Vol. 50 No. 5 (2004): Revista Mexicana de Física.

Published: 2004-01-01

Articles

  • Excitable chaos in diffusively coupled FitzHugh-Nagumo equations

    Abstract:
    A prototypic model of three coupled FitzHugh-Nagumo oscillators is shown to exhibit spatio-temporal hyperchaos. With increasing the number of coupled units the number of positive Lyapunov exponents increases. A system in two spatial dimensions shows two types of excitable spatio-temporal (hyper-)chaos depending on which variable is chosen for the coupling. Some implications for excitable cardiac tissue are discussed.
    ⬇️ Scroll down to see the full summary
    G. Baier, M. Müller
    422-0

  • Theoretical study of $e^-$ - He scattering using the Schwinger variational principle with plane waves as a trial basis set

    Abstract:
    We report an application of the Schwinger variational principle with plane waves as a trial basis set. Differential cross sections are obtained for e$^-$ - He from 15 to 100~eV. Our differential cross is found to be in reasonable agreement with experimental data.
    ⬇️ Scroll down to see the full summary
    J.L.S. Lino, G.G.Z. Torres
    427-0

  • Diseño óptimo y realización de celdas solares de silicio para producción industrial. Estado del arte de la investigación en México

    Abstract:
    We present here the work that the author and his group have developed during several years for the design and fabrication of silicon solar cells with selective emitters with potential for industrial production. The solar cell design methodology is based on the solution of the carrier transport equations in semiconductor devices, taking into account the variation of mobility and lifetime in non-uniformly doped regions (Gaussian profiles as obtained from diffusion or ion implantation processes), and phenomena such as Auger recombination and bandgap shrinkage in highly doped silicon. A computer program was also developed for the design of the metal grid, taking into account the lateral flux of current in the emitter, and also the technological limitations for the fabrication of the grid fingers. In addition, another computer program was developed for optimizing the thickness of anti-reflection layers, given the index of refraction, in order to have a complete set of programs for the design of silicon solar cells. We describe the processes required for making the silicon solar cells, and several experiments that have allowed the improvement of the conversion efficiency of this kind of solar cells. For example, silicon nitride layers were deposited by Plasma Enhanced Vapor Deposition (PECVD) from SiH$_{4}$/NH$_{3}$, and the deposition conditions were optimized for achieving surface state passivation at the emitter, and at the same time working as optimal anti-reflection layer. Similarly, a new process of aluminum alloying at the back of the cells was added to cell fabrication, leading to an increase in the efficiency by more than 20% with respect to cells made by conventional processes, due to effects of metallic impurity gettering. Finally, the benefit due to an optimum double anti-reflection layer TiO$_{2}$/SiO$_{2}$ is also shown.
    ⬇️ Scroll down to see the full summary
    A. Morales-Acevedo
    431-0

  • Ecuaciones de Hamilton-Jacobi y de Schrödinger en la dinámica relativista de tiempo propio

    Abstract:
    The dynamics of a relativistic point particle is formulated using the proper time as evolution parameter on the hyperbolic $p_0^2-{\vec p}^2=M^2c^2$ and spheric $p_4^2+{\vec p}^2=\ce_0^2/c^2$ shells. This last case corresponds to considering the motion under a Lorentz invariant potential. The Hamilton-Jacobi equations of motion under this Lorentz scalar potential are formulated both for massive ($M^2=m^2,~m>0$) and massless ($M=0,~m>~0$) particles, and for the neutrino. We present additionally a first quatization version of the model following the Schrödinger canonical quatization scheme.
    ⬇️ Scroll down to see the full summary
    R.M. Yamaleev, A.L. Fernández Osorio, A.R. Rodríguez Dgz., .
    443-0

  • Aplicación de redes neuronales de aproximación a una línea de luz para reconstrucción 3D de objetos

    Abstract:
    A technique for 3D object shape detection based on light line image processing is presented. In this process, an approximation neural network is used to reconstruct the 3D object shape. This neural network is generated using images of a light line projected onto the objects, whose dimensions are known. These images are obtained in the scanning step of the light line onto the objects. The profilometric method used by the neural network is based on the light line deformations. These deformations are measured by the Gaussian approximation method. In this technique, the 3D shape is obtained without use the parameters of the experimental set-up. It is an advantage over conventional methods of the light line projection. In this manner, the accuracy is improved due to the errors are not introduces in the system. The accuracy in this technique is deduced by the rms value. This technique is tested with simulations and real objects. Also, the time processing and accuracy results are presented.
    ⬇️ Scroll down to see the full summary
    J.A. Muñoz Rodríguez, A. Asundi, R. Rodríguez Vera
    453-0

  • Multiple beam interference with near-grazing waves in dielectric wedges: polarization dependence

    Abstract:
    Multiple beam interference of near-grazing waves has been recognized as responsible for bright interference patterns in dielectric wedges. Near-grazing waves are traveling waves having wave vectors forming an angle of nearly $\pi $/2 with the normal of a planar surface. When two such surfaces form a wedge with near-grazing waves within, two main features determine interference pattern formation. First, the values of the Fresnel reflection coefficients are close to unity for near-grazing waves. Second, the phase dependence as a function of the gap between surfaces is step-wise nearly constant for the same kind of waves. Both features have polarization dependence, so polarization dependence of interference patterns are also expected. Numerical results for $s$ and $p$ polarizations are presented under monochromatic and polychromatic illuminations. Experimental observations are also shown.
    ⬇️ Scroll down to see the full summary
    G. Rodríguez-Zurita, J. Vázquez-Castillo, J. Pedraza-Contreras
    462-0

  • Similarity between optical response kinetics of conducting polymer thin film based gas sensors and electrochromic devices

    Abstract:
    Chemically deposited polyaniline (PANI) thin films were used as optically active materials in optochemical gas sensors (OGS) as well as in electrochromic devices (ECDs). An electrochemically deposited poly(3-methylthiophene) (P3MT) thin film based ECD was also prepared to be compared to PANI based ECDs. In all these optical devices, the optical response kinetic curves of PANI and P3MT films can be fitted quite well with two terms of the same exponential expression: one faster and one slower. The faster one could be attributed to a rapid specific surface or interface reaction process, and the slower one to a volumetric or bulk diffusion-reaction process. The physical meanings of the fitting parameters come from the Langmuir adsorption theory and Freundlich isotherm in the case of OGS and the Butler-Volmer equation for polymer based ECDs.
    ⬇️ Scroll down to see the full summary
    Hailin Hu., L. Hechavarría, M.E. Nicho
    471-0

  • Quasi-surfaces waves under drift and diffusion mechanism in nonlinear interfaces$^\dag$

    Abstract:
    We study numerically and theoretically the behaviour of one-dimensional bright spatial soliton in an interface formed by a nonlinear media under drift and diffusion nonlinearities, and a linear one in the second media. The mechanism of diffusion causes self-bending effect on the soliton, and in consequence it is launched to nonlinear interface; after that the soliton is reflected to nonlinear medium and self-bending by diffusion newly launched the soliton to the interface. In consequence, a quasi-surface wave is formed. We present details about the trajectory, coefficient of saturation and energy during the dynamics of the spatial soliton.
    ⬇️ Scroll down to see the full summary
    E. Alvarado-Méndez, J.A. Andrade-Lucio, R. Rojas-Laguna, J.M. Estudillo-A, ala., J.G. Aviña-Cervantes, O.G. Ibarra-Manzano, V. V, sloukh.
    478-0

  • Spectral reflectance estimation of ancient mexican codices, multispectral images approach

    Abstract:
    Mexican codices are an ancient reading and writing system developed by the pre-Hispanic cultures of Mesoamerica. These pictorial documents are a cultural legacy, part of which dates from the 16$^{th}$ century. The collection known as ``Collection of Original Mexican Codices'' is one of the most important codices collection around the world and it is under the custody of the National Library of Anthropology and History~(BNAH, Biblioteca Nacional de Antropologia e Historia) in Mexico City. For preservation of the documents, the collection is kept under limited access and controlled illumination conditions, only tungsten lamps are allowed for illumination of codices and light intensity should no exceed 1000 Lux. In order to achieve accurate color reproduction of codices we have proposed to the BNAH a multispectral aproach. Our color reproduction method is based on the estimation of the spectral reflectance for every pixel in the image from a set of 16 bands multispectral images. Multispectral approach enables us to remove the original tungsten capture illumination and to carry out a color simulation under any arbitrary illumination, for instance CIE-D65, hence this approach enables us to reproduce the true colors of original codices. In this paper we present the results of several approaches for spectra estimation considering the illumination conditions established by the BNAH, and we show a color simulation of a codice replica under CIE-D65.
    ⬇️ Scroll down to see the full summary
    J. Conde, H. Haneishi, M. Yamaguchi, N. Oh, ama., J. Baez
    484-0

  • Baryon magnetic moments in the SU(3) and the SU(2)$\times$U(1) flavor groups

    Abstract:
    Working within the non relativistic quark model, a two parameter fit to the magnetic moments of baryons is presented. The fit has an excellent~$\chi^2$. The model is based on taking different flavor groups to describe the different magnetic moments. The selection of which group to assign to each baryon is guided by the structure of its wavefunction. The model corresponds to assigning different effective masses to a quark depending on which baryon is being considered. Using the values extracted from the fit, the magnetic moments of the $\Omega^-$ and the $\Delta^{++}$ have been predicted, and the comparison to the existing experimental values is quite satisfactory.
    ⬇️ Scroll down to see the full summary
    J. G, R. Huerta, L.R. Quintero
    490-0

  • Fabricación y caracterización de materiales compuestos de matriz metálica Al-Cu y Al-Mg reforzados con partículas de TiC

    Abstract:
    The pressureless melt infiltration of Al-Cu and Al-Mg alloys into particulate 56 vol.% TiC preforms was studied. The infiltration of aluminum alloys varying the Mg and Cu content was compared with the infiltration rate of pure aluminum and were carried out in a thermogravimetric analyzer. It was found that the infiltration rate of Al-Mg$_{x}$ increases as the magnesium content increases. On the contrary, infiltration rate of Al-Cu$_{x}$ increases with decreasing copper content. For both systems the higher the temperature the faster the infiltration rate. Comparing the results at the infiltration temperature of 900$^\circ$C, the infiltration rate was greater with the Al-Cu$_{x}$ alloys than Al-Mg$_{x}$ alloys, but never was greater than pure Al. Moreover, the infiltration of pure magnesium into the preforms was faster than the Al-Mg$_{x }$alloys. XRD studies of composites containing copper confirmed the formation of CuAl$_{2}$ phase, the amount increasing with the Cu content in the composites. For the AlMg$_{x}$ matrix composites, no reaction phase was detected.
    ⬇️ Scroll down to see the full summary
    A. Contreras, E. Bedolla
    495-0

  • (Ba,Sr)TiO$_{3}$ ferroelectric thin films for tunable microwave applications

    Abstract:
    The dielectric properties of ferroelectric thin films were investigated for tunable microwave applications. We have observed that epitaxially grown Ba$_{1 - X}$Sr$_{X}$TiO$_{3}$ (BST, 0.4$ \le $ X $ \le $1) films are distorted from the normal cubic symmetry of the corresponding bulk at room temperature. This structural distortion caused by film strain has a strong impact on the microwave dielectric properties. For compressive strain, the dielectric constant and tuning were decreased and the films showed high dielectric Q. However for tensional strain, the opposite effect was observed. This observation has been interpreted based on phenomenological thermodynamics and strain-induced polarization physics. Two experimental examples, strain-relieved films and strain-enabled films, are presented to show how film strain affects the tunable microwave properties.
    ⬇️ Scroll down to see the full summary
    Wontae Chang., Steven W. Kirchoefer, J.A. Bellotti, J.M. Pond, D.G. Schlom, J.H. Haeni
    501-0

  • Elongational properties and crystallization of poly (isobutylene) melts probed by synchrotron radiation

    Abstract:
    In this paper, we describe a phase transition in poly(isobutylene) of various molecular weight under elongational stress. Rheometry and wide angle X-ray diffraction have been performed simultaneously to correlate mechanical and structural properties of the material. The low molecular weight samples did not crystallize under the flow in the range of the strain rates applied. On the other hand, as the molecular weight increases the crystallization is always preceded by a strain of the samples under stretching.
    ⬇️ Scroll down to see the full summary
    A. Gonzalez-Alvarez, M. Arellano, O. Diat, J.F. Legr, J.M. Piau, .
    506-0

  • Micro-structures made with a capillary

    Abstract:
    A method to elaborate micro-structures of ionic crystals based on the deposition of drops on a surface with a capillary is presented. The typical dimension of these structures is in the micron range. The possible applications of such structures in optics are discussed.
    ⬇️ Scroll down to see the full summary
    T. Lopez-Rios, F. Ga, R.A. Vazquez-Nava, A. Barbara, S. Pairis, .
    515-0

  • Application of the total internal reflection phenomenon as a gas sensing technique using evaporated SnO$_{2}$ thin films

    Abstract:
    In this work it is shown that the Total Internal Reflection (TIR) phenomenon can be exploited for gas detection. Total internal reflectance measurements for evaporated SnO$_{2}$ thin films on quartz substrates are measured at ambient conditions. Changes in the reflectance measurements are measured when CO$_{2}$ or N$_{2}$ gas flows on the surface of the SnO$_{2}$ films. The observed shift in the measured reflectance is defined as the signal of the sensor. Preliminary results show that the maximum response takes place at incident angles close to the critical angle of the quartz-air system.
    ⬇️ Scroll down to see the full summary
    L. Martínez Pérez, V. Altuzar, N. Muñoz Aguirre, V. Gariba, -Febles., M. Lozada-Cassou, M. Aguilar-Frutis, O. Zela, a-Angel.
    518-0

  • Scattering of periodic solitons

    Abstract:
    Through numerical simulations we study $N$-soliton scattering ($N$=3,4) in the ($2+1)$-dimensional $CP^1$ model with periodic boundary conditions. Solitons colliding from symmetrical configurations scatter at $\pi/N$, as observed in the usual model with standard boundary conditions. When the initial configurations are not symmetric the angles differ from $\pi/N$. We describe our observed patterns based on a properly formulated geodesic approximation.
    ⬇️ Scroll down to see the full summary
    R.J. Cova, W.J. Zakrzewski
    527-0

  • Surface deformations induced by CH$_3$S adsorption on Au(111) and Cu(111): a DFT study

    Abstract:
    Surface deformations induced by methylthiolate adsorption on the (111) faces of Au and Cu are investigated by means of state of the art DFT slab calculations. We find that a significant surface rearrangement takes place when the molecules are adsorbed. Surface deformations are found to be larger for Au than for Cu and the magnitude of the deformations depends on both the coverage and the site of adsorption. Methanethiol adsorption on both Cu(111) and Au(111) \cite{ourjpc} is stronger for partial than for full coverage.
    ⬇️ Scroll down to see the full summary
    M.C. Vargas, A. Selloni
    536-0

  • Diseño y construcción de un esparcímetro de luz

    Abstract:
    In this paper, we present the design and construction of a light scatterometer optimized to carry out measurements of high angular precision and weak signals, to study random rough surface of thin films. The scatterometer has the advantages of great ease of alignment and high angular precision as compared with commercial one. To prove the equipment angular precision, we report experimental results of reflection measurements using the technique of attenuated total reflection (ATR) in the Kretschmann configuration of two systems which were excited electromagnetic modes. The first system used is BK7 glass-prism/Ag/air and the second is BK7 glass-prism/MgF$_{2}$/ZnS/MgF$_{2}$/air. With this apparatus, reflection minima could be measured which in the 50% of it's depth have a width of 0.1$^{\circ }$. As detection sensibility test of the equipment, we report scattering measurements carry out to the second system. Fitting the experimental results, a good coincidence between theory and experiment was found. We report values of thickness, refractive index, absorption index, height of the roughness and correlation length obtained from the fitting. These values were compared with those reported in the literature.
    ⬇️ Scroll down to see the full summary
    A.S. Ramírez-Duverger, R. García-Llamas
    541-0