Vol. 50 No. 6 (2004): Revista Mexicana de Física.

Published: 2004-01-01

Articles

  • Comments on area spectra in loop quantum gravity

    A. Corichi
    549-0
    Abstract:
    We examine and compare different area spectra that have been recently considered in Loop Quantum Gravity (LQG). In particular we focus our attention on an Equally Spaced (ES) spectrum operator introduced recently by Alekseev et al that has gained some attention. We show that such operator is not well defined within the LQG framework, and comment on the issues regarding area spectra and QNM frequencies.
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  • The semiclassical theory of quantized fields in classical electromagnetic backgrounds

    J. Haro
    553-0
    Abstract:
    We formulate mathematically the process of pair production in electromagnetic fields for spinless particles. We compute the probability that~$n$ pairs are created in the semiclassical approximation, and herein we prove that the pair creation phenomenon is a stochastic Poisson process. Finally, we prove rigorously and interpret suitably the Schwinger formula.
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  • Qualitative analysis of the capillary flow stability of spurting materials by using transmitted light intensity measurements

    B.M. Marín Santibañez, F. Rodríguez González, J. Pérez-González, L.A. Vega Acosta M., E. Moreno García
    562-0
    Abstract:
    The stability in the capillary flow of spurting materials was analyzed by following the temporal variation of their average birefringence. Two different fluids were analyzed, a high-density polyethylene melt, and an aqueous micellar solution of Cetylpyridinium Chloride and Sodium Salicylate. Birefringence changes were detected through measurements of the transmitted light intensity and video images of the flow channel. Transmitted light intensity measurements were more sensitive and provided better information about the flow stability than pressure ones. An unstable flow region was present in the micellar solution before the onset of spurt. Also, there were periodic changes in the optical properties of both fluids, in the spurt region and at higher shear rates, which indicate that the flow is also unstable in the high shear rate branch, in contrast to the generalized assumption of the stable flow in such regime. Several frequency components of the transmitted light intensity were observed, coincident with different spurts, in the unstable flow regimes for both fluids. Finally, an outstanding decrease in the transmitted light intensity was observed in both fluids under flow conditions where slip was present.
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  • High resolution spectroscopy in gases and its application on the frequency stabilization of semiconductor lasers

    E. de Carlos López\(^{a, b}\)., J.M. López Romero\(^{a}\)
    569-0
    Abstract:
    The gas spectroscopy is a standard technique for frequency stabilization in semiconductor lasers. The Doppler effect is what exerts most influence on the linewidths of atomic spectrum increments. In the present work, the saturation spectroscopy, whose main characteristic is to cancel the undesirable consequences of this effect, is studied. Different techniques of dispersion-like signals generation are revised, and are used as error signals in the stabilization of semiconductor lasers. From a different point of view, these techniques are reviewed and being compared. Moreover, the experimental results of Cesium 133 spectroscopy made at Centro Nacional de Metrología, CENAM, are exhibited herein.
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  • Segmentación de imágenes de color

    J.J. Báez Rojas, M.L. Guerrero, J. Conde Acevedo, A. Padilla Vivanco, G. Urcid Serrano
    579-0
    Abstract:
    In this work we propose a modification to the color system $I1$, $I2$. $I3$. This color system was developed by Ohta et al. We named $C1 C2 C3$ the modified components, those were experimentally obtained. We use $C1$ $C2$ $C3$ to segment and generate maps. During the segmentation process we obtain binary images. In this work we propose two arithmetic operations with the binary images, product and sum. Multiplication reduces the noise aroused during the thresholding segmentation process.
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  • Band gap energy in Zn-rich $\pmb{Zn_{1-x}Cd_xTe}$ thin films grown by r.f. sputtering

    M. Becerril, H. Silva-López, O. Zela, a-Angel.
    588-0
    Abstract:
    Ternary $Zn_{1-x}Cd_{x}Te$ semiconducting thin films were grown on 7059 Corning glass substrates at room temperature by co-sputtering from a ZnTe-Cd target. The visible Cd-area onto ZnTe target was varied to cover 0% - 4% of the total area. The optical and structural properties of the films were analysed as a function of the Cd concentration ($x$) on the layers. The sharp diffraction lines indicate mainly the ZnTe in cubic phase. When the cadmium was incorporated into the ZnTe lattice, the band gap energy $(E_g)$ decreased from 2.2748 eV ($x$=0) to 2.2226 eV ($x$= 0.081). We found a linear relationship between $E_g$ and ($x$) in the interval of ($x$) studied, that predicts a value of the bowing parameter b in the $E_g(x) = E_{go} + ax + bx^{2}$ relationship, which coincides with the value of b calculated by making the same study for Cd-rich CdZnTe. This result becomes interesting given the large number of values reported for b and $a$ for $E_g$($x$) of this ternary material.
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  • Theoretical basis for the study of the effect of base composition on DNA melting

    L. Dagdug, L. Young
    594-0
    Abstract:
    We extend the ideas used to describe the glass transition in strong glasses employing the stochastic matrix method, giving a theoretical framework for the study of the configurational changes and the melting temperature of DNA. Our theoretical model enables a systematic study of the melting transition and the melting temperature dependence on the sequence differences in vertical stacking. Taking into account the fractional composition in a single strand, exact analytic results are given for the fraction of bonds intact and denatured at a particular temperature. This method is applicable to long DNA as well as RNA.
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  • CIE-XYZ fitting by multispectral images and mean square error minimization with a linear interpolation function

    J. Conde, H. Haneishi, M. Yamaguchi, N. Oh, ama., J. Baez
    601-0
    Abstract:
    We present a proposal to achieve accurate color reproduction by fitting the CIE-XYZ tristimulus values of a given color stimulus, and using the 16-band multispectral images, as well as the CIE-XYZ tristimulus values of a known color test chart. We propose a simple linear combination of the multispectral images as interpolation function, which is equivalent to fitting the data to a straight line in a 16-dimentional space. By using this interpolation function, we minimize the merit function of the mean error between the measured and estimated CIE-XYZ tristimulus values of the color test chart. We show, by making a visual comparison between the results achieved using this proposal and the results achieved using a spectral reflectance estimation technique, that the proposed interpolation function when using 16-channel multispectral images produces high quality results in terms of color reproduction fidelity.
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  • Symplectic structures and dynamical symmetry groups

    G.F. Torres del Castillo, M.P. Velázquez Quesada
    608-0
    Abstract:
    Apart from the total energy, the two-dimensional isotropic harmonic oscillator possesses three independent constants of motion which, with the standard symplectic structure, generates a dynamical symmetry group isomorphic to SU(2). We show that, by suitably redefining the symplectic structure, any of these three constants of motion can be used as a Hamiltonian, and that the remaining two, together with the total energy, generate a dynamical symmetry group isomorphic to SU(1,1). We also show that the standard energy levels of the quantum two-dimensional isotropic harmonic oscillator and their degeneracies are obtained making use of the appropriate representations of SU(1,1), provided that the canonical commutation relations are modified according to the new symplectic structure. Whereas in classical mechanics the different symplectic structures lead to equivalent formulations of the equations of motion, in quantum mechanics the modifications of the commutation relations should be accompanied by modifications in the interpretation of the formalism in order to obtain results equivalent to those found with the common relations.
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  • Subband structure comparison between $n$- and $p$-type double delta-doped GaAs quantum wells

    I. Rodriguez-Vargas, L.M. Gaggero-Sager
    614-0
    Abstract:
    We compute the electron level structure ($n$-type) and the hole subband structure ($p$-type) of double $\delta$-doped GaAs (DDD) quantum wells, considering exchange effects. The Thomas-Fermi (TF), and Thomas-Fermi-Dirac (TFD) approximations have been applied in order to describe the bending of the conduction and valence band, respectively. The electron and the hole subband structure study indicates that exchange effects are more important in $p$-type DDD quantum wells than in $n$-type DDD. Also our results agree with the experimental data available.
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  • Corriente eléctrica de portadores calientes en semiconductores intrínsecos: desequilibrio en la concentración

    G. Espejo-López, O.I. L, ubimov., O. Yu, Yu. G
    620-0
    Abstract:
    The theory of nonlinear Current Voltaje Characteristics (CVC) of hot electrons which take into account not only the nonequilibrium energy of carriers (heating of carriers), but the nonequilibrium concentration due to heating is presented for the first time.
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  • Medición del ángulo de desviación en un pentaprisma usando un interferómetro de Sagnac

    A. Jaramillo Núñez
    625-0
    Abstract:
    A technique to measuring the error in the deviation angle of a pentaprism, which makes use of a Sagnac interferometer, is described. The beams of the interferometer are employed as reference beams in order to align the pentaprism. The deviation angle is measured with the aid of a reference flat attached to a goniometer mounting. Deviation angles with an error of 15 arcsec have been measured with a precision of 2 arcsec. The measurement precision can be improved substituting the goniometer for one of higher precision.
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  • Scaling of the optical phonon frequency in diamond-like elements and III-V group semiconductor compounds

    J.L. Escamilla Re, es., E. Haro Poniatowski
    629-0
    Abstract:
    In this work, the homology in the vibrational properties of solids with diamond and zincblende crystal structure is studied. Introducing the Tolpygo´s dimensionless frequency we scale the phonon dispersion curves along high-symmetry directions for diamond-like elements and some III-V semiconductor compounds (AB compounds); also, starting from the diamond-like structure element phonon frequencies, we calculate the optical phonon frequencies for such AB compounds. The agreement between theoretical and experimental data is outstanding considering the simplicity of the calculations needed to obtain these results.
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  • Brownian motion of optically anisotropic particles in weak polymer gels

    P. Díaz-Le, va., E. Pérez, J.L. Arauz-Lara
    633-0
    Abstract:
    The translational and rotational dynamics of optically anisotropic colloidal particles in viscoelastic polymer gels is studied by dynamic light scattering. Polymer gels are formed by polymerization of acrylamide and bis-acrylamide, as cross-linking agent, in water. Particles with crystalline internal structure are made by emulsification of liquid crystal at the nematic phase and a further consolidation. In this work we consider only low fractions of cross-linking, where the systems are ergodic and gaussian statistics is valid. Experimental results for the rotational and translational mean squared displacements, showing the effect of the system's viscoelasticity on both diffusion modes, can be fitted to an extended model of Brownian particles harmonically bounded to a multicomponent of weak elastic restoring forces.
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  • Simultaneous measurement of the (d,d) and (d,p) resonant reactions using thick deuterated polyethylene targets

    E. Chávez, A. Huerta, M.E. Ortiz, G. Murillo, R. Policroniades, A. Varela, E. Moreno, A. Galindo-Uribarri, J. Gómez-del-Campo, F. Liang, D. Shapira
    639-0
    Abstract:
    The thick target technique is extended here to study reactions induced by deuterons. Deuterated target production and characterization is reviewed in detail. The experimental setup used in a previous work, to study proton induced resonances, is complemented here with an additional thin transmission detector for standard particle identification with the E-$\Delta $E technique. Through the measurement of the excitation functions of the $^{2}$H($^{12}$C,d) and $^{2}$H($^{12}$C,p) reactions, resonant states in $^{14}$N are studied. Using known energies and widths for resonances in the relevant energy range, good fits were obtained with R-matrix calculations.
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