    Using a sine voltage sweep C-V, DLTS, gettering and defect revealing techniques the thermal carrier generation in MOS (Metal - Oxide - Semiconductor) structures was investigated. The thermal generation from a field dependent becomes as field independent due to the gettering. The activation energy of generation - recombination centers, $E_{a}$, the generation lifetime, $\tau_{g}$, and Poole-Frenkel factor, $\alpha$, were determined. Dislocations with an average density of $5\times10^6 cm^{-2}$ were observed in all wafers. In the samples with field enhanced carrier generation a linear dependence between $E_{a}$ and $\alpha$ was found, where the Poole-Frenkel factor increases with the increase of the activation energy. On the other hand $E_{a}$, $\tau_{g}$ and $\alpha$ were found to be strongly affected by the gettering, thereby suggesting that they depend on the level of decoration of the dislocations with impurities.
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  • Particle motion near the resonant surface of a high-frequency wave in a magnetized plasma

    J. J
    239-0
    Abstract:
    The motion of single charged particles in a magnetized plasma is studied using a Lagrangian formalism, applied to the average over a gyro-period, in the presence of an electromagnetic wave that is being resonantly absorbed by the medium. Near the resonance surface the wave amplitude has a gradient and the resulting particle drifts are shown to be consistent with the existence of a ponderomotive force, produced by the wave-field-gradient. The orbits are also obtained numerically to show how the drifting motion arises, and to study the case of high field-gradients, which is not described with the formal analysis.
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  • Application of Magnus series for polarization evolution in fibers

    N.Korneev.
    250-0
    Abstract:
    We apply the technique of Magnus series to build approximations for polarization evolution in fibers with varying twist and birefringence. Conditions for trajectory mixing on Poincaré sphere are identified. The technique relates realistic perturbation parameters to the parameters of popular coarse step method of numerical simulation. We demonstrate that liner polarization mode dispersion, if it is a dominant process, leads for big propagation lengths to a speckle-like output pulse shape.
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  • A centrifuge for studies of fluid dynamics phenomena in a rotating frame of reference

    M. Vargas, E. Ramos, G. Ascanio, R. Espejel, G. Esquivel, G. Hernández-Cruz
    255-0
    Abstract:
    A centrifuge for the study of fluid mechanics phenomena in a rotating frame has been constructed at the Center for Energy Research at the National University of Mexico (UNAM). The centrifuge has a 1.5 $m$ long arm and is designed to rotate at a maximum rate of $86$ $rpm$ which corresponds to a maximum centrifugal acceleration of $13$ $g$. This paper describes the mechanical characteristics of the centrifuge and its instrumentation. The potential of the system for performing fluid mechanics experiments in a rotating frame of reference is also discussed and sample results of natural convection in a cavity are presented.
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  • Directional properties of a modified Bonner spectrometer

    R. Policroniades, A. Varela, E. Moreno, M. F, J. Calvillo
    267-0
    Abstract:
    The study of the directional properties of a modified Bonner type spectrometer, are reported in this paper. It was established in a previous publication [1], that a small radial hole on each polyethylene sphere gives rise to a directional sensitivity when used on this apparatus. Now, the directional properties are further improved by adding a thin cylindrical cadmium shell inserted in the radial holes of each sphere. The angular response for a set of six spheres is reported using, both, a fast (non-moderated) and a moderated neutron source provided by the $^{9}$Be(d,n)$^{10}$B reaction by bombarding a thick beryllium target with 4 MeV deuterons.
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  • Thomson scattering revisited

    R. Cuevas, A. Queijeiro
    271-0
    Abstract:
    We review Thomson scattering, as the low energy limit of Compton scattering, for target particles of spin 0, {\small 1/2}, 1, and {\small 3/2 }. We show how, at the scattering amplitude level, Thomson result emerges; then, the computation of the cross section is quite simple.
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  • Spheroidal functions revisited: matrix evaluation and generating functions

    N. Aquino, E. Castaño, E. Le, -Koo.
    277-0
    Abstract:
    A matrix method to evaluate the angular spheroidal functions is presented as an alternative to the orthodox method.Plane waves are identified as generating functions of the spheroidal functions showing their explicit expansions.
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