Vol. 50 No. 2 (2004): Revista Mexicana de Física.

Published: 2004-01-01

Articles

  • Sobre el estado excitado 2$\pmb{\sigma_g}$ del ion molecular H$\pmb{_2^+}$ en un campo magnético intenso

    F.B. Alarcón, A.V. Turbiner, J.C. López Vie, ra.
    93-0
    Abstract:
    A variational study of the excited state $2\sigma_g$ of the $H_2^+$ molecular ion in a parallel configuration with a strong magnetic field of strength $B=1$ a.u. ($=2.35\times 10^9$ G) is presented. It is shown that a proper choice of a simple trial function gives accurate results in the total and binding energies.
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  • Considerations about the variability of the Bragg's law fulfilment

    L. Pérez-Arrieta, C. Tabares-Muñoz
    96-0
    Abstract:
    On working with an X-ray powder diffractometer, due to practical difficulties, the surface of the specimen some times is not accurately placed on the working plane of the goniometer. This disagreement produces an asymmetric broadening of the diffraction line profile, and also a shift in the peaks positions. In this work we expose some considerations about the way each diffracted beam fulfils the Bragg's law, addressed to its possible application for correcting the $2\theta $ shifts caused by the specimen-displacement error of polycrystalline samples.
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  • Calculo del ángulo de desviación en un pentaprisma, en función de los errores en los ángulos de 90 y 112.5 grados y el error piramidal

    A. Jaramillo Núñez, A. Vera Marquina
    99-0
    Abstract:
    An expression is derived showing the relationship between the errors in the 90 and 112.5 degree angles of a pentaprism and the angle formed between entrance and exit beams (angle of deviation). A relationship between the pyramidal errors of each prism surface and the deviation of the exit beam from the plane defined by the entrance and ideal exit beams is also deduced. Finally, a method for constructing a pentaprism with a particular error making use of these derivations is discussed. The fabricated pentaprism is used to characterize a coordinate measuring machine of large dimensions.
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  • Some comments on unitarity gauge

    M.A. López--Osorio, E. Martínez--Pascual, J.J. Toscano
    107-0
    Abstract:
    A pedagogical discussion on the unitarity gauge within the context of Hamiltonian path integral formalism is presented. A model based on the group $O(N)$, spontaneously broken down to the subgroup $O(N-1)$, is used to illustrate the main aspects of this gauge--fixing procedure. Among the issues, discussed with some extent, are: (1) the structure of model's constraints following the Dirac's method, (2) the gauge--fixing procedure, using the unitarity gauge for the massive gauge fields and the Coulomb one for the massless gauge fields, (3) the absence of BRST symmetry in this gauge--fixing procedure and its implications on the renormalizability of the theory, and (4) the static role of the ghost and anti--ghost fields associated with the massive gauge fields and how their contributions can be eliminated by using the dimensional regularization scheme.
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  • Self-normalized photoacoustic techniques for thermal diffusivity measurements in metals

    J. A
    120-0
    Abstract:
    The analytical solution for the one-dimensional heat diffusion problem involving an harmonic heat source in a single layer, in the surface absorption limit, is used to provide self-normalized methodologies for thermal diffusivity measurements in metals, by using the photoacoustic technique. The self-normalized procedure involves the photoacoustic phase lag between the rear and front configurations. Three methodologies are described. Two of them involving linear fits in the photoacoustic thermally thin and thermally thick regimes. Comparison between the theoretical normalized equations and the corresponding normalized experimental data allows for the development of criteria on the selection of an appropriate modulation frequency range where a reliable analysis can be done. Computer simulations and thermal diffusivity values measured for some materials are also provided. The values of thermal diffusivity for each material, obtained by using the different reported methodologies, were found to be close to each other, showing self-consistency between the different methodologies described in this paper.
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  • Estabilidad robusta de la posición vertical de dos péndulos acoplados por un resorte

    E. Muñoz Aguirre, O. Alex, rova.
    127-0
    Abstract:
    In the present work we find the equations governing the movement of two coupled pendulums with a spring of elasticity constant $K$ and we make a study of the robust stability when the system is subjected to a vertical acceleration. Likewise we make an brief analysis of the influence of the spring constant on the stability of the spring.
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  • Layer-by-layer analysis of second harmonic generation at a simple surface

    J.E. Mejía, C. Salazar, B.S. Mendoza
    134-0
    Abstract:
    We present a general scheme, based on a a microscopic formulation, to obtain the second harmonic signal produced by each atomic layer of a semi-infinite crystal. Using the simple Si(111):H $(1\times 1)$ surface as an example, we obtain that the nonlinear polarization in the bulk does not decay to zero due to the lack of centrosymmetry of the individual layers. However, the sum of this polarization follows the physically correct picture that the second harmonic signal is zero at the centrosymmetric bulk and finite in the selvedge as the surface is approached and the centrosymmetry is broken. The results show that the selvedge region includes the surface and just a few layers below it.
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  • All-optical performing of logic-based operations due to a two-phonon light scattering

    A.S. Shcherbakov, E. Tepichin Rodríguez, A. Aguirre López.
    140-0
    Abstract:
    We describe physical principles of a two-phonon light scattering in crystals and present the feasibility of exploiting such a phenomenon for the creation of an all-optical logic. Both performing the functionally complete set of logic operations and fulfilling one bit memory have been algorithmically analyzed and estimated.
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  • Order/disorder in brain electrical activity

    O.A. Rosso, A. Figliola
    149-0
    Abstract:
    The processing of information by the brain is reflected in dynamical changes of the electrical activity in time, frequency, and space. Therefore, the concomitant studies require methods capable of describing the quantitative variation of the signal in both time and frequency. Here we present a quantitative EEG (qEEG) analysis, based on the Orthogonal Discrete Wavelet Transform (ODWT), of generalized epileptic tonic-clonic EEG signals. Two quantifiers: the Relative Wavelet Energy (RWE) and the Normalized Total Wavelet Entropy (NTWS) have been used. The RWE gives information about the relative energy associated with the different frequency bands present in the EEG and their corresponding degree of importance. The NTWS is a measure of the order/disorder degree in the EEG signal. These two quantifiers were computing in EEG signals as provided by scalp electrodes of epileptic patients. We showed that the epileptic recruitment rhythm observed for generalized epileptic tonic-clonic seizures is accurately described by the RWE quantifier. In addition, a significant decrease in the NTWS was observed in the recruitment epoch, indicating a more rhythmic and ordered behavior in the brain electrical activity.
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  • Velocity autocorrelation function of a dispersion of heavy particles in a turbulent flow: on the effect of interparticle collisions

    R. Avila, M.A. Rodríguez-Meza
    156-0
    Abstract:
    The effect of particle-to-particle interactions on the dispersion and on the velocity auto-correlation function of heavy particles in a turbulent flow, is presented. The inter-particle collision process is based on a direct numerical simulation approach, which requires that all the particles be simultaneously tracked through the flow field. In the first part of the paper, the turbulent characteristics of the velocity of non-colliding heavy particles which disperse in a vertical, nearly isotropic, grid generated decaying turbulence air flow, are presented. In the second part of this investigation, the solid particles are allowed to collide. The numerical predictions confirm the fact that the inter-particle collisions promote a decrease of the lateral particle dispersion, the particle velocity autocorrelation function and the mean lateral velocity of the particles.
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  • Análisis del proceso de propagación de secuencias de pulsos ultracortos en fibras con inhomogeneidades periódicas

    R. Parada-Alfonso, V. V, sloukh., E. Martí-Panameño
    162-0
    Abstract:
    Ultrashort pulses secuences of picosecond duration are propagated in two differents modes through a waveguide which is assumed to have a refraction index that varies periodically along its length. The interaction between the two modes is analized using Jacobi functios as analitical solutions serving like a model of periodical pulse trains. Linear and nonlinear effects are considered to solve the nonlinear Schrödinger coupled equations system which let us study the propagation of pulse trains.
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  • Opo-time of flight system for multiphoton ionization and dissociation studies

    E. Mejia-Ospino, I. Alvarez, C. Cisneros
    170-0
    Abstract:
    The design and operation of a time-of-flight mass analyzer for study of ionization and dissociation of atoms and molecules produced by interaction with pulsed laser generated by an OPO are described. This apparatus allows the investigation of atomic and molecular excited states no accessible by single-photon absorption. The description of the main parts as well as the associated instrumentation, and results of multiphoton absorption of xenon and acetone are presented.
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  • Signal separation with almost periodic components: a wavelets based method

    O.A. Rosso, A. Figliola, S. Blanco, P.M. Jacovkis
    179-0
    Abstract:
    Natural time series usually show either a combination of periodic phenomena with stochastic components or chaotic behavior. In many cases, when nonlinear characteristics are computed, they will essentially indicate the most remarkable effects and the results will underestimate or overestimate the real complexity of the system. For that reason signal separation of the frequency bands representing well known phenomena, like periodic or almost periodic behaviors, allows comprehension of the hidden nonlinear or stochastic phenomena involved. In this work a signal separation method based on trigonometric wavelet packets is described. The method has been applied, as an example, to a time series of daily mean discharges of the Atuel river in Argentina, that presents strong annual and semiannual oscillations due to meteorological effects. The correlation dimension and the maximum Lyapunov exponent of the residual time series were obtained taking away its known almost periodic components.
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  • Cerámicas bioeutécticas W-TCP

    A. Tejeda, C. Piña, S. Martínez, G. Avila
    187-0
    Abstract:
    To obtention of biocompatible ceramics based in calcium phosphate it was chosen the eutectic binary system Wollastonite- Tricalcium Phosphate (W-TCP) and were employed different process of cooling. The obtained eutectic ceramics from calcium phosphate systems are called bioeutectics because their potential applications in biological areas. The relation between cooling rate process and the resultant structure is reported. The characterization of these materials was made with X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Differential Thermal Analysis (DTA).
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  • Influence of indium segregation on the light emission of piezoelectric InGaAs/GaAs quantum wells grown by molecular beam epitaxy

    C.M. Yee-Rendón, M. López-López, M. Meléndez-Lira
    193-0
    Abstract:
    Pseudomorphic $In_{0.2}Ga_{0.8}As/GaAs$ quantum wells (QWs) were grown by molecular beam epitaxy (MBE) on GaAs substrates oriented along the (11n) direction, with n=1,2,3,4. The optical and structural properties of the heterostructures were studied by photoluminescence spectroscopy (PL) at 14, 77 and 300 K, and atomic force microscopy (AFM) measurements. The emission wavelength from the QWs has two contributions, a blue shift due to the compressive strain, and a red shift due to the quantum confined Stark effect produced by the piezoelectric field present in these materials. A traditional theoretical interpretation of the QWs emission employing a simple well model shows discrepancies with the experimental results. In order to satisfactorily explain the emission wavelength we proposed to include segregation effects of In at the wells interfaces. The matrix transfer method was implemented to solve numerically the Schrödinger equation taking into account In segregation effects by including an asymmetric potential well with a profile depending on the details of the In incorporation. With segregation effects included in the emission calculations, the theoretical predictions reproduce very well the experimental values of PL emission. Our results demostrate that in order to have efficient InGaAs QWs-based optoelectronic devices is very important to take into account interfacial segregation effects.
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  • Electroweak radiative corrections to semileptonic $\tau$ decays

    J. Erler
    200-0
    Abstract:
    I present an update on the electroweak radiative correction factor to semileptonic $\tau$ decays, including a next-to-leading order resummation of large logarithms. My result differs both qualitatively and quantitatively from the one recently obtained by Davier~\etal. As two consequences, (i) the discrepancy between the predictions for the muon $g-2$ based on $\tau$ decay data and $e^+ e^-$ annihilation data increases, and (ii) the $g-2$ prediction based on $\tau$ decay data appears to be consistent (within about one standard deviation) with the experimental result from BNL.
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  • Grabado anisotrópico de silicio para aplicación en micromaquinado usando plasmas de SF$_{\pmb 6}$/CH$_{\pmb 4}$/O$_{\pmb 2}$/Ar y SF$_{\pmb 6}$/CF$_{\pmb 4}$/O$_{\pmb 2}$/Ar

    C. Re, es-Betanzo., S. A, ov., J.W. Swart
    203-0
    Abstract:
    We investigated the reactive ion etching of silicon using S$F_{6}$/C$H_{4}$(C$F_{4}$)/$O_{2}$/Ar gas mixtures containing fluorine for MEMS applications. Etch rates and anisotropy of etch profiles were examined as a function of gas composition, material of electrode, and RF power. Etch depths were measured using a profilometer, and etch profiles were analyzed by scanning electron microscope. As a mask material, an aluminium film deposited by evaporation, was used. High anisotropy of etching of ~0.95 was achieved at etch depths up to 20-30 micrometers and etch rates of approximately 0.3-0.6 $\mu$m/min. Highly anisotropic etching is based on a mechanism that enhance the ion bombarding and protects the sidewalls due to polymerization and/or oxidation mechanisms in order to avoid the lateral etch. However, under the anisotropic etching conditions, considerable damages of the etched surfaces (roughness formation), were observed. After etching experiments, wet / dry cleaning procedures were applied to remove surface residues resulting from the reactive ion etching and to improve the etched surface morphology.
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  • Sensor diferencial de fibra óptica

    J. Tapia-Mercado, A. García-Weidner, M.L. Galindo-Cavazos, A. V, M. A
    208-0
    Abstract:
    A differential temperature sensor employing a low-coherence interferometry technique with signal processing based on phase scanning is described. The sensor is composed of a compensated interferometer formed by two sections of a high-birefringence fiber and a polarimetric scheme, which includes Wollaston prism and polarizer, permitting the interference between two orthogonally polarization modes. The sensor allows the measurements of the difference in temperature, between two points with high sensitivity. The sensor can be used in calorimetry for laser power measurements. The details of the sensor configuration and signal processing method along with experimental results are described.
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