Vol. 53 No. 4 (2007): Revista Mexicana de Física.

Published: 2007-01-01

Articles

  • Estudio de la conductividad del sistema amorfo Li$_{2}$S-Sb$_{2}$S$_{3}$-P$_{2}$S$_{5}$

    Z. Nagamedianova, E.M. Sánchez
    228-0
    Abstract:
    There is a great interest in sulfide glasses because of their high lithium ion conductivity. This work presents the study of new glasses based on Li$_{2}$S-Sb$_{2}$S$_{3}$-P$_{2}$S$_{5}$ system which are ionic lithium conductors with possible application in solid state batteries of lithium. The synthesis of glasses has been performed by classical quenching technique and glassy nature was confirmed by various techniques as X-Ray Diffraction (XRD), Microscopy and Differential Scanning Calorimetry (DSC). Conductivities have been determined by Impedance Spectroscopy (IS). Some compositions of high lithium content have presented ionic conductivities close to 10$^{ - 6}$ S/cm at room temperature that permits to consider them as solid lithium conductors. In addition, the values of activation energy, E$_{a}$, pre-exponential factors,${\rm g}\sigma _{0}$, and circuit equivalent model R(RQ)(RQ) were reported. The results are comparable to those ones of other lithium glasses based on P$_{2}$S$_{5}$, SiS$_{2}$, GeS$_{2}$, As$_{2}$S$_{3}$ etc. Despite ionic conductivities are insufficient for application on common batteries, those new glasses based on Li$_{2}$S-Sb$_{2}$S$_{3}$ and prepared as thin films could be used on solid state microbatteries of lithium.
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  • Hologramas tipo Lohmann multiplexados

    M. Araiza E., S. Guel S., C. Sifuentes G., A. Lastras, J. Nieto
    235-0
    Abstract:
    Based on Lohmann´s method to create computer generated holograms (CGH) a multiplexing scheme is presented by making some modifications to basic cells, known as type I cells. With this variation, it is possible to storage up to 3 images in a hologram. At the reconstruction stage, the images are recovered simultaneously in the same direction and same diffraction order. Results show that the proposed procedure is effective for applications in image processing with some limitations.
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  • La cuantificación de los sitios activos en las bases de DNA y RNA utilizando las funciones Fukui condensadas

    M. Virginia Popa
    241-0
    Abstract:
    In this paper the dipolar moment, the Fukui functions by considering the Mulliken charges and the energies of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) are computed in order to determine the most active centers of DNA and RNA (adenine, guanine, cytosine, timine and uracile), being them in gas phase and the solvent. The DNA and RNA molecules are optimized by using the theory levels AM1 in gas phase, HF/6-31G, LSDA/6-31++G, B3LYP/LANL2DZ, PBE/6-31++G and, for uracil MP2/6-31++G has also been used, in gas phase and the solvent phase ($\epsilon$=78.39) with the Tomasi model of continued polarizable. The \textit {larger} dipole moment induced on the molecule is due to the presence of the solvent and become even large when the electronic correlation is introduced with the PBE (Perdew-Burke-Ernzernhof) and the local functional LSDA (Local Spin Density Approximation). The first four active sites found indiscriminately of the theory level employed coincide with the experimental data already reported in the literature. The HOMO-LUMO (gap) is small when the DFT (Density Function Theory) is used along with LSDA/6-31++G, comparing this value with the rest levels of theory. If the energies of the frontier orbital are known, then such energy levels can be compared to those of the reactants, making it possible to determine whether a bound exists or not, and if so what type of bound it is.
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  • Scattering of elastic waves by shallow elliptical cracks

    A. Rodríguez-Castellanos, R. Avila-Carrera, F.J. Sánchez-Sesma
    254-0
    Abstract:
    Comprehensive studies in engineering have dealt with diffraction phenomena in unbounded elastic domains containing cracks, while some others have been carried out to investigate diffraction by discontinuities located near a free surface. In this last case, the presence of cracks significantly affects wave motion and, in some cases, large resonant peaks may appear. In order to study these resonant peaks and describe how they respond, we propose the use of the Indirect Boundary Element Method to simulate 2D scattering of elastic P- and SV-waves. The geometry considered for the cracks is elliptical, but in some cases comparison of its behavior is made with that of planar cracks or cavities. This method establishes a system of integral equations that allows us to compute the diffracted displacement and traction fields. We present our results in the frequency domain. In the case of planar cracks located near the free surface, we validate the method by comparing our results to those of a previously published study. We develop examples of various elliptical crack configurations to show resonance effects, where one can observe important variations in the resonance peaks in the frequency domain. The results shown here can be used to detect the presence of subsurface cracks. Nevertheless, it is difficult to determine the shape (planar or elliptical) of the discontinuity that is embedded in the halfspace.
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  • On the boundary conditions in tracer transport models for fractured porous underground formations

    M. Coronado, J. Ramírez-Sabag, O. Valdiviezo-Mijangos
    260-0
    Abstract:
    A boundary condition traditionally used in analytical models for tracer or contaminant pulse transport in porous underground formations gives the tracer concentration at the injection border as a discontinuous function in time. It has recently been shown that this condition leads to a physically improper pulse behavior. Models using sounder boundary conditions are already available for non-fractured porous media, but not for fractured media, where the traditional condition is commonly employed which can potentially lead to errors. We develop two new formulations to describe tracer tests in fractured media. They set conditions (i) on the total amount of injected tracer and (ii) on the tracer flow. The new formulations are compared against the traditional debatable model by examining tracer breakthrough curve differences. It has been found that they are important at small Peclet numbers. Differences are analyzed in two ways, by (a) employing typical model parameter values, and (b) fitting the three models to the same field tracer data set, and comparing the resulting model parameter values. In the first case the breakthrough curve difference has been quantified at 25%, and in the field tests considered in the second case it was from 1% to 10%. In general these discrepancies are small, but could become significant in some cases.
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  • Ecuaciones de fuerza de Lorentz como ecuaciones de Heisenberg para un sistema cuántico en el espacio euclidiano 4D

    A.R. Rodríguez-Domínguez
    270-0
    Abstract:
    In an earlier work, the dynamic equations for a relativistic charged particle under the action of electromagnetic fields were formulated by R. Yamaleev \cite{Yamal1} in terms of external, as well as internal momenta. Evolution equations for external momenta, the Lorentz-force equations, were derived from the evolution equations for internal momenta. The mapping between the observables of external and internal momenta are related by Viète formulae for a quadratic polynomial, the characteristic polynomial of the relativistic dynamics. In this paper we show that the system of dynamic equations, constructed in Ref. 1, can be cast into the Heisenberg scheme for a four-dimensional quantum system. Within this scheme the equations in terms of internal momenta play the role of evolution equations for a state vector, whereas the external momenta obey the Heisenberg equation for an operator evolution. The solutions of the Lorentz-force equation for the motion inside constant electromagnetic fields are presented via pentagonometric functions.
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  • Construccion de un inclinómetro óptico

    A. Jaramillo-Núñez, M. Lucero-Alvarez
    281-0
    Abstract:
    In this work an optical instrument for measuring tilt is described. The device is compact, requires no special handling and can measure variations in tilt of the order of 0.5 arcsec, although this value can be suitably modified by changing the setup parameters. The instrument was developed as a tiltmeter using a liquid surface as reference surface. The inclination is obtained by measuring the longitudinal displacement of a laser beam after being reflected from the surface of the liquid. The application of the instrument for measuring the tilt of the spindle of a polishing machine of large dimensions is presented.
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  • Caracterización de un amplificador de fibra dopada con erbio a partir de sus parámetros experimentales

    M. Bello-Jiménez, E.A. Kuzin, B. Ibarra-Escamilla, R. Tellez-García
    289-0
    Abstract:
    In this paper we describe a method to characterize the gain of an erbium-doped fiber amplifier (EDFA) through the numerical simulation of the signal beam along the amplifier. The simulation is based on a model constituted by the propagation and rate equations for an erbium-doped fiber. The manipulation of these equations allows us to regroup the parameters present in an EDFA, which we have named the A, B, C, D parameters, and they can be obtained experimentally from an erbium-doped fiber. Experimental results show that the measurement of these parameters allow us to estimate with very good correspondence the amplifier gain.
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  • Capacitance of a plate capacitor with one band-limited fractal rough surface

    N.C. Bruce, A. García-Valenzuela
    296-0
    Abstract:
    The problem of the capacitance between a band-limited, zero-mean, fractal shaped-rough surface and a plane electrode is investigated. Five parameters are required to define the rough surface: $\sigma $, the rms height, $D$ (1 $<$ $D$ $<$ 2), the fractal dimension of the roughness; $K_0 $, the fundamental spatial frequency; $b$ ($b$ $>$ 1), the spatial frequency scaling parameter; and $N$, the number of spatial frequency components in the surface structure. We find that the graph of inverse capacitance against nearest electrode separation depends on $\sigma $ and $D$, whereas it is nearly independent of $K_0 $, $b$, and $N$ for $N >$ 4. The numerical results also indicate that the surface roughness can be interpreted as an equivalent dielectric film with an effective dielectric constant and effective thickness for surprisingly small minimum electrode separations. Our findings in this paper can be used to complement established techniques for the experimental determination of the statistical parameters of the surface roughness of conducting surfaces.
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  • Effect of giant electric fields on the optical properties of GaN quantum wells

    G. Gonzalez de la Cruz, H. Herrera, A. Calderon Arenas
    303-0
    Abstract:
    Spontaneous and piezoelectric fields are known to be the key to understanding the optical properties of nitride heterostructures. This effect modifies the electronic states in the quantum well (QW) and the emission energy in the photoluminescence (PL) spectrum. These fields induce a reduction in the oscillator strength of the transition energy between the confined electron and hole states in GaN/Al$_{x}$Ga$_{1 - x}$N QWs, and dramatically increase the carrier lifetime as the QW thickness increases. In this work, we solve analytically the Schrödinger equation for moderate electric fields when the electron-hole transition energy in the QW is larger than the energy gap of the GaN. Furthermore, the large redshifts of the PL energy position and the spatial separation of the electrons and holes several greater times than the Bohr radius caused by the strong piezoelectric fields are explained using a triangular potential, instead of a square one, in the Schrödinger equation. The transition energy calculations between the electron-hole pair as a function of the well width with the electric field as a fitting parameter are in agreement with the measured photoluminescence energy peaks.
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  • Decaimientos del meson B en $K_0^* \pi, F_0 k$

    J.A. Mendoza S., C.A. Ramírez
    307-0
    Abstract:
    We calculate the branching ratios for the decays $B \to K_0^* \pi$ and $ B \to f_0 K$, in the \lq Naive Factorization (NFA) \rq approximation, The dipole penguin operator ${\cal O}_{11}$ contributions are included and the scalar mesons are considered as $q \bar q$ bounded states. We compare, favorably our results with the experimental available data.
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  • Contribución del momento magnético a la fuerza de reacción a la radiación en forma relativista

    G. Ares de Parga, M. Ortiz-Domínguez, R. Mares
    314-0
    Abstract:
    Starting with the magnetic Smirnov reaction force, a relativistic equation which describes the motion of a charged particle with constant magnetic moment of spin $\overrightarrow{\mu }$, is obtained. A Landau-Lifshitz-like equation is deduced with magnetic moment. A fundamental difference between the classical and relativistic radiation rate of energy is found.
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  • Vacuum oven to control the annealing process in alloyed nanolayers

    J.E. Corona, R.D. Maldonado, A.I. Oliva
    318-0
    Abstract:
    The design and construction of a vacuum quartz cylindrical oven to diffuse two or more metallic thin films for alloying is discussed. The oven has a gas-inert ambient (Ar) anda heater plate whose temperature is controlled by means of a home-made software development in LabView 7.0. The heater area of $10 \times 5$ cm$^{2}$ raises the temperature to 600$^\circ$C over time due to the water-cooled extremes implemented. The oven is heated and controlled with a DC power supply, and a K-type thermocouple is used for monitoring the temperature with a $\pm 0.1^{\circ}$C resolution. Experiments done with the oven to obtain nanostructured AuCu alloys are detailed.
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  • Thermal simulation of breast tumors

    F.J. González
    323-0
    Abstract:
    It is well known that differences in energy consumption exist for normal and cancerous tissue. These differences lead to small but detectable local temperature changes, which is why infrared imaging has been used in the detection of different types of cancer; however, the early instrumentation was not sensitive enough to detect the subtle changes in temperature needed to accurately diagnose and monitor the disease. In recent years the sensitivity of infrared instruments has greatly improved. In this paper the bioheat transfer equation is solved for a simplified model of a female breast and a cancerous tumor in order to quantify the minimum size of a tumor or the maximum depth of a certain sized tumor that a modern state-of-the-art infrared imaging system can detect. Finite Element simulations showed that current state-of-the-art imagers are capable of detecting 3 cm tumors located deeper than 7 cm from the skin surface, and tumors smaller than 0.5 cm can be detected if they are located close to the surface of the skin.
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