Vol. 51 No. 3 (2005): Revista Mexicana de Física.

Published: 2005-01-01

Articles

  • Analysis of some atmospheric mesoscale models

    M.A. Nuñez
    217-0
    Abstract:
    Computational mesoscale models have become an important tool for air pollution studies and operational weather analyses. In this work, we analyze some of these mesoscale models. The results show that some of these models have inconsistencies that limit their use in current applications in Mexico. Some modifications are suggested to improve these models.
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  • InAs quantum dots grown on GaAs (100) surfaces subjected to novel in-situ treatments

    V.H. Méndez-García
    230-0
    Abstract:
    Novel in-situ treatments were performed to GaAs(100) surfaces in order to improve the size homogeneity of self-assembled InAs quantum dots (QDs). The treatments consisted in exposing the GaAs surfaces at high temperature for 10 seconds with the As$_{4}$- shutter closed. In the first experiment, the GaAs surface was just kept under no fluxes during 10 seconds, while in another growth the Si shutter was opened during the As$_{4}$ flux interruption. Both experiments were compared with a conventionally grown sample. Remarkable differences in the growth kinetics were observed when the InAs deposition was performed on different treated GaAs surfaces. The thermal treatment performed under no Si-flux extended the two to three-dimensional growth transition at much larger InAs thickness. On the contrary, the Si-treated sample showed an earlier lattice relaxation as compared with the reference sample. As for the final topology of the samples both treatments decreased the QDs diameter and height dispersion as compared with the conventionally grown sample. Therefore, a significant improvement on the size distribution of QDs was induced by the novel in-situ treatments, which also reduced the full width at half maximum (FWHM) of the photoluminescence (PL) emission spectra. Additionally, PL experiments showed a clear correlation between the dots size increase and the emission peak redshift observed for the QDs grown on GaAs surfaces subjected to the different treatments.
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  • Un algoritmo no iterativo para la tomografía de capacitancia eléctrica

    A. Fraguela, J. Oliveros, L. Cervantes, M. Morín, S. Gómez
    236-0
    Abstract:
    A novel method is described to solve the 2D inverse parameter identification problem, for image reconstruction of permittivity distribution in the cross section of a pipeline that contain multiphase dielectric fluid, using Electrical Capacitance Tomography. This method is non-iterative and consists of the decomposition of the inverse problem in several simpler problems, each one of them can be solved with a numerically stable procedure, with the necessary theoretical justification. The method takes into account important characteristics of the problem: its non-linearity, its ill posedness and the need to perform a theoretically justified regularization because the errors in data may not allow getting the images of the cross section with all the desired precision. In this paper three of the five problems of the novel method are studied and results for algorithms used for each one are established.
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  • Microscopic behavior of carbon stripper foils prepared by a mixed ion beam sputtering method under heavy ion beam bombardment

    Hideshi Muto., J. López Monro, .
    243-0
    Abstract:
    A mixed ion beam sputtering method has been used for preparing long lifetime stripper foils. The microscopic transformations of these originally amorphous carbon foils under heavy ion beam irradiation are described. The structure changes of the carbon foils, irradiated by 3.2 MeV Ne$^{ + }$ ion beams, were examined by a 200 keV electron microscope. The larges sizes of grains of the foils were composed of graphite like polycrystallites that had isotropic configurations, with graphite layers nearly perpendicular to the foil surface.
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  • Sistema para la simulación y análisis de radiografía y tomografía de rayos gamma

    J.C. Palacios, J. Santos, L.C. Longoria, R.T. Perr, .
    246-0
    Abstract:
    In this work, we present the development of a system to reconstruct the inner contend of industrial objects using gamma rays. This system is conformed by two subsystems. The first subsystem obtains radiographic gamma images and the second reconstructs 2D images corresponding to slices of the object, using transmission and of emission tomography. Both subsystem has a module to simulate gamma radiography and industrial gamma tomography, respectively. The developed simulation module can be used to design gamma tomography for specific applications, determining parameters like the number of projections and the necessary energy for the reconstruction of the involved object.
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  • Conversión paramétrica en un cristal fotónico no-lineal

    M. Torres-Cisneros, J.W. Haus, P. Powers, P. Bojja, M. Scalora, M.J. Bloemer, N. Akozbek, L.A. Aguilera-Cortés, R. Guzmán-Cabrera, R. Castro-Sánchez, M.A. Meneses-Nava, J.A. Andrade-Lucio, J.J. Sánchez-Mondragón
    258-0
    Abstract:
    In this work we present a novel coherent source of millimeter wavelength waves based on parametric down-conversion in an one-dimensional photonic crystal. Our proposal is based either on the band edge or on defect-mode field enhancement phenomena near a photonic band gap. The numerical results have been showed that a wide range of intensities and bandwidths of coherent radiation can be obtained as we vary either the number periods or the refraction index contrast between layers of the crystal array.
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  • A note on chaos-based communication schemes

    D. López-Mancilla, C. Cruz-Hernández
    265-0
    Abstract:
    In this work, a modified chaos-based communication scheme is presented. In particular, we extend the basic scheme for chaotic masking using a single transmission channel proposed by Cuomo and coworkers in 1993. The substantial differences between the traditional scheme and this modified one, significantly affect the reception quality of the sent message, such that this one can faithfully be recovered without filtering at the receiver end, without considering the effect of noise in the transmission channel. Nevertheless, if noise is present, this can be eliminated using a filtering stage. We use two Lorenz systems unidirectionally coupled, the first like a master/transmitter system and the other like a slave/receiver system in order to illustrate with some numerical simulations the effectiveness of the modified scheme.
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  • Theoretical study of electron - SiH$_4$ collisions using the Schwinger variational principle with plane waves as a trial basis set

    J.L.S. Lino, A. Eiras
    270-0
    Abstract:
    We report an application of the Schwinger variational principle with plane waves as a trial basis set. Differential cross sections are obtained for electron-SiH$_4$ collisions from 7.5-40~eV. Our differential cross sections are found to be in reasonable agreement with experimental data.
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  • Lock-in amplifier-based rotating-analyzer spectroscopic ellipsometer with micro-controlled angular frequency

    J.M. Flores-Camacho, O.F. Núñez-Olvera, G. Rodríguez-Pedroza, A. Lastras Martínez, L.F. Lastras-Martínez
    274-0
    Abstract:
    We report on the development of a full operational rotating analyzer spectroscopic ellipsometer. This instrument employs a phase-sensitive amplifier to process the optical signal as an alternative to Fast Fourier Transform analysis. We describe electronic hardware designed to stabilize the rotation frequency of the analyzer prism as well as to drive the device for the positioning of the polarizer prism azimuth. The ellipsometer allows for dielectric function measurement in the energy range from 1.7-5.5 eV, in both ambient air and Ultra High Vacuum (UHV). UHV measurements can be carried out at a temperature as low as 150 K. To evaluate the ellipsometer performance we present results of the determination of the complex dielectric function of a number of semiconductors, namely, GaSb, GaAs, InGaAs, CdTe and CdHgTe.
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  • Stoichiometric hydroxyapatite obtained by precipitation and sol gel processes

    C. Guzmán Vázquez, C. Piña Barba, N. Munguía
    284-0
    Abstract:
    Three methods for obtaining hydroxiapatite (HA) are described. HA is a very interesting ceramic because of its many medical applications. The first two precipitation methods start from calcium and phosphorous compounds, whereas the third method is a sol-gel process that uses alcoxides. The products were characterized and compared. The observed differences are important for practical applications.
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  • Time-resolved electron temperature measurement in a multiwire Z-pinch plasma

    A. Robledo-Martínez
    294-0
    Abstract:
    A simplified technique to measure the plasma temperature in multiwire Z-pinch experiments is reported. X-ray PIN, diodes fitted with different transmission filters, were employed to detect the X-radiation in different energy bands. The comparison of the relative intensities in each band gives a time-resolved measurement of the electronic temperature. The method consists in applying a simple model of the continuum spectrum that is convoluted with the filter-detector response to obtain predicted amplitude ratios. The computed ratios are then compared to the experimental ones to obtain the time-resolved temperature. When the method was applied to experiments carried out with 1-4 wire bundles, the temperature was found to be in the interval 1 to 7~keV.
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  • Short range order in new rock salt type structures of the system Li$_{3}$NbO$_{4}$-CoO

    M. Vega-Glez, M.A. Castellanos Román, A. Huanosta-Tera
    299-0
    Abstract:
    Synthesis of compounds belonging to the system Li$_{3}$NbO$_{4}$-CoO has been obtained by a solid state reaction, the products are appropriately described by the formula Li$_{(3 - 3x)}$Co$_{4x}$Nb$_{(1 - x)}$O$_{4}$; 0 $ \le $ x $ \le $ 1.$_{ }$These new compounds have been studied in their diffraction features. Their electron diffraction patterns are characterized by the existence of a well defined diffuse intensity distribution in addition to sharp Bragg reflections of a rock salt type structure. This has been attributed to the presence of atomic short range order in the structure. As the amount of Co$^{2 + }$ ions increases in the structure, the amount of grains which produces diffuse scattering becomes larger therefore, we propose that the presence of Co$^{2 + }$ drives local spatial cation distribution which would rise electron diffuse scattering. An explanation, in terms of a modulated structure, has been put in advance depicting a probable atomic arrangement to explain the detected short range order.
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  • On the necessity of the postulate of the collapse of the wave function: an explicit calculation of the ``double-slit" problem

    R. Martínez-Galicia, V. Romero-Rochín
    304-0
    Abstract:
    Based on numerical calculations of the two-slit problem, we object van Kampen´s proposal that the collapse of the wave function is an unnecessary postulate of Quantum Mechanics if the measuring apparatus is included in a unitary evolution of the system-apparatus. We argue that van Kampen's interpretation does not substitute the collapse postulate, it is not precise and it appears impracticable.
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  • Recubrimientos de circonia depositados por electroforesis sobre acero 316L

    I. Espitia-Cabrera, M.E. Contreras-García, P. Bartolo-Pérez, J.L. Peña, J. Re, es-Gasga., L. Martínez
    311-0
    Abstract:
    The present research involved zirconia coatings prepared using electrophoretic deposition (EPD) on 316L stainless steel, via hydrolysis of ZrOCl$_{2}$ aqueous solution. Initially, a first zirconia thin film was obtained and treated at 400 $^\circ$C for consolidation. Then a second zirconia film was deposited to obtain a homogeneous and fully covered 316L stainless steel plate. The XPS analyses show that on the first zirconia film, the elements Fe, Cr, O and Zr are present. In this first film the compounds Cr$_{2}$O$_{3}$, Fe$_{2}$O$_{3}$ and ZrO$_{2}$ are formed. While in the second film only the Zr and O are observed so that the surface is formed by ZrO$_{2}$.
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  • Riccati nonhermiticity with application to the Morse potential

    O. Cornejo-Pérez, R. López-S, oval., H.C. Rosu
    316-0
    Abstract:
    A supersymmetric one-dimensional matrix procedure, similar to the relationships of the same type between Dirac and Schrödinger equations in particle physics, is described in the general terms. By this means, we are able to introduce a nonhermitic Hamiltonian having the imaginary part proportional to the solution of a Riccati equation of the Witten type. The procedure is applied to the exactly solvable Morse potential introducing, in this way, the corresponding nonhermitic Morse problem. A possible application is the molecular diffraction in evanescent waves over nanostructured surfaces.
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  • Nuevas aplicaciones decorativas de aleaciones a base de aluminio-molibdeno

    V.H. Mesa-Laguna, C.D. Hernández-Pérez, M.A. Alvarez-Pérez, J. Guzmán, M. García-Hipólito, J. A, C. González, O. Alvarez-Fregoso
    320-0
    Abstract:
    Decorative properties of aluminum-molibdenum alloys have been analyzed as a function of chemical composition and applied heat treatment. These decorative application are due exclusively for their nanostructure nature. The alloys were prepared at room temperature by DC magnetron sputtering technique in argon atmosphere at different deposition time to obtain several thickness and chemical compositions in the range 3 to 30% of molibdenum metal.
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