Vol. 56 No. 2 (2010): Revista Mexicana de Física.

Published: 2010-01-01

Articles

  • The problem of glass formation and the low frequency vibrational modes anomalies

    G.G. Naumis, J.R. Romero-Arias
    97-0
    Abstract:
    In this article, we give a short review of the theoretical problems concerning the formation of disordered materials (known as glasses). Then we show how this problem is related to the anomalies in the low frequency vibrational modes observed in glasses. Rigidity theory and the energy landscape topology picture is used to decode how this relationship is built for network glasses. Finally, the problem of the speed of cooling and the ability to reach thermal equilibrium in glasses is explored using a simple model with non-linear interactions. The results are also interesting in soft-matter and protein folding.
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  • Microreología de polímeros asociativos multiuniones en solución acuosa

    E. Robles-Avila, M.A. Valdez-Covarrubias, R. Gámez-Corrales, J.I. Acedo-Carrillo, E.J. Jiménez-Regalado
    106-0
    Abstract:
    The experimental techniques of optical microrheology and classical rheology have been applied to study the viscoelastic properties of aqueous solution of an associative copolymer (AP) multisticker in the unentangled semidiluted regimen of concentration. Polymer chains consists of polyacrylamides (99 %mol), hydrophobically modified with low amounts ($ \le $1 % mol) of N,N-dioctylacrylamide (DOAM). Under certain physicochemical conditions these AP self assemble in aggregates of nanometric size to form physical transitory microgeles, resulting in viscoelastic fluids. The passive microrheological technique uses dynamic light scattering to obtain the information of the local dynamic properties of aqueous solutions of associating polymers. With these experimental techniques the range of frequencies is extended up to 50,000~s$^{ - 1}$, allowing to obtain more information of the moduli G'($\omega )$ y G''($\omega )$. In this work, a study of the concentration and diameter of tracer particles also the concentration of AP on the viscoelastic properties has been performed. The viscoelastic behavior of aqueous solutions of AP has been interpreted using the theoretical model of Rubinstein and Semenov [1].
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  • The action of canonical transformations on functions defined on the configuration space

    G.F. Torres del Castillo, D.A. Rosete Álvarez, I. Fuentecilla Cárcamo
    113-0
    Abstract:
    The effect of an arbitrary canonical transformation on functions defined on the configuration space is defined in such a way that a solution to the time-independent Hamilton--Jacobi equation is mapped into another solution if the Hamiltonian is invariant under the canonical transformation.
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  • Preparación de películas delgadas del sistema Ti-Al-O mediante rf-sputtering

    J.A. Montes de Oca, J. Ceballos-Alvarez, J. Galaviz-Pérez J.-P, M. Laha, e., J. Muñoz-Saldaña
    118-0
    Abstract:
    In the present work Ti-Al-O thin films were synthesized by rf-sputtering technique on glass and silicon (Si) substrates using TiAl and Ti$_{3}$Al targets in a sputtering chamber with an Ar-O$_{2}$ atmosphere. Ti-Al-O thin films were obtained varying experimental parameters such as oxygen percent fed to the reaction chamber, plasma power density and substrate temperature. The films deposited on glass substrates were used to evaluate their optical properties, while those deposited on Si substrates were used to evaluate mechanical and morphological properties. The crystalline structure, morphology, chemical composition and optical properties of the films were evaluated by X-ray diffraction (XRD), high-resolution scanning electron microscopy (HR-SEM), Auger Electron Spectroscopy (AES) and Visible UV Spectroscopy (UV-VIS). Films thicknesses were measured using a profiler. The roughness and mechanical properties such as hardness and Young's modulus were analyzed by atomic force microscopy (AFM) and nanoindentation technique, respectively.
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  • Performance characteristic of energy selective electron (ESE) refrigerator with filter heat conduction

    Zemin Ding., Lingen Chen., Fengrui Sun.
    125-0
    Abstract:
    The cooling load and coefficient of performance (COP) characteristic of an energy selective electron (ESE) refrigerator with filter heat conduction operating in the maximum cooling load regime and the intermediate regime (i.e. between maximum cooling load operation and reversible operation) are analyzed in this paper. In the analyses, the analytical formulae for cooling load and COP of the ESE refrigerator are derived for the two operation regimes, respectively. The performance characteristics are obtained by numerical calculations. The influence of the resonance widths on the performance of the ESE refrigerator in the intermediate regime is discussed. It is shown that, in the maximum cooling load operation regime, when the filter heat transfer is taken into account, the cooling load versus COP characteristic change from monotonic curves (without filter heat conduction) to parabolic-like ones (with filter heat conduction); while in the intermediate regime, the cooling load versus COP characteristic are always loop-shaped ones and the heat conduction of the filter does not change the shape of the characteristic curves. The cooling load is a parabolic-like function of resonance width while the COP is a monotonic one of resonance width. With the increase in thermal conductivity, the COP decreases in both operation regimes. The results obtained herein have theoretical significance for the understanding of thermodynamic performance of the micro-nano devices.
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  • Unstable states of In$_{x}$Al$_{y}$Ga$_{1 - x -y}$N quaternary alloys

    S.F. Díaz Albarrán, P. Rodríguez Peralta
    132-0
    Abstract:
    Regions of unstable states characterized by the spinodal decomposition of high-Al-content In$_{x}$Al$_{y}$Ga$_{1 - x - y}$N quaternary alloys lattices matched to the GaN $\left({001}\right)$ substrate are described. The alloy are represented as strictly regular solutions. In the analysis we take into account the internal deformation and elastic strain energies. The regions of unstable states or ranges of spinodal decomposition considering the compositions $0.06 \le x \le 0.18$ and $0 \le y \le 0.95$ with and without elastic energy are demonstrated up to 580$^{\circ}$C. We conclude that high concentrations of aluminum dramatically increased the phase separation temperature.
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  • Optimal ratios of the piston speeds for a finite speed endoreversible Carnot heat engine cycle

    Huijun Feng., Lingen Chen., Fengrui Sun.
    135-0
    Abstract:
    The performance of an endoreversible Carnot heat engine cycle is analyzed and optimized using the theory of finite time thermodynamics based on Agrawal and Menon's$^{i}$ model of finite speed of the piston on the four branches and Curzon and Ahlborn's$^{ii}$ model of finite rate of heat transfer. The finite speeds of the piston on the four branches are further assumed to be different, which is unlike the model of constant-speed of the piston on the four branches. The analytical formula between power and efficiency of the cycle is derived for a fixed cycle period. There exist optimal ratios of the finite piston speeds on the four branches. The effects of the temperature ratio of the heat reservoirs on the dimensionless power versus efficiency of the cycle and isothermal expansion ratio are obtained by numerical examples.
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  • Two methods to determine the Hermite-Gaussian beam radius by means of aperiodic rulings

    O. Mata-Mendez
    141-0
    Abstract:
    We study the diffraction of Hermite-Gaussian beams by aperiodic rulings by means of the Rayleigh-Sommerfeld theory in the scalar diffraction regime. We extend to Hermite-Gaussian beams the results of a previous paper where Gaussian beams were considered [J. Opt. Soc. Am. A 25 (2008) 2743]. The transmitted power and the normally diffracted energy are analyzed as a function of the beam radius. Two methods to determine the Hermite-Gaussian beam radius by means of aperiodic rulings are proposed. These two methods are based on the maximum and minimum transmitted power, and in the normally diffracted energy.
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  • Análisis cuantitativo de interacciones moleculares proteína-proteína mediante la combinación de microarreglos y un lector óptico basado en el fenómeno de resonancia de plasmones superficiales

    V. Altuzar, C. Mendoza-Barrera, M.L. Muñoz, J.G. Mendoza-Alvarez, F. Sánchez-Sinencio
    147-0
    Abstract:
    La resonancia de plasmones superficiales (SPR) encuentra una aplicación directa como sensor óptico en la lectura de interacciones biomoleculares. En este trabajo empleamos un microscopio de resonancia de plasmones superficiales para la lectura simultánea y en tiempo real de interacciones proteína-proteína mediante su combinación con microarreglos. La detección de dichos eventos moleculares se hacen sin necesidad de marcado fluorescente de la molécula bajo estudio. Los microarreglos fabricados contienen 144 elementos microscópicos activos de proteínas de (1) albumina de suero de humano HSA (0.1, 05, 1.0 mg/ml), (2) lisozima Lyz (0.1, 05, 1.0 mg/ml), (3) anti albúmina de suero de bovino anti BSA (1.0 mg/ml) y (4) albúmina de suero de bovino BSA (0.1, 05, 1.0 mg/ml). Determinamos simultáneamente las densidades de concentración superficial de interacción de anti albumina de suero de bovino a 1.0 mg/ml inyectada sobre los micrositios de proteínas previamente descritos, a través de las curvas de cinética de interacción SPR. Encontramos que para concentraciones altas (1.0 mg/ml) de los micrositios impresos interactuando con solución conteniendo anti BSA (1.0 mg/ml), la densidad de concentración superficial de anti BSA anclada fue de 660 ng/cm$^{2}$ para BSA, 366 ng/cm$^{2}$ en el caso de HSA, 22.3 ng/cm$^{2 }$ en Lyz y de 8.3 ng/cm$^{2 }$ para anti BSA.
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  • Geometric associative memories applied to pattern restoration

    B. Cruz, R. Barrón, H. Sossa
    155-0
    Abstract:
    Two main research areas in Pattern Recognition are pattern classification and pattern restoration. In the literature, many models have been developed to solve many of the problems related to these areas. Among these models, Associative Memories (AMs) can be highlighted. An AM can be seen as a one-layer Neural Network. Recently, a Geometric Algebra based AM model was developed for pattern classification, the so-called Geometric Associative Memories (GAMs). In general, AMs are very efficient for restoring patterns affected BY either additive or subtractive noise, but in the case of mixed noise their efficiency is very poor. In this work, modified GAMs are used to solve the problem of pattern restoration. This new modification makes use of Conformal Geometric Algebra principles and optimization techniques to completely and directly restore patterns affected by (mixed) noise. Numerical and real examples are presented to test whether the modification can be efficiently used for pattern restoration. The proposal is compared with other reported approaches in the literature. Formal conditions are also given to ensure the correct functioning of the proposal.
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  • Anisotropic cosmology in Sáez-Ballester theory: classical and quantum solutions

    J. Socorro, M. Sabido, M.A. Sánchez, M.G. Frías Palos
    166-0
    Abstract:
    We use the Sáez-Ballester theory on anisotropic Bianchi I cosmological model, with barotropic fluid and cosmological constant. We obtain the classical solution by using the Hamilton-Jacobi approach. Also the quantum regime is constructed and exact solutions to the Wheeler-DeWitt equation are found.
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  • Calorimetric measurements of AC looses in Bi-2213/Ag multifilament tape

    M. Ramos, J. Schwartz, U. Trociewitz, S. Pamidi, S. Flores
    172-0
    Abstract:
    We present research work collaboration between the University of Texas at El Paso and The National High Magnetic Field Laboratory. This work presents a series of measurements of AC losses in multi-filamentary Bi-2223/Ag tapes using a calorimetric method as described by Ashworth and Suenaga. The AC losses were investigated at liquid nitrogen temperatures (77 K) for different AC currents and in frequency range form 50 Hz to 500 Hz. The results are compared with the Norris ellipse model for calculating self-field loss in high temperature superconducting tape.
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  • Prototipo de microtomógrafo por emisión de positrones. Resolución espacial y estudios metabólicos

    H. Alva-Sánchez, T. Murrieta, C. Ruiz-Trejo, M.E. Br, an., A. Martínez-Dávalos, M. Rodríguez-Villafuerte
    175-0
    Abstract:
    El Grupo de Dosimetría y Física Médica del Instituto de Física de la UNAM ha desarrollado, en los últimos 4 años, un prototipo de microtomógrafo por emisión de positrones (microPET) para animales pequeños. El sistema está formado por arreglos de cristales centelladores de oxiortosilicato de lutecio con itrio dopados con cerio acoplados a tubos fotomultiplicadores sensibles a la posición. Las señales electrónicas de los detectores son procesadas con módulos de instrumentación nuclear y son digitalizadas con una tarjeta de adquisición de datos multicanal. A partir de un conjunto de proyecciones a diferentes ángulos, corregidas por distorsión geométrica y no uniformidades, se realiza la reconstrucción tomográfica utilizando retroproyección filtrada. En este trabajo se presentan medidas de la resolución espacial de reconstrucción utilizando fuentes lineales obteniendo un valor promedio de 2.36 $\pm $ 0.44 mm. Adicionalmente, se muestran los primeros estudios metabólicos de ratones sanos de 30 g inyectados con fluorodesoxiglucosa y fluoruro de sodio (ambos marcados con $^{18}$F) con los cuales se obtuvo información de la captación normal de la glucosa y la identificación de la estructura ósea, respectivamente. El microPET es una herramienta de gran utilidad para desarrollar proyectos de investigación básica en física de detectores y su aplicación en medicina nuclear.
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