Vol. 51 No. 6 (2005): Revista Mexicana de Física.

Published: 2005-01-01

Articles

  • Solitones en la teoría Einstein--Maxwell--dilatón--axión

    A. Herrera--Aguilar, J.O. Téllez--Vázquez
    549-0
    Abstract:
    In the present research work we obtain solitonic solutions by means of two of the most successful nonlinear techniques of the modern physics: the inverse scattering method and the application of Lie--Bäcklund symmetries. We show that these techniques can be applied in the framework of the four--dimensional low--energy effective field theory of the heterotic string better known as Einstein--Maxwell--dilaton--axion theory. Thus, we obtain an exact soliton solution for the field equation of the chiral matrix $P$, which involves the $g_{tt}$--component of the tensor field, the dilaton and a electric field, starting from pure flat space--time. We further apply a nonlinear symmetry of the full effective theory on this soliton solution in order to generate a new field configuration which also involves the pseudoscalar field axion, a magnetic field and the $g_{t \phi}$--component of the metric tensor (which describes gravitational rotation). We analize as well some properties of these solitonic configurations.
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  • Reduction of multipath effect through a critical scattering zone in microcell environments

    C.A. López Mir, a., D.H. Covarrubias Rosales
    558-0
    Abstract:
    In this work, we investigate a critical region (CR) in microcell elliptical environments, an area between the mobile and the base station (BS) containing multipaths whose angles and times of arrival possess the acceptable angle and delay spreads of the channel. The focus of the paper is to estimate the theoretical rates of multipath reduction expected from the CR. These rates illustrate the convenience of adjusting or not the antenna's beamwidth (aperture) according to the angle spread. Also, to confirm the model's results; the angle and time of arrival statistics deduced from the elliptical model are validated through simulation. Results closely agree with theoretical values expected from the model.
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  • Two-dimensional treesph simulations of choked flow systems

    J. Klapp, L. Di G, S. Galindo, E. Sira
    563-0
    Abstract:
    It is well-known that the flow of gas, liquid, and their mixtures through restrictors installed in pipeline systems is of great practical importance in many industrial processes. In spite of its significance, numerical hydrodynamics simulations of such flows are almost non-existent in the literature. Here we present exploratory two-dimensional calculations of the flow of a viscous, single-phase fluid through a wellhead choke of real dimensions, using the method of Smoothed Particle Hydrodynamics (SPH) coupled with a simple isothermal equation of state for description of the flow. The results indicate that an approximately stationary mean flow pattern is rapidly established across the entire tube, with the density and pressure dropping and the flow velocity rising within the choke throat. If the downstream flow is inhibited at the outlet end of the tube, a pressure drop of about 12% occurs across the choke when the mean flow reaches an approximate steady state. If, on the other hand, the flow is not inhibited downstream, the pressure drop is reduced to about 8% or less. The flow across the choke throat remains subsonic with typical velocities of $\sim 0.1c$, where $c$ denotes the sound speed. In contrast, the flow velocities in the upstream and downstream sections of the pipe are on the average factors of $\sim 6$ and $\sim 3.5$ times lower, respectively. Correlation studies based on experimental data indicate that the pressure drop is only 3% or even less for gas flow through wellhead chokes at a speed of $0.1c$. This discrepancy reflects the inadequacy of the isothermal equation of state to describe realistic gas flows.
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  • Investigación experimental del espejo de lazo óptico no lineal con un acoplador simétrico y una placa retardadora de un cuarto de onda en el lazo

    F. Gutiérrez-Zainos, B. Ibarra-Escamilla, E.A. Kuzin, U. Ruíz-Corona, O. Pottiez
    574-0
    Abstract:
    We research experimentally the configuration of an non linear optical fiber loop mirror, with a symmetrical coupler, highly twisted low-birefringence fiber and a quarter-wave retarder plate inserted into the loop as polarization controller. The retarder plate is used to create an asymmetry in polarization and the nonlinear polarization rotation is used to produce a differential phase shift between the counter-propagating beams in the fiber loop. It is possible to change the transmission behavior with the rotation of the retarder plate. Some advantages that we get with this configuration, in comparison with others similar devices are high contrast between the maximun and minimun of the transmission and flexibility in the adjustment of its transmission characteristic curve. With this, we have the possibility to use it to reduce the amplitude fluctuations in a train of optical pulses.
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  • On the stabilization of bubble solitons

    E. Villagran, J. Bernal, M. Aguero
    580-0
    Abstract:
    We analyze the instability of bubble soliton solutions for the cubic quintic nonlinear Schrödinger equation. This equation, for instance, can be obtained studying the nonlinear excitations of the DNA model. We have found that under specific restrictions concerning the main parameter of the model and soliton velocities, these solutions are stable.
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  • An extension of some classic results on species competition in structured habitats

    H. del Faro Odi, J. Leonel Torres
    585-0
    Abstract:
    We extend some results on population dynamics in a structured habitat, obtained in a classic paper by J.G. Skellam [1], regarding competition between two species differing in fitness, in a habitat with two unequal sectors. Using the discrete spatial and temporal method introduced by him, we generalize his results to an arbitrary number of species ranked according to fitness, competing in a habitat with any number of sectors. We show particular instances of complete species segregation, and of segregation in some sectors, and coexistence in others. We also consider the case of two species, when fitness superiority is not absolute in the sense originally defined by Skellam. Then we briefly ponder the relevance of his discrete method to subsequent research, based on a survey of the fundamental literature on this subject.
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  • Interferometric sinograms of transparent objects with stepping wise shifted Ronchi ruling for optical tomography

    C. Meneses-Fabian, G. Rodriguez-Zurita, V. Arrizón
    591-0
    Abstract:
    An experimental set-up to achieve tomographic images of slices belonging to transparent objects is presented. The proposed system is based on an interferometer which employs two windows in the object plane and a translating absorptive grating (Ronchi ruling) in the frequency plane. In the image plane, replicated windows can be brought to superposition by proper adjustment of the windows' spacing and the grating period. After adaptation of this interferometer to tomographic measurements, the phase of the projections result encoded as interference fringes, thereby forming a composite interference pattern in the plane of the corresponding sinogram (interfero-sinograms). Using phase-stepping techniques, four phase-shifted interfero-sinograms are found after proper translation of the grating. Then wrapped a and unwrapped phase is extracted as a conventional interferogram to obtain a sinogram to be subjected to a filtered backprojection algorithm for reconstruction. As experimental results, the reconstructions of some transparent samples are presented.
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  • Estudio del comportamiento ante la corrosión de películas delgadas de Ni y óxido de Ni obtenidas por espurreo catódico frente a un medio amargo

    C.R. Magaña-Zavala, F.J. Rodríguez-Gómez, D.R. Acosta, R. Ávila-Godo, .
    596-0
    Abstract:
    The aim of this work is to study the behavior of nickel and nickel oxide thin films obtained by cathodic sputtering on steel AISI 1018 (UNS G 10180), with the purpose of diminishing the corrosion rate of low carbon steel when it is exposed to a sour medium (sulphidric acid and chlorides) and to extend the life of metallic petrochemical structures which would result in an important amount of monetary and human resources savings. The characterization of the surfaces was carried out by electrochemical techniques, such as polarization curves and electrochemical impedance spectroscopy.
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  • Variación del espectro de transmitancia de una fibra óptica con recubrimiento de polímero por influencia de hidrocarburos y cambios de temperatura

    A. Treviño Santo, o., M.G. Shl, agin., F.J. Mendieta J., V.V. Spirin, L. Niño de Rivera
    600-0
    Abstract:
    We develop an analysis of the behavior of an evanescent field fiber optic sensor under different conditions for its optimization. This paper presents results of an experimental study of the spectral characteristics of a polymer cladding optical fiber exposed to different analytes. The measurements were performed in the spectral interval from 1100 to 1800 nanometers in a temperature range from 5 to 50$^\circ$C. Influence of ambient temperature on the optical fiber transmittance was found to be strongly dependent on wavelength.
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  • Spinodal decomposition ranges of Al$_x$Ga$_{1-x}$N$_y$P$_{1-y}$ and Al$_x$Ga$_{1-x}$N$_y$Sb$_{1-y}$ alloys

    S.F. Díaz Albarrán, V.A. El, ukhin.
    605-0
    Abstract:
    Spinodal decomposition ranges of low $N$-content Al$_x$Ga$_{1-x}$N$_y$P$_{1-y}$ and Al$_x$Ga$_{1-x}$N$_y$Sb$_{1-y}$ alloys grown on GaP (001) and GaSb (001) substrates, respectively, are described. The alloys are considered according to the strictly regular approximation. In the analysis we take into account the transformations of the bonds and strain, coherency strain, and elastic energies. The spinodal decomposition ranges of the alloys with compositions $0\le x\le0.4$, $0\le y\le0.05$ are demonstrated up to temperatures of 1100$^\circ$C. It is shown that the enlargement of Al and N concentrations dramatically increases the phase separation temperature.
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  • Synthesis of nonuniform long-period optical gratings with a continuous refractive index profile

    O.V. Kolokoltsev, V.A. Sv, r., d., I.F. Llamas, C.L. Ordóñez Romero
    610-0
    Abstract:
    In this work we present a new method for such a non-trivial problem as the synthesis of optical filters based on non-uniform Long-Period Fiber Gratings (LPGs). The method is based on a real-coded genetic algorithm (GA) with a new quasi-analytical procedure for solving the Zakharov-Shabat Inverse Problem. The method possesses an improved computational stability compared to classic numerical algorithms. The main peculiarity of the non-uniform LPG transmission gratings, compared to the Bragg gratings, is that their optical fields oscillate rapidly. However, the quasi-analytical approach proposed allows one to reduce to a minimum the number of sampling points, without, at the same time, losing accuracy in the solution, i.e. the method possesses an improved efficiency. The algorithm convergence was also improved by using the relations of the law of conservation of energy between the interacting waves. It has been shown that the algorithm works adequately even for the case of strongly over-coupled co-propagating lightwave modes, as we demonstrate in several examples of the synthesis of ultra-wide pass-band optical filters (FWHM of 100-200 nm).
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  • Diseño de una superficie cónica en silicio cristalino

    A.G. Rojas H., F. Renero C., W. Calleja A., R. Pérez R., O. Totolhua P., F. Gracia T.
    620-0
    Abstract:
    A fabrication design of a conic surface ($k=-6.5$), in monocrystalline silicon substrate and based on anisotropic etching with KOH, is presented. The conical surface is approximated by means of spherical windows; comparisons between diameter and separation of the spherical windows, as well as the parameters that govern the selection are discussed.
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  • Two-dimensional delta potential wells and condensed-matter physics

    M. de Llano, A. Salazar, M.A. Solís
    626-0
    Abstract:
    It is well-known that a delta potential well in 1D has only one bound state but that in 3D it supports an infinite number of bound states with infinite binding energy for the lowest level. We show how this also holds for the less familiar 2D case, and then discuss why this makes 3D delta potential wells unphysical as models of interparticle interactions for condensed-matter, many-body systems. However, both 2D and 3D delta wells can be ``regularized'' to support a single bound level which in turn renders them conveniently simple single-parameter interactions, e.g., for modeling the pair-forming dynamics of quasi-2D superconductors such as the cuprates, or in 3D of other superconductors and of neutral-fermion superfluids such as ultra-cold trapped Fermi gases.
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  • Figure of merit of a europium type 1:2:3 superconductor compound

    G.M. González, R. Rodríguez-Mijangos, G. Vázquez-Polo
    633-0
    Abstract:
    In this work we evaluate the potentialities of the compound EuBa$_{2}$Cu$_{3}$O$_{7-\delta}$ for thermoelectric applications by showing the curves of thermoelectric figure of merit versus temperatures at different oxygen concentrations. Today those kinds of data are not accessible in current literature for these types of compounds. The curves indicate that the best figure of merit corresponds to a sample with an electric conductivity related to a de-oxygenation parameter $\delta=0.7$, when the material is not in a superconducting state (tetragonal phase). For this oxygen concentration, the values achieved for the electrical conductivity, together with data of thermoelectric power and thermal conductivity, are acceptable for starting a search in thermoelectric applications, which allow us to suppose a possible basement to improve the figure of merit in this compound.
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  • Reconstrucción 3D de la sagita de una superficie en rotación

    V.I. Moreno Oliva, C.I. Robledo Sánchez, A. Cordero Dávila, P. Nanco Hernández, A. Jaramillo Núñez
    636-0
    Abstract:
    An instrument of optical contact is described to obtain the values of zagita in 3D of concave and convex nonreflecting surfaces. The system measure the zagita imaging a thin variable slit over the surface test. To obtain 3D information the instrument is translated in $X$ direction and the surface test is rotated a constant angular velocity. With this system is reached a resolution of 2.5$\mu$m in $Z$ and 1$\mu$m in $X$.
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