Vol. 50 No. 4 (2004): Revista Mexicana de Física.

Published: 2004-01-01

Articles

  • Charged pore matrices prepared with and without template particles by computer simulations

    E. Nuñez, M. Rivera, H. Dominguez
    331-0
    Abstract:
    Series of Molecular Dynamics simulations to study the structure of different porous matrices were investigated. Matrices were prepared by two different processes. The first method consisted of charged particles simulated at a fixed density, and after equilibration, the matrix structure was taken from the last configuration. The second method was prepared from a binary mixture of charged particles where one of the components served as the template material and the other as the matrix. The final porous matrix configuration was obtained by removing template particles from the equilibrated mixture, and therefore only the matrix particles remained in the system. The volume distribution, the cluster formation, and the porosity in the pore matrix were investigated. The present results were compared with previous results of pore matrices prepared without electrostatic interactions. Therefore, electrostatic effects were analyzed in terms of the pore matrix preparation method. As a general trend, it was observed that matrices prepared with template presented smaller voids in the structure in comparison with matrices prepared without template. It was also found that porosity was higher in matrices without template than in matrices with template. When comparisons were made between matrices with and without electrostatic interactions, the porosity was higher in matrices with charge than in matrices without charge. Finally, at low and high temperatures, the porosity values were nearly the same.
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  • Possible cosmological implications in electrodynamics due to variations of the fine structure constant

    J.L. Martínez-Ledesma, S. Mendoza
    337-0
    Abstract:
    Astronomical observations are suggesting that the fine structure constant varies cosmologically. We present an analysis on the consequences that these variations might induce on the electromagnetic field as a whole. We show that under these circumstances the electrodynamics in vacuum could be described by two fields, the ``standard'' Maxwell's field and a new scalar field. We provide a generalised Lorentz force which can be used to test our results experimentally.
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  • Pinch technique prescription to compute the electroweak corrections to the muon anomalous magnetic moment

    L.G. Cabral-Rosetti, G. López Castro, J. Pestieau
    340-0
    Abstract:
    We apply a simple prescription derived from the framework of the Pinch Technique formalism to check the calculation of the gauge-invariant one-loop bosonic electroweak corrections to the muon anomalous magnetic moment.
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  • An echo of an exciting light pulse in quantum wells

    I.G. Lang, L.I. Korovin, D.A. Contreras-Solorio, S.T. Pavlov
    343-0
    Abstract:
    It is shown that non-sinusoidal oscillations appear in transmitted, reflected and absorbed light fluxes when light pulses irradiate a semiconductor quantum well (QW) containing a large number of equidistant energy levels of electronic excitations. The oscillation amplitude is comparable to the flux values for short pulses with the duration $\gamma_l^{-1} \leq \hbar/\Delta E$. A damped echo of the exciting pulse appears in the time intervals $2\pi\hbar/\Delta E$ for very short light pulses $\gamma_l^{-1}\ll \hbar/\Delta E$. Symmetric and asymmetric pulses with a sharp front are considered. Our theory is applicable to narrow QW's in a quantizing magnetic field when the equidistant energy levels correspond to the electron-hole pairs with different Landau quantum numbers.
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  • A nonlinear $R_\xi$--gauge for the electroweak theory

    J.G. M\' endez, J.J. Toscano
    346-0
    Abstract:
    A Gauge--fixing procedure for the electroweak theory, based in the BRST symmetry and covariance under the electromagnetic group, is proposed. It is found that in order to have a renormalizable theory, four--ghost interactions must be included in the BRST invariant action, since in this class of gauges these couplings are induced at the one--loop level. This type of gauges allows us to remove several unphysical vertices appearing in conventional linear gauges, which greatly simplifies the loop calculations, since the resultant theory satisfies QED--like Ward identities. Explicit expressions for the Lagrangian of the bosonic sector, including the corresponding ghost term, are presented.
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  • Influencia del tratamiento térmico con CdCl$_{2}$ sobre las características finales de las celdas solares CdS/CdTe procesadas por transporte de vapor en espacio cercano

    O. Vigil-Galán, G. Santana-Rodríguez, J. Vidal-Larramendi, G. Contreras-Puente, R. Mendoza-Perez, M. Hernández-Bojórquez, A. Morales-Acevedo
    353-0
    Abstract:
    In this paper we present a study on the influence of CdCl$_{2}$ thermal annealing on the characteristics of CdS/CdTe solar cells. Particularly the short circuit current (Isc), the fill factor (FF) and the open circuit voltage (V$_{oc})$ are reported and discussed as a function of the annealing conditions. We show that the annealing improves substantially the open circuit voltage and the fill factor of the cells.
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  • Uso de programación lineal para conocer los parámetros geométricos de superficies cónicas convexas

    A. Santiago-Alvarado, S. Vázquez-Montiel, R. Nivón-Santiago, C. Castañeda-Roldán
    358-0
    Abstract:
    In this paper we proposed a method to obtain the analytic shape of an optical convex surface starting from the coordinates of some points measured on the surface. In other words, we want to find the analytic shape of the surface that best fits a distribution of points measured on a surface (in particular, we want to apply the method to the secondary mirror of the Large Millimeter Telescope; this mirror is a convex surface of 2.57 m diameter, conic constant K= -1, and f/0.4). The method consists of solving the adjustment problem as a problem of polynomial approximation in uniform norm and it is solved by means of linear programming. Finally, we present the result obtained when we evaluate some conical surfaces with the proposed method.
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  • Compactification with $\pmb{U(1)}$ magnetic field within Dirac supersymmetry

    J. Besprosvan, M. Moreno, .
    366-0
    Abstract:
    We consider Dirac-supersymmetric interactions, which produce CP-conserving separation of positive and negative energy solutions in the Dirac equation in order to investigate an alternative to the Kaluza-Klein mechanism. We review conditions under which separation is possible into free particle and compactified behaviors in different dimensions, with attention to spin degrees of freedom. We show a $U(1)$ constant magnetic field produces such kind of behavior; an explicit treatment is given to the 6-$d$ to 4-$d$ and 4-$d$ to 2-$d$ breaking cases and the spectrum is obtained. A dynamical mass-creation mechanism is suggested from the procedure.
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  • Láser de fibra óptica compuesto por dos cavidades acopladas: aplicación como sensor de fibra óptica

    R.A. Vázquez-Sánchez, E.A. Kuzin, B. Ibarra-Escamilla, M. Ma, Alarcón., M. Shl, agin., I. Márquez Borbón
    374-0
    Abstract:
    We show an optical fiber laser sensor which consist of two cavities coupled and three fiber Bragg gratings. We used one Bragg grating (called reference) and two Bragg gratings (called sensors), which have the lower reflection wavelength. The reference grating with the two sensors grating make two cavities: first one is the internal cavity which has 4230 m of length and the another one is the external cavity which has 4277 m of length. Measuring the laser beating frequency for a resonance cavity and moving the frequency peaks when the another cavity is put in resonance, we prove that the arrangement can be used as a two points sensor for determining the difference of temperature or stress between these two points.
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  • The Lagrangians of a one-dimensional mechanical system

    G.F. Torres del Castillo
    379-0
    Abstract:
    Starting from the fact that for an arbitrary autonomous mechanical system any constant of motion can be used as Hamiltonian, the expression for the Lagrangians of a one-dimensional mechanical system previously found by other authors is derived.
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  • Structural and morphological properties of TiO$_{2}$thin films prepared by spray pyrolysis

    N. Castillo, D. Olguín, A. Conde-Gallardo, S. Jiménez-S, oval.
    382-0
    Abstract:
    In the present work the ultrasonic spray pyrolysis technique was employed to produce TiO$_{2}$ thin films on fused quartz and on silicon substrates. An aerosol, generated ultrasonically, of titanium diisopropoxide was employed in the deposition experiments. The crystallization process of the as deposited samples was studied by X-Ray Difraction (XRD), Raman spectroscopy (RS), Atomic Force Microscopy (AFM), and optical transmission spectroscopies (TS). The results show that spray pyrolysis technique is able to produce films with smooth surface and good crystalline properties. When deposition temperatures were below 400$^\circ$C, the films grow with a flat surface (roughness $ \sim $5{\AA}) but in amorphous phase; while for equal or higher values to this temperature, the films develop a crystalline phase corresponding to the TiO$_{2}$ anatase phase, but the surface roughness is increased up to $ \sim $225{\AA}. After annealing at 750$^\circ$C, the samples deposited on Si show a partial transition to rutile phase preferentially oriented in (111) direction; while, under a similar annealing, those films deposited on fused quartz do not show any phase transition.
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  • Coarse graining in genetic dynamics: A renormalization group analysis of a simple genetic system

    C.R. Stephens, C. Chr, ssomalakos., Adolfo Zamora.
    388-0
    Abstract:
    We show how the idea of coarse graining can be applied fruitfully to the area of genetic dynamics, both in the context of ``effective'' theories - leading to more appropriate effective degrees of freedom with which to describe the dynamics - as well as in terms of integrating out degrees of freedom, using the Renormalization Group as a systematic calculational scheme. Specializing to dynamics in the presence of selection and mutation we show how the Renormalization Group can be implemented at the level of a transfer matrix-type description. Further, we present an explicit exact RG transformation for the simple case of a one gene-two allele system, solving for its fixed points and the asymptotic behaviour of the system in the vicinity of these fixed points.
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  • Some considerations regarding the exit pupil location in some visual systems

    V.M. Durán-Ramírez, D. Malacara-Doblado, J.M. López Ramírez
    397-0
    Abstract:
    The proper location of the eye with respect to a visual optical instruments is not a trivial problem. It depends on many practical factors not widely known. This paper describes some of the many factors that determine the best position for the pupil of the observer with respect to the optical system which is not necessarily the exit pupil of the system. The practical consequences of this problem are very important for the design of visual systems.
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  • Higgs mass and grand unification

    S.R. Juárez W., A. Morales S., P. Kielanowski
    401-0
    Abstract:
    The knowledge of the solution of the Renormalization Group Equation (RGE) for the quartic self-interaction of the Higgs scalar $\lambda _{H}$ is crucial for the determination of the energy limits on the Higgs mass. $\lambda _{H}$ is also important in order to solve the RGEs at the two loop level for other observables like the quark masses or the Cabibbo-Kobayashi-Maskawa (CKM) matrix. We obtain an analytical and numerical (considering Grand Unification) solution to the one loop renormalization group evolution of the Higgs quartic coupling $\lambda _{H}$ in the energy range $[m_{t},E_{GU}]$, where $m_{t}$ is the mass of the top quark and $E_{GU}=10^{14}$~GeV. We find that depending on the value of $\lambda _{H}(m_{t})$ the solution for $ \lambda _{H}(E)$ may have singularities or zeros and become negative, in which case the Standard Model (SM) becomes inconsistent. We obtain that for $0.65\leq \lambda _{H}(m_{t})\leq 0.69$ the SM is valid in the whole range $[m_{t},E_{GU}]$. These values of $\lambda _{H}(m_{t})$ correspond to the following Higgs mass $198\leq m_{H}\leq 205\,$GeV.
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  • Low temperature S-shaped heat capacities in finite nuclei

    Osvaldo Civitarese., Peter O. Hess, Jorge G. Hirsch
    406-0
    Abstract:
    While in the thermodynamic limit a phase transition is signaled by the presence of a sharp peak in the specific heat, in finite systems a bump is usually found. However, there are relevant cases in which the presence of a low-temperature bump in the canonical specific heat of atomic nuclei is linked to the existence of isolated low energy states through a local Schottky effect, and do not represent a phase transition. Examples are presented for light and heavy deformed nuclei, by using in the calculations experimental and theoretical energy levels.
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  • Excitones confinados en puntos cuánticos esferoidales prolatos

    A. Corella-Madueño, R.A. Rosas, J.L. Marín, R. Riera
    412-0
    Abstract:
    The variational method is used to solve in approximately way the Schrödinger's wave equation associated to a Wannier-Mott exciton confined within a spheroidal quantum dot. The confinement effect on the ground-state energy of the electron-hole pair trapped inside a crystallite with this geometry, and with soft or hard walls, is analyzed. The walls can be modeled as finite or infinite potential barriers with suitable border conditions, which will depend on the considered case. The results of this work are compared with those obtained by other authors through more sophisticated methods. A comparison with experimental data of CdS crystallites embedded in materials of different composition is made, too. For a finite potential barrier, a critical size of the crystallite from which the exciton escapes of the quantum dot, is predicted. This is in opposition with the infinite potential barrier model where the exciton never can leave the region where it is confined.
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