Vol. 49 No. 3 (2003): Revista Mexicana de Física.

Published: 2003-01-01

Articles

  • Dynamic specific heat of frustrated Ising spin rings

    G. Ismail
    194-0
    Abstract:
    The dynamic specific heat C($\omega )$ is calculated exactly for rings of six coupled Ising spins within Glauber dynamics. We used the response of the internal energy to small temperature oscillations to find C($\omega )$. The spin glass (SG) and disordered ferromagnetic (DFM) rings showed here have four energy minima and thus four diverging relaxation times in the time evolution of magnetization and three such times in the evolution of energy. The properties of the real and imaginary parts of dynamic specific heat are investigated for different temperatures and frequencies. The dynamic susceptibility is affected by the longest relaxing mode while the dynamic specific heat is not. Our results confirm that C($\omega )$ is sensitive only to rapidly relaxing processes for ferromagnetic~(FM) and anti-ferromagnetic (AFM) cases.
    ⬇️ Scroll down to see the full summary

  • Quintessence-like dark matter in spiral galaxies

    F. Siddhartha Guzmán, Tonatiuh Matos., Darío Núñez., Er., Ram{\'\i}rez., .
    203-0
    Abstract:
    Through the geodesic analysis of a static and axially symmetric space time, we present conditions on the state equation of an isotropic perfect fluid $p=\omega d$, when it is considered as the dark matter in spiral galaxies. The main conclusion is that it can be an exotic \mbox{fluid ($-1<\omega <-1/3$)} as it is found for Quintessence at cosmological scale.
    ⬇️ Scroll down to see the full summary

  • Two-particle correlations in the one-dimensional Hubbard model: a ground-state analytical solution

    E. Vallejo, O. Navarro, J.E. Espinosa
    207-0
    Abstract:
    A solution to the extended Hubbard Hamiltonian for the case of two-particles in an infinite one-dimensional lattice is presented, using a real-space mapping method and the Green function technique. This Hamiltonian considers the on-site ($U$) and the nearest-neighbor ($V$) interactions. The method is based on mapping the correlated many-body problem onto an equivalent site-impurity tight-binding one in a higher dimensional space. In this new space we obtained the analytical solution for the ground state binding energy. Results are in agreement with the numerical solution obtained previously \cite{nw92}, and with those obtained in the reciprocal space \cite% {mh90}.
    ⬇️ Scroll down to see the full summary

  • The structure of dimerizing fluids from ``experimental'' diffraction data by reverse Monte Carlo modelling

    L. Pusztai, H. Dominguez, O.A. Pizio
    212-0
    Abstract:
    Molecular dynamics simulations of systems consisting of monoatomic and diatomic species, which mimick equilibrium distributions of particles in dimerizing associating fluids under given external conditions have been performed. The results of these simulations are given in terms of pair distribution functions of atoms and of the corresponding structure factors. The data are considered as ``experimental input'' for the reverse Monte Carlo (RMC) modelling focused in the evaluation of the (a priori, unknown) composition of the system, and at getting insight into structural properties that are implicitely contained in the experimental structure factor. We discuss usefulness of the simulation scheme, its accuracy and limitations to intend the application of the RMC modelling for more complex associating fluids.
    ⬇️ Scroll down to see the full summary

  • $\alpha - $Fe$_{2}$O$_{3}$ films grown by the spin-on sol-gel deposition method

    A. Avila-García, G. Carbajal-Franco, A. Tiburcio-Silver, E. Barrera-Calva, E. Andrade-Ibarra
    219-0
    Abstract:
    $\alpha - $Fe$_{2}$O$_{3}$ polycrystalline films with grains larger than 31 nm were grown by the spin-on sol-gel deposition method. The particular sol used was prepared starting from two distinct precursor reagents. Both precursors leaded to similar films. Order within the films was altered by adding tin to the samples. Transmittance measurements confirmed that the hematite phase is obtained by annealing the samples above 400~$^{\circ}$C and yielded an optical gap of about 2.2 eV, but additional transitions at 2.7 eV were also observed. From RBS measurements it was found that tin inclusion decreases iron content as expected, but also increases oxygen concentration within the films. This last observation was associated to the disorder rise when introducing tin atoms.
    ⬇️ Scroll down to see the full summary

  • Estudio de fotoluminiscencia de películas de InGaP crecidas sobre substratos de GaAs por el método de epitaxia de fase líquida

    T. Prutskij, P. Díaz-Arencibia, F. Silva, A. Mintairov, J. Merz
    224-0
    Abstract:
    We have studied the luminescent properties of InGaP films grown on GaAs substrates by liquid phase epitaxy (LPE). Photoluminescence~(PL) measurements were performed in wide temperature (4 -- 250 K) and exciting power density (4 orders of magnitude) ranges for different polarization of the emitted radiation along the [011] and [0$\overline 1 $1] directions. It was found that donor-acceptor transition dominates at low temperature (4K) while the band-to-band transition at higher temperature (250K). The dependence with the excitation intensity shows the characteristic behavior of donor-acceptor recombination. The difference in the spectral peak position for different polarizations is a result of valence-band splitting into a heavy and light hole bands due to lattice mismatch strain. Moreover, the difference of the line shape of the spectra for different polarizations indicates the presence of anisotropy for different crystallographic directions.
    ⬇️ Scroll down to see the full summary

  • Hydrodynamic model for 2D degenerate free-electron gas for arbitrary frequencies

    M. del Castillo-Mussot, G.J. Vázquez Fonseca, G.H. Cocoletzi
    231-0
    Abstract:
    Following Halevi's procedure for 3D degenerate free-electron gas (3D-DEG), we investigate the response function in the hydrodynamic model (HM) for 2D-DEG confined in low dimensional systems when collisions are included. For small wavevectors we found from the two-dimensional Boltzmann-Mermin model a useful expression for the HM complex stiffness parameter of the nonlocal dielectric function~$\beta $, which is $\beta ^{2}= [\left( (% 3\omega /4) +i(\nu /2)\right) /(\omega +i\nu )]v_{F}^{2}$, where $% \omega $ and $\nu $ are the circular and collisional frequencies and $v_{F}$ is the Fermi velocity.
    ⬇️ Scroll down to see the full summary

  • Efecto de la radiación en las propiedades termoluminiscentes de lavas

    V. Correcher, D. Molina, J. Garcia-Guinea
    235-0
    Abstract:
    Blue thermoluminescence (TL) emission from different lavas of many places (Costa Rica, Canary Islands, Hawaii Islands, Iceland and Italy) corresponding to different eruptions has been studied to know its potential use in the field of dating and retrospective dosimetry. Due to the light emission is linked to the point defects of the crystalline lattice structure, X-ray diffraction analyses were performed to determine the components of this polymineral material that mostly are cristobalite, plagioclase and phyllosilicates. Exposures to different doses (in a range of 1-25 Gy) were given to each sample to determine the evolution of the TL signal with the irradiation under laboratory conditions. In all cases, a linear response could be observed and no saturation has been detected in this range of doses. Both natural (NTL) and induced (ITL) TL signal exhibit a complex glow curve structure associated to a continuous trap distribution over 100$^{\circ}$C that could be attributed to the formation-annihilation [AlO$_{4}$/alcali]$^{ + }$ and [AlO$_{4}$]$^{o}$ centres. In ITL signal a discrete electron trap distribution (at lower temperature $\sim $85$^{\circ}$C) could be detected, thus the coexistence of both trap structure could be appreciated in this material.
    ⬇️ Scroll down to see the full summary

  • Decaimientos de esneutrinos

    J.E. Barradas Guevara, M. Gómez Bock
    242-0
    Abstract:
    Based on the Minimal Supersymmetric Standard Model (MSSM), we discussed possible two body decay modes for the sneutrino $\widetilde{\nu}$, one charged and other neutral: $\widetilde{\nu}\rightarrow l \widetilde{\chi}_1^\pm$ and $\widetilde{\nu}\rightarrow \nu \widetilde{\chi}_1^0$, respectively. Exploring a range of possible values for its mass, and for the chargino $\widetilde{\chi}_i^\pm$ and neutralino $\widetilde{\chi}_j^0$ masses as well. We present the specific calculation for branching ratios, which are analyzed numerically.
    ⬇️ Scroll down to see the full summary

  • Optical tomography of phase objects with parallel projection differences and ESPI

    C. Meneses-Fabian, G. Rodríguez-Zurita, J.F. Vázquez-Castillo
    251-0
    Abstract:
    This paper describes a technique that obtains border-enhanced tomographic images of a slice belonging to a phase object. This is obtained experimentally by means of the sequential storage of differences of parallel projections from two adjacent angular positions of the object and with appropriate techniques for their reconstruction. An ESPI (Electronics Speckle Pattern Interferometry) system is used in a Mach-Zenhder configuration to obtain data. Computer simulations are shown as well as experimental results.
    ⬇️ Scroll down to see the full summary

  • Diseño y construcción de un autocorrelador de pulsos de femtosegundos usando absorción de dos fotones en un diodo luminiscente

    M.A. García Arthur, R. Rangel Rojo, N. Jamasbi, M. Mohebi
    258-0
    Abstract:
    In this work, we present the design and construction of a femtosecond laser pulse autocorrelator, based on the two-photon absorption process in a light emitting diode (LED). Compared with traditional autocorrelators based on nonlinear crystals, this autocorrelator has the advantages of lower cost and greater ease of alignment. The intensity and interferometric autocorrelation traces obtained with this device for a titanium-sapphire laser with pulses that are less than 100 fs long, are also presented.
    ⬇️ Scroll down to see the full summary

  • Determinación de la energía de activación para la reacción de H+H$_\mathbf{2}$ mediante el cálculo de superficie de energía potencial

    F. Galindo Hernández, F. Méndez Ruiz.
    264-0
    Abstract:
    The study of the reaction path of the H + H$_{2}$ system was considered, the system consist of pairs of interacting hydrogen atoms. The energies of each pair of atoms (H$_{2})$ and the triatomic system (H$_{3})$ were calculated by means of the Heitler, London and Sato equations. We developed a calculation method that allowed us to obtain a mesh of potential energy values based on the interatomic distances, and with the aid of computer software we obtained a potential energy surface. This procedure provided the reaction path in energetic terms from the reagents to products. We found that the value of the activation energy is close to the experimental value and to those values obtained by quantum ab-initio calculations.
    ⬇️ Scroll down to see the full summary

  • Obtaining the gravitational force corresponding to arbitrary spacetimes. The Schwarzschild's case.

    T. Soldovieri, A.G. Muñoz S.
    271-0
    Abstract:
    Making use of the classical Binet's equation a general procedure to obtain the gravitational force corresponding to an arbitrary 4-dimensional spacetime is presented. This method provides, for general relativistic scenarios, classics expressions that may help to visualize certain effects that Newton's theory can not explain. In particular, the force produced by a gravitational field which source is spherically symmetrical~(Schwarzschild's spacetime) is obtained. Such expression uses a redefinition of the classical reduced mass, in the limit case it can be reduced to Newton's universal law of gravitation and it produces two different orbital velocities for test particles that asimptotically coincide with the Newtonian one. As far as we know this is a new result.
    ⬇️ Scroll down to see the full summary

  • Fórmulas y teoremas de adición de las funciones elípticas de Jacobi

    G. Bautista, E. Piña, E. Soto
    276-0
    Abstract:
    This paper is dedicated to the systematic study of the formulae and addition theorems of the Jacobi's functions. Starting from fundamental properties, we show most known equations and, at the same time, we classify and sort them in the most useful form, in order to get a satisfactory formulary. The addition theorems are expressed in vectorial language, as five parallel vectors in four dimensions. We also discover 16 orthogonal vectors to the above mentioned direction, with a very simple structure, notwithstanding only three of them are linearly independent. We group them in sets of four vectors, also orthogonal to one different vector of the standard basis. In each group of four vectors, only two of them are linearly independent, therefore we associate an antisymmetric tensor to each quartet.
    ⬇️ Scroll down to see the full summary

  • El análisis de Fourier de las trayectorias planetarias y el modelo copernicano del sistema solar

    J.A. Peralta, A. Calles, E. Yépez
    283-0
    Abstract:
    The most important models before newtonian mechanics and the law of gravitation were stablished by Kepler and Copernicus. The relation between Newton's theory and Kepler's laws of planetary motion is widely discussed in textbooks; however, the relation with the model of Copernicus, where the position of a planet as a function of time is described as combination of circular motions, is usually avoided. In this work we use two simple and useful numerical techniques to show that this relation is easily performed. We use the algorithm of Verlet to solve the differential equations, not in polar coordinates as is usually done, but in cartesian coordinates, we also use the fast Fourier transform method to analyse the time series that in a natural way generate the deferent and epicicles of the Copernicus' model.
    ⬇️ Scroll down to see the full summary

  • The Wigner function in paraxial optics II. Optical diffraction pattern representation

    C.J. Román-Moreno, R. Ortega-Martínez, C. Flores-Arvizo
    290-0
    Abstract:
    The Wigner distribution function is a tool to visualize a signal in the space-frequency domain. Moreover, it can be produced by purely optical means. We describe the Brenner-Lohmann optical setup with monochromatic light, which produces the Wigner function. A signal composed of rectangle functions (optically produced by slits) has a Wigner function with a ``sand clock" form. We point out the strong oscillations of the Wigner function between two interfering components, which has been called the smile function\/ of a ``Schrödinger's cat" state. This bears interesting optical diffraction patterns in our figures.
    ⬇️ Scroll down to see the full summary