Vol. 48 No. 5 (2002): Revista Mexicana de Física.

Published: 2002-01-01

Articles

  • Cálculos ab initio de la estructura electrónica del superconductor $MgB_2$

    Roberto Núñez Gonzalez., Arm., o Re., es Serrato.
    391-0
    Abstract:
    We present ab-initio calculations of the electronic properties of superconductor MgB$_2$ using a WIEN97 computational code, which use a FLAPW (Full-Potential Linearized Augmented Plane Waves) method to calculate electronic properties. Total energy, band structure, density of states and charge density are calculated employing experimental lattice parameters. This work has special interest in the analysis of charge density. From total energy calculations we obtain the lattice parameters in equilibrium, and they agreed to 1% with the experimental ones; from charge density calculations we observe charge transference to boron atoms planes and the covalent character of the bond B-B, besides we observe the presence of type metallic and partially ionic bonds.
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  • Simulación numérica de señales de radiometría fototérmica en mono cristales de silicio

    I. Campos-Cantón, M.E. Rodríguez, Facundo Ruiz., .
    397-0
    Abstract:
    By using the theoretical model proposed by Mandelis et al. and a numerical simulations. We have analysed the generation of photoinduced black body radiation (photothermal radimometry signal) on monocrystalline silicon wafers. We report the particular role of each one of the main parameters involved on the phototermal signal. The parameter values were taken of the reported values for industrial silicon wafers. We show a discuss the obtained results.
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  • Low energy neutrons from a $^{239}$PuBe isotopic neutron source inserted in moderating media

    Héctor René Vega-Carrillo., Celia Torres-Muhech.
    405-0
    Abstract:
    Several neutron applications share a common problem: the neutron source design. In this work MCNP computer code has been used to design a moderated $^{239}$PuBe neutron source to produce low energy neutrons. The design involves the source located at the center of a spherical moderator. Moderator media studied were light water, heavy water and a heterogeneous combination of light water and heavy water. Similar moderating features were found between the 24.5 cm-radius container filled with heavy water (23.0-cm-thick) and that made with light water (3.5-cm-thick) plus heavy water (19.5-cm-thick). A $^{239}$PuBe neutron source inserted in this moderator produces, at 27 cm, a neutron fluence of 1.8 x 10$^{ - 4}$ n-cm$^{ - 2}$ per source neutron, with an average neutron energy of 0.34 MeV, where 47.8 % have an energy $ \le $ 0.4 eV. A further study of this moderator was carried out using a reflector medium made of graphite. Thus, 15-cm-thickness reflector improves the neutron field producing a neutron fluence of 1.1 x 10$^{ - 3}$ n-cm$^{ - 2}$ per source neutron, whose average neutron energy is 0.098 MeV, where 75.8 % have an energy $ \le $ 0.4 eV.
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  • Production of the MSSM higgs bosons at next generation linear $e^+e^-$ colliders

    A. Gutiérrez-Rodríguez, M. A, O. A, o.
    413-0
    Abstract:
    We study the production of the Higgs bosons predicted in the Minimal Supersymmetric extension of the Standard Model $(h^0, H^0, A^0, H^\pm)$, with the reactions $e^{+}e^{-}\rightarrow b\bar b h^0 (H^0, A^0)$, and $e^+e^-\to \tau^-\bar \nu_\tau H^+, \tau^+\nu_\tau H^-$, using the helicity formalism. We evaluate the cross section of $h^0, H^0, A^0$ and $H^\pm$ in the limit when $\tan\beta$ is large. The numerical computation is done considering two stages of a possible Next Linear $e^{+}e^{-}$ Collider: the first with $\sqrt{s}=500$ $GeV$ and design luminosity 50 $fb^{-1}$, and the second with $\sqrt{s}=1$ $TeV$ and luminosity 100-200 $fb^{-1}$.
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  • Quantum bits and superposition of displaced Fock states of the cavity field

    L.M. Arévalo Aguilar, H. Mo, a-Cessa.
    423-0
    Abstract:
    We study the effects of counter rotating terms in the interaction of quantized light with a two-level atom, by using the method of small rotations. We give an expression for the wave function of the composed system atom plus field and point out one initial wave function that generates a quantum bit of the electromagnetic field with arbitrary amplitudes.
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  • The depolarization field in polarizable objects of general shape

    A. Zehe, A Ramírez.
    427-0
    Abstract:
    The polarization of particles or biological cells is commonly investigated by measuring the impedance of suspensions or by a variety of single particle methods, that exploit different force effects. For biological cells the most striking frequency-dependent changes in polarizability result from structural (Maxwell-Wagner) polarization phenomena. Explicit solutions of the Laplace equation are available only for objects with finite surfaces of the second degree. Thus, dielectric models consider the structural properties of cells by assuming spherical or ellipsoidal geometries, since only in very few cases is the effective local field $E_{i}(r)$ in the presence of a dielectric object known. This concerns dielectric bodies of special shape, which are exposed to a special electric field $E_{0}(r)$. In the present paper an approximation procedure is presented for the general case, allowing to calculate the depolarization field $E_{i}(r)$, which is generated in the presence of an arbitrarily shaped dielectric object, introduced into a field space $\vec {E}_0 (\vec {r})$. Contrary to recent numerical methods (finite element technique), which require extensive computer resources due to the unavailability of analytical solutions, the here presented approach results in closed analytical expressions. The applicability of the method is demonstrated for a non-ellipsoidal cylindrical dielectric by measuring its dipole moment in a microwave field. The accordance with the calculated results is found to be one order of magnitude better than it would be in the commonly practiced procedure, where the cylinder is substituted by a spheroid of the same axis relation.
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  • PD molecule: transition probabilities and dissociation energy

    N. Prithivi Kumaran, N. Rajamanickam
    432-0
    Abstract:
    The Franck-Condon (FC) factors (transition probabilities) and r-centroids have been evaluated by the more reliable numerical integration procedure for the bands of A$^{3} \Pi_{i}- $X$^{3}\Sigma ^{-}$ system of molecule PD, using a suitable potential. The dissociation energy for the electronic ground state of PD molecule has been estimated by fitting the empirical potential function to the experimental potential energy curve, using correlation coefficient.
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  • Sensor láser de fibra óptica con una cavidad de 8.6 km formada por dos rejillas de Bragg usadas como espejos.

    M. Ma, -Alarcón., E. A, R. A, M. A, M. G, agin., I. Márquez-Borbón
    434-0
    Abstract:
    We report the operation of a fiber laser sensor made by an Erbium Doped Fiber pumped at $980nm$, an $8.67km$ passive fiber and two fiber Bragg gratings placed at the ends of the laser cavity. Under normal conditions, the Bragg gratings have different reflection wavelengths and laser emission is not generated. The two Bragg gratings can be placed at the same reflection wavelength when the Bragg grating with the lowest reflective wavelength is strained which can be used as a sensor element. The laser generation thus shows that the Bragg grating is under strain. Furthermore, our configuration gives us the possibility for knowing the distance between two Bragg gratings when the laser beating frequency is measured. A measurement precision better than $25m$ in $8.67Km$ is shown to be feasible.
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  • Determination of phases of $\alpha $-Fe$_{2}$O$_{3}$:SiO$_{2}$ compound by the rietveld refinement

    S.A. Palomares Sánchez, S. Ponce-Castañeda, J.R. Martínez, Facundo Ruiz.
    438-0
    Abstract:
    We use a variation of the Rietveld refinement method to calculate the amorphous content of composites formed by a silica xerogel amorphous matrix and iron particles embedded into. In order to apply the Rietveld refinement to amorphous structures an initial crystalline model is assumed with the same composition as the material to be modelled. In this work we try to refine the structure of compounds using the program MAUD. It is shown how this program can be used to determine the amorphous and crystalline fractions in composites consisting of an amorphous matrix and incorporated iron oxide particles. The analysed compounds underwent different thermal treatments.
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  • Periodic orbits for the elliptic case of the Sun-Earth-Moon problem in new coordinates

    A. Escalona-Buendía, E. Piña
    443-0
    Abstract:
    We present a set of periodic and quasi-periodic orbits for the bidimensional case of the Sun-Earth-Moon problem using the coordinates recently introduced by Piña and Jiménez-Lara. Eliminating the restriction we used in a previous work that Earth-Moon system describes a circular orbit around the Sun, we recover the periodic orbits we have found, and we find periodic orbits for the elliptic case. We also find quasi-periodic orbits closer to the real case.
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  • Melting of sodium clusters

    Juan A. Re, es-Nava., Ignacio L. Garzón, Marcela R. Beltrán, Karo Michaelian.
    450-0
    Abstract:
    Thermal stability properties and the melting-like transition of Na$_N$, $N$ = 13-147, clusters are studied through microcanonical molecular dynamics simulations. The metallic bonding in the sodium clusters is mimicked by a many-body Gupta potential based on the second moment approximation of a tight-binding Hamiltonian. The characteristics of the solid-to-liquid transition in the sodium clusters are analyzed by calculating physical quantities like caloric curves, heat capacities, and root-mean-square bond length fluctuations using simulation times of several nanoseconds. Distinct melting mechanisms are obtained for the sodium clusters in the size range investigated. The calculated melting temperatures show an irregular variation with the cluster size, in qualitative agreement with recent experimental results. However, the calculated melting point for the Na$_{55}$ cluster is about 40 $%$ lower than the experimental value.
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  • Sensor de fibra óptica distribuido para la localización de fugas de hidrocarburo basado en mediciones de transmisión / reflexión

    R.M. López, V.V. Spirin, S.V. Miridonov, M.G. Shl, agin., G. Beltrán, E.A. Kuzin
    457-0
    Abstract:
    A novel type of distributed fiber optic sensor for petroleum hydrocarbon leak detection and localization based on the measurement of transmitted and Rayleigh backscattered power is presented. The sensor structure includes a sensitive polymer, which reversibly expands in hydrocarbon presence and induces the bending losses in the fiber. The location of the loss region is determined from unique relationships between normalized transmitted and Rayleigh backscattered powers for different positions of the disturbance along the test fiber. The localization of a strong disturbance with an estimated accuracy of $\pm 1$ m along a 2.844 km-length single-mode fiber was demonstrated.
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  • Modos de oscilación en cuerdas homogéneas por tercios

    G. Rodríguez Zurita, Ramón Alvarado Bustos., Rubén Alvarado Bustos., L. E
    463-0
    Abstract:
    Along this work, some solutions for string oscillation modes with constant linear mass density each third (homogeneous in thirds) fixed in both ends and under constant tension are shown. Solutions are found following well-known methods for piecewise constant potentials in one dimension as is usual in introductory Quantum Mechanics. The analysis justifies the procedure to construct piecewise homogeneous strings from cooper wires of adequate gauges by solding wire pieces together. This case have similarities with the Schödinger equation for a potential barrier. Experimental results are presented and possibilities of use for pedagogical purposes are discussed.
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  • Aparato para la medición del efecto Faraday

    S. Galindo, S. Cruz
    475-0
    Abstract:
    A Faraday rotation demonstration apparatus for an undergraduate laboratory is described. The main features of the instrument are: The use of light emitting diodes as the monochromatic source, the employment of a cooling water system to keep the sample and the magnetic field coil at a constant temperature, and the use of the half shadow effect for angular rotation measurements. The apparatus can be built from scraps.
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  • Measurement of forces between two parallel conducting cylinders

    A. Corona Cruz, E. Le, -Koo.
    485-0
    Abstract:
    We describe the experimental arrangement and method to measure the force between pairs of parallel conducting cylinders at different separations and with fixed potential differences. The results of the measurement for diverse values of the geometrical and electrical parameters are reported. The measured forces are compared with their values evaluated on the basis of Ref.~1, finding a good agreement for cylinders of the same length, and recognizing the edge effects for cylinders of different lengths.
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