Vol. 48 No. 2 (2002): Revista Mexicana de Física.

Published: 2002-01-01

Articles

  • Tricritical behavior in stationary double diffusive convection with cross diffusion

    R. Becerril, J. Mendoza
    88-0
    Abstract:
    We derive an amplitude equation for the stationary instability for the isothermal double diffusive system with cross diffusion. A quintic term is computed in terms of rescaled cross-diffusion constants. This quintic term is stabilizing in the vicinity of the tricritical point. The existence of a tricritical point indicates the presence of a hysteresis loop whose parameters are also presented.
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  • Fabricación y caracterización de diodos electro-luminiscentes de silicio poroso

    G. Romero Paredes R., R. Peña-Sierra, G. Castillo-Cabrera
    92-0
    Abstract:
    Electroluminescent diodes with emission in the visible region of the electromagnetic spectrum were developed using thin porous silicon layers (PSL). The device structure consists of a metal-porous silicon junction formed with gold and PSL with porosity of 50 %. The photoluminescence (PL) spectra of the PSL and the resulting electroluminescence (EL) spectra of the devices were studied. The maximum of the PL- and EL spectra is located at 800 nm and 560 nm, respectively. The origin of the electro luminescent signal is ascribed to the silicon oxide covering the silicon filaments of the PSL.
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  • Phenomenon of primary and secondary extinction in textured material

    T.G. Kr, shtab., J. Palacios Gómez., M.O. Mazin
    100-0
    Abstract:
    A new X-ray diffraction method is proposed for a more exact calculation of pole figures to determine the crystallite orientation distribution function (CODF) in textured materials, by the introduction of a correction of the integrated intensity of the diffracted beam due to the phenomenon of extinction. Besides, for the case of a symmetrical Bragg reflection from an ``infinitely thick'' flat parallel plate textured sample a simple solution is developed for the problem of separation and determination of primary and secondary extinction parameters, which can be present simultaneously. The determination of these parameters gives additional information about crystallite structure and allows us to evaluate the average subgrains size and their disorientation, respectively [1]. In this work, according to the dynamic diffraction theory, it is shown that the extinction length, which is directly connected with the phenomenon of primary extinction, in the Bragg geometry for $\sigma $ polarization (perpendicular) is independent on the wavelength used for a given reflection. On the other hand, the additional contribution from secondary extinction depends on X-rays wavelength due to the change of effective absorption coefficient. Considering this fact, the calculations of pole density and the parameters of primary and secondary extinction are performed using the same strong reflection for two different wavelengths and, for one of these wavelengths, a second order of reflection. For confirmation of the proposed method a partially cold rolled aluminum sample and an aluminum powder standard sample were measured. The corrected pole densities, the values of primary and secondary extinction and the average sizes of perfect coherent areas and average angle of disorientation of subgrains in the selected directions were obtained. The obtained results show that the precise X-ray diffraction characterization of textured materials requires the consideration, in general, of primary as well as secondary extinction phenomenon.
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  • Diffusion of a dipolar tracer in a salty surface

    M. Hernández-Contreras
    107-0
    Abstract:
    The two dimensional translational and rotational diffusion coefficients of a dipolar tracer in a charged surface are determined with the generalized Langevin equation approach. The electrolyte friction effect on the translational brownian movement displays a maximum as a function of ionic strength showing similar trends to its three dimensional counterpart. However, its effect on the rotational motion is important only at low and moderate salt concentration and negligible at high ionic concentration.
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  • Supersymmetric quantum solution for FRW cosmological model with matter

    J. Socorro
    112-0
    Abstract:
    Using the technique of supersymmetric quantum mechanics we present new cosmological quantum solutions, in the regime for FRW cosmological model using a barotropic perfect fluid as matter field.
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  • Magnetotransport properties of Hg$_{1 -x}$Re$_{x}$Ba$_{2}$Ca$_{2}$Cu$_{3}$O$_{8 + \delta }$ superconductors

    J. Roa-Rojas, M.T.D. Orl, o., E. Baggio-Saitovich, P. Pureur
    118-0
    Abstract:
    Systematic measurements on Hall and longitudinal resistivities as a function of temperature in Hg$_{1 - x}$Re$_{x}$Ba$_{2}$Ca$_{2}$Cu$_{3}O_{8 + \delta }$ (x=0.18) ceramic samples were effectuated. These measurements were performed in magnetic fields H=0, 10, 20, 40 and 50 kOe, applied perpendicularly to the transport current orientation. In the normal phase, the Hall coefficient is positive and may be accurately fitted to $R_H = 1/aT + b$. The Hall angle varies approximately as predicted by Anderson's formula. When the temperature approaches from above the mean-field critical temperature $T_{c}$, the Hall resistivity decreases abruptly as a consequence of thermal fluctuations. Below $T_{c}$ and for $H=10 $and 20 kOe, the Hall response changes signal, going passing through a minimum and becoming positive again before the zero resistance state is obtained. For stronger applied fields, the Hall resistivity remains positive in the mixed phase, though its qualitative shape, preserving the local minimum below $T_{c}$. We ascribe the double sign reversal feature to combined effects of thermal fluctuations and vortex motion. Close to the zero resistance state, the Hall resistivity varies as a power law of the longitudinal resistivity, with a field-independent exponent. This result may be interpreted as evidence of a vortex-glass transition.
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  • Non-minimal coupling for spin 3/2 fields

    Victor M. Villanueva, Juan A. Nieto, Octavio Obregón.
    123-0
    Abstract:
    The problem of the electromagnetic coupling for spin $3/2$ fields is discussed. Following supergravity and some recent works in the field of classical supersymmetric particles, we find that the electromagnetic coupling must not obey a minimal coupling in the sense that one needs to consider not only the electromagnetic potential but also the coupling of the electromagnetic field strenght. This coupling coincides with the one found by Ferrara et al by requiring that the gyromagnetic ratio be 2. Coupling with non-Abelian Yang-Mills fields is also discussed.
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  • Proton rapidity distribution in nucleus-nucleus collisions at high energy

    Fu-Hu Liu.
    128-0
    Abstract:
    The proton rapidity distributions in nucleus-nucleus collisions at the Alternating Gradient Synchrotron (AGS) and the Super Proton Synchrotron (SPS) energies are analyzed by the revised thermalized cylinder model. The calculated results are compared and found to be in agreement with the experimental data of Si-Al and Si-Pb collisions at~$14.6 $AGeV/$c$, Pb-Pb collisions at~$158 $AGeV/$c$, and S-S collisions at~$200 $AGeV/$c$.
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  • Body motion in a resistive medium at temperature $T$

    M. I
    132-0
    Abstract:
    We consider a macroscopic body propagating in a one-dimensional resistive medium, consisting of an ideal gas at temperature $T$. For a whole family of collisions with varying degree of inelasticity, we find an exact expression for the effective force on the moving body as a function of the body's speed and the value of the restitution coefficient. At low and high speeds it reduces to the well-known Stoke's and Newton's law, respectively.
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  • Formulación del segundo axioma de la termodinámica mediante motores báricos

    M. A
    135-0
    Abstract:
    In the normal teaching of thermodynamics as well as in the traditional textbooks, the second axiom is postulated in terms of thermal refrigerators and motors according to the Clausius and Kelvin-Planck's formulations, leaving open the possibility for generalizing the statement of the second axiom by means of motors and ``refrigerators'' associated to other intensive variables than temperature. In particular it is possible to employ pressure motors and vacuum pumps in the formulation of a corresponding second axiom, such that the maximum work is dependent on the pressure differences between two fixed ``pressure reservoires''. It is hoped that the teaching of this generalization of the second axiom leads the student to the understanding of two facts: first, that his perception of the fundamentals of thermodynamics is more profound, and second, that these fundamentals are not unalterable so that he could be responsible for its change. It might then be so that his learning of thermodynamics is more meaningful.
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  • Mapeo estructural de sílica xerogel utilizando espectroscopía infrarroja

    J.R. Martínez, F. Ruiz
    142-0
    Abstract:
    Infrared spectroscopy has been used to analyse structural changes in samples prepared by the sol-gel method. Silica xerogels powders were prepared from tetraethylorthosilicate (TEOS), distilled water, and ethanol with different H$_{2}$O/TEOS molar ratios. A low water to TEOS molar ratio gives a incomplete hydrolysis of the TEOS molecules and yielding to oligomeric structures, whereas a high water to TEOS molar ratio yield to a full hydrolyzation faciliting the cross-linking of polymeric chains and producing a three-dimensional interconnection of siloxanes.
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  • Superluminal rates of separation and EPR photons

    J. Karles, { H. Perez Rojas}
    150-0
    Abstract:
    We consider rates of separation between two particles greater than $c$ (which is not in contradiction with special relativity) in understanding some conflict between special relativity and quantum mechanics found by moving observers of correlated EPR pairs of photons. The photon frequencies observed in the moving frame have opposite shifts than those found when the detectors are fixed in that frame. By observing from two different frames the arrival of a photon from an EPR pair to a given detector, it is illustrated how the measurement of the simultaneous arrival of the other photon of the EPR pair in one frame disturbs the measurement in the other frame.
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  • Introducción al modelo estándar en el background field method electrodebil

    Luis G. Cabral-Rosetti
    155-0