Vol. 53 No. 5 (2007): Revista Mexicana de Física.

Published: 2007-01-01

Articles

  • First-principles predictor of the location of ergodic/non-ergodic transitions

    P.E. Ramírez-González, R. Juárez-Maldonado, L. Yeomans-Re, na., M.A. Chávez-Rojo, M. Chávez-Páez, A. Vizcarra-Rendón, M. Medina-No, ola.
    327-0
    Abstract:
    This letter presents a remarkably simple approach to the first-principles determination of the ergodic/non-ergodic transition in monodisperse colloidal suspensions. It consists of an equation for the long-time asymptotic value $\gamma$ of the mean squared displacement of the colloidal particles, whose finite real solutions signal the non-ergodic state, and determines the non-ergodic parameter $f(k)$. We illustrate its concrete application to three simple model colloidal systems, namely, hard-spheres, hard-spheres plus repulsive (screened Coulomb) Yukawa interaction, and hard-sphere plus attractive Yukawa tail. The results indicate that this is quite a competitive theory, similar in spirit to, but conceptually independent from, the well-known mode coupling theory.
    ⬇️ Scroll down to see the full summary

  • A generalized chaotic encryption system for multimedia applications

    R. Hasimoto-Beltrán
    332-0
    Abstract:
    Recently, several encryption schemes based on 1, 2, and 4 one-dimensional chaotic maps have been presented in the literature (see Ref. 1). The idea of increasing the number of maps is to provide more security and robustness to chaos-based cryptographic systems. In this paper, we propose a generalized encryption scheme based on $N$ one-dimensional coupled chaotic maps, where only a window of $m$ maps at a time (m $ \le $ N) is active for the encryption process. The m-map window is randomly updated by shifting it circle-wise one map at a time through the entire N-map system. To add more security to the system, we randomly perturb the coupling parameter using current cyphertext output (spatiotemporal or global feedback); so that any change in the input data (plaintext) is reflected instantly in the total system (not only locally). Our generalized implementation is based on the scheme presented in Ref. 1, but it can also be implemented in any chaos based encryption scheme. Our approach adds more robustness to the system, maintaining excellent statistical properties and fast performance for real-time multimedia applications.
    ⬇️ Scroll down to see the full summary

  • Theory of thermoelectric cooling in semiconductor structures

    Yu. G, G.N. Logvinov
    337-0
    Abstract:
    A new approach is suggested to explain the Peltier effect. This approach is based on the idea of the occurrence of induced thermal diffusion fluxes in any non-uniform medium through which a d.c. electric current flows, in particular in a structure composed of two different uniform semiconductors. These induced thermal diffusion fluxes arise to compensate for the change in thermal fluxes carried out by an electric current (drift thermal fluxes) during their driving through the junction in accordance with the general Le Châtelier-Braun principle. The occurrence of these thermal diffusion fluxes leads to temperature non-uniformity in the structure and, as a result, to the junction's cooling or heating. The general heat balance equations are obtained. It is shown that only two sources of heat exist: the Joule source of heat, and the Thomson source of heat. They have commensurable magnitudes in the problem considered. There is no Peltier's source of heating or cooling present. The new equation for the Thomson heat is obtained and its physical interpretation is made. New boundary conditions for the heat balance equation are derived. The analysis of these boundary conditions shows that the Peltier sources of heat are also absent at the junctions. It is shown that, in the general case, the thermoelectric cooling represents the superposition of two effects, the isothermal Peltier effect and the adiabatic Peltier effect. Both essentially depend on the junction surface thermal conductivity. The isothermal Peltier effect disappears in the limiting case of a very small surface thermal conductivity while the adiabatic Peltier effect disappears in the limiting case of a very large surface thermal conductivity. The dependence of thermoelectric cooling on the geometrical dimensions of the structure is discussed. It is shown that the thermoelectric cooling (heating) is a thermodynamically reversible process in the linear approximation of the electric current applied.
    ⬇️ Scroll down to see the full summary

  • Una prueba empírica de generadores de números pseudoaleatorios mediante un proceso de decaimiento exponencial

    H.F. Coronel-Brizio, A.R. Hernández-Monto, a., M.A. Jiménez-Montaño, L.E. Mora-Forsbach
    350-0
    Abstract:
    Empirical tests for pseudorandom number generators based on the use of processes or physical models have been successfully used and are considered as complementary to theoretical tests of randomness. In this work a statistical methodology for evaluating the quality of pseudorandom number generators is presented. The method is illustrated in the context of the so-called exponential decay process, using some pseudorandom number generators commonly used in physics.
    ⬇️ Scroll down to see the full summary

  • Accurate absolute measurement of trapped Cs atoms in a MOT

    M. Talavera O., M. López R., E. de Carlos L., S. Jiménez S.
    358-0
    Abstract:
    A Cs-133 Magneto-Optical Trap (MOT) has been developed at the Time and Frequency Division of the Centro Nacional de Metrología, CENAM, in Mexico. This MOT is part of a primary frequency standard based on ultra-cold Cs atoms, called CsF-1 clock, under development at CENAM. In this Cs MOT, we use the standard configuration ($\sigma ^ + - \sigma ^ - )$ 4-horizontal 2-vertical laser beams 1.9 cm in diameter, with 5 mW each. We use a 852 nm, 5 mW, DBR laser as a master laser which is stabilized by saturation spectroscopy. Emission linewidth of the master laser is 1 MHz. In order to amplify the light of the master laser, a 50 mW, 852 nm AlGaAs laser is used as slave laser. This slave laser is stabilized by light injection technique. A 12 MHz red shift of the light is performed by two double passes through two Acusto-Optic Modulators (AOMs). The optical part of the CENAMs MOT is very robust against mechanical vibration, acoustic noise and temperature changes in our laboratory, because none of our diode lasers use an extended cavity to reduce the linewidth. In this paper, we report results of our MOT characterization as a function of several operation parameters such as the intensity of laser beams, the laser beam diameter, the red shift of light, and the gradient of the magnetic field. We also report accurate absolute measurement of the number of Cs atoms trapped in our Cs MOT. We found up to $6\times 10^7$ Cs atoms trapped in our MOT measured with an uncertainty no greater than 6.4%.
    ⬇️ Scroll down to see the full summary

  • Monte Carlo simulation for fragment mass and kinetic energy distributions from the neutron-induced fission of $^{235}U$

    M. Monto, a., E. Saettone, J. Rojas
    366-0
    Abstract:
    The mass and kinetic energy distribution of nuclear fragments from the thermal neutron-induced fission of $^{235}U$ have been studied using a Monte Carlo simulation. Besides reproducing the pronounced broadening on the standard deviation of the final fragment kinetic energy distribution ($\sigma_{e}(m)$) around the mass number $m$ = 109, our simulation also produces a second broadening around $m$ = 125 that is in agreement with the experimental data obtained by Belhafaf et al. These results are a consequence of the characteristics of the neutron emission, the variation in the primary fragment mean kinetic energy, and the yield as a function of the mass.
    ⬇️ Scroll down to see the full summary

  • Bandas prohibidas gigantes en heteroestructuras fotónicas

    R. Archuleta-García, B. Manzanares-Martínez, J. Manzanares-Martínez
    371-0
    Abstract:
    In this work we show that it is possible to design giant photonic band gaps in heterostructures via the determination of the group velocity. A photonic heterostructure is composed by the union of two or more distinct photonic crystals. We present the calculation of the heterostructure band structure implementing the supercell technique in the Wave Plane Method. We show that even if the heterostructure present a very complicated photonic band diagram, it is possible to discriminate the regions of low (high) transmission obtaining the group velocity. We verify the existence of the forbidden (allowed) regions with the theoretical calculations of the light transmission.
    ⬇️ Scroll down to see the full summary

  • Formación del ion negativo de carbón por asociación radiativa

    J. Campos, A. Lipovka, J. Saucedo, V. Zalkind
    376-0
    Abstract:
    In the present paper the problem of the radiative association of atoms of carbon $C$ with electrons $e^{-}$ at an interval of kinetic temperatures $ T_{c}$ of 100 $\leq$ T$_{c}$ $<$3000$^{\circ }$K is considered. The calculation of the rate of these associations has been made by using the principle of detailed balance. It is shown that the rate has the expected behavior (it increases with the temperature). This behavior is very similar to the one found for the $ H^{-}$ formation rate coefficient, which also increases with the temperature.
    ⬇️ Scroll down to see the full summary

  • Láser de fibra de amarre de modos de figura ocho basado en un espejo de lazo óptico no lineal simétrico

    R. Grajales-Coutiño, B. Ibarra-Escamilla, E.A. Kuzin, J. Gutiérrez-Gutiérrez, O. Pottiez, P. Zaca-Morán
    380-0
    Abstract:
    In this work we investigate a figure-eight passive mode-locked fiber laser which is formed by a Nonlinear Optical Loop Mirrror (NOLM) using a symmetrical coupler and a Quarter-Wave Retarder (QWR) in the loop. The NOLM has the function of a saturable absorber, which is a non-lineal element. Thus, the response for an optic signal of entrance is dependent of the intensity, so the NOLM output pulse is narrower than the NOLM input pulse. The advantage of this configuration is that we need to adjust only one element to get the mode-locked pulses when the pump power is up to 70 mW, this element is the QWR angle. The laser generated pulses around 30 ps by a fundamental repetition frequency approximately of 0.8 MHz.
    ⬇️ Scroll down to see the full summary

  • Concepción y preparación de un recubrimiento protector en forma de película compósito a base de carburo de titanio

    J.A. Montes de Oca, J.A. Galaviz, J.R. Vargas-García, Y. LePetitcorps, J.P. Manaud
    386-0
    Abstract:
    Transition metal nitrides and carbides have widely been used as refractory materials due to their high melting point, high hardness and high Young's modulus. However, its fragility induces a low cracking resistance, mainly when these materials are deposited on substrates with a very different thermal expansion coefficient. Several techniques have been widely used for depositing nitride and titanium carbide coatings. In order to obtain a hard refractory coating for protecting a tungsten device exposed to an aggressive liquid metal, in this study we report the design and synthesis of TiC coatings obtained by CVD at 1000$^\circ$C, and TiC-based coatings and Ti$_{1 - x}$W$_{x}$/TiC$_{x}$ composite films obtained by activated reactive evaporation process (ARE) at 250 and 500$^\circ$C respectively. The effect of different experimental parameters was correlated with crystalline structure, texture, morphology and chemical composition of the deposits. The microstructure, crystal size and morphology of the films were evaluated by x-ray diffraction (XRD) technique and scanning electron microscopy (SEM) respectively. The chemical composition of the films was evaluated by Auger electron spectroscopy (AES) and electron probe microanalysis (EPMA). Finally, cracking behavior of the films prepared by the ARE technique was evaluated by a heat treatment at 1000 $^\circ$C during 30 min in an He atmosphere.
    ⬇️ Scroll down to see the full summary

  • Electroweak-Higgs unification in the two Higgs doublet model: masses and couplings of the neutral and charged Higgs bosons

    J.L. Díaz-Cruz, A. Rosado
    396-0
    Abstract:
    We obtain the mass spectrum and the Higgs self-couplings of the two Higgs doublet model (THDM) in an alternative unification scenario where the parameters of the Higgs potential $\lambda_i$ ($i=1,2,3,4,5$) are determined by imposing their unification on the electroweak gauge couplings. An attractive feature of this scenario is the possibility of determining the Higgs boson masses by evolving the $\lambda_i$ from the electroweak-Higgs unification scale $M_{GH}$ down to the electroweak scale. The unification condition for the gauge ($g_1,g_2$) and Higgs couplings is written as $g_1=g_2=f(\lambda_i)$, where $g_1=k_Y^{1/2} g_Y$, and $k_Y$ isthe normalization constant. Two variants for the unification condition are discussed: Scenario I is defined through the linear relation: $g_1=g_2=k_H(i)\lambda_i(M_{GH})$, while Scenario II assumes a quadratic relation $g^2_1=g^2_2=k_H(i)\lambda_i(M_{GH})$. In Scenario I, by setting ad hoc $-k_H(5)=\frac{1}{2} k_H(4)=\frac{3}{2} k_H(3)=k_H(2)=k_H(1) =1$, taking $\tan\beta=1$ and using the standard normalization ($k_Y=5/3$), we obtain the following spectrum for the Higgs boson masses: $m_{h^0} = 109.1$ GeV, $m_{H^0} = 123.2$ GeV, $m_{A^0} = 115.5$ GeV, and $m_{H^{\pm}} = 80.3$ GeV, with similar results for other normalizations such as $k_Y=3/2$ and $k_Y=7/4$.
    ⬇️ Scroll down to see the full summary

  • The hydrogen atom via the four-dimensional spherical harmonics

    G.F. Torres del Castillo, J.L. Calvario Acócal
    407-0
    Abstract:
    Using the fact that the Schrödinger equation for the stationary states of the hydrogen atom is equivalent to an integral equation on the unit sphere in a four-dimensional space, the eigenvalues, the eigenfunctions, and a dynamical symmetry group for this problem are obtained from the four-dimensional spherical harmonics and the group of rotations on the sphere. It is shown that the four-dimensional spherical harmonics separable in Euler angles correspond to solutions of the time-independent Schrödinger equation that are separable in parabolic coordinates.
    ⬇️ Scroll down to see the full summary

  • Optimal stabilization of unstable periodic orbits embedded in chaotic systems

    C.A. Cruz-Villar
    415-0
    Abstract:
    A gradient-flow-based approach is proposed in this paper for stabilizing unstable periodic orbits (UPO) embedded in chaotic systems. In order to obtain an on-line stabilizing solution, the stabilization problem is considered to be an optimal control problem, and system state sensitivities with respect to the control input are introduced. The resulting feedback controller is able to stabilize UPO embedded in both kind of systems, with or without an odd Floquet number. Moreover, the proposed approach is easily extended to identifying the period of the UPO to be stabilized when it is unknown. Simulation experiments of the proposed controller are carried out on the Rössler and the Lorenz systems.
    ⬇️ Scroll down to see the full summary

  • Geometric analysis of the null screens used for testing convex optical surfaces

    L. Carmona-Paredes, R. Díaz-Uribe
    421-0
    Abstract:
    A geometric analysis of the screens used for testing convex optical surfaces is presented. Some properties of the transformation between the image plane and the screen plane, such as symmetry and magnification, are presented for the case when the reflecting surface is a sphere. Due to the intrinsic variations in the magnification, the geometric relationship between the image and the object shows some unexpected behavior. Two cases are analyzed in detail: i) a set of concentric circles centered on the origin of coordinates of the image plane with a set of radial straight lines; and ii) a square grid and a square array of circles of the same size.
    ⬇️ Scroll down to see the full summary