Vol. 51 No. 5 (2005): Revista Mexicana de Física.

Published: 2005-01-01

Articles

  • Identificación del sistema de Rössler: enfoque algebraico y algoritmos genéticos

    C. Aguilar Ibáñez, J. Sánchez H., M.S. Suárez C., F. Flores A., R. Garrido M., R. Martínez G.
    437-0
    Abstract:
    This article presents a method to determine the parameters of Rössler's attractor in a very approximated way, by means of observations of an available variable. It is shown that the system is observable and identifiable algebraically with respect to the chosen output. This fact allows to construct a differential parametrization of the output and its derivatives. Using this parametrization an identification scheme based on least mean squares is established and the solution is found with a genetic algorithm.
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  • Modulation of spatial coherence of optical field by means of liquid crystal light modulator

    A.S. Ostrovsk, E. Hernández García, .
    442-0
    Abstract:
    The theory of modulation of spatial coherence of optical field with the aid of a dynamic diffusion screen is given. Some useful models of a random phase screen are considered. The possibility of modulation of spatial coherence by means of liquid crystal light modulator is demonstrated with a physical experiment.
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  • Local induction of spatio-temporal chaos

    G. Baier, A. Ramírez, I. Amaro, M. Müller
    447-0
    Abstract:
    A periodic perturbation of one excitable system causes a symmetry-breaking instability in two reversibly coupled neighbors. The result is applied to a two-dimensional extended system. Tuning of the local perturbation frequency causes a regular target pattern to switch first to circular chaotic waves and then to chaotic wave fragments. Thus a global order-disorder transition can be induced by local control in an otherwise homogeneous medium.
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  • Anharmonic coefficients in high symmetry directions for the diamond structure

    J.L. Escamilla-Re, es., E. Haro-Poniatowski
    452-0
    Abstract:
    In this work, the cubic anharmonic coefficients are computed analytically in high symmetry directions considering central potential interactions up to fifth nearest neighbors for crystals with the diamond structure. It is found that certain channels of decay are forbidden. Furthermore particular relations between the different polarizations of the optical phonon for the cubic anharmonic coefficients are presented. Using these results, the validity of the so-called Peierls approximation is discussed.
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  • Analogy betwen dislocation creep and relativistic cosmology

    J.A. Montema, or-Aldrete., J.D. Muñoz-Andrade, A. Mendoza-Allende, A. Montema, or-Varela.
    461-0
    Abstract:
    A formal, physical analogy between plastic deformation, mainly dislocation creep, and Relativistic Cosmology is presented. The physical analogy between eight expressions for dislocation creep and Relativistic Cosmology have been obtained. By comparing the mathematical expressions and by using a physical analysis, two new equations have been obtained for dislocation creep. Also, four new expressions have been obtained for Relativistic Cosmology. From these four new equations, one may determine the neutron energy, $u_{N}$, by knowing of the present value of the universe radius and the Einstenian gravitational constant. Another new expression gives the neutron radius, $r_{N}$, as the present value of the universe radius, $R_{OU}$ divided by 10$^{40}$.
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  • Analytic partition function for plasmas

    O. Cardona, E. Simonneau, L. Crivellari
    476-0
    Abstract:
    The partition function for atoms in a gas with partial ionization is obtained by assuming that the internal and atomic linear energy densities are equal, and considering that the kinetic energy per degree of freedom is sufficient to ionize the upper levels of the atoms, and, at the same time, approximating the sum by an integral, which we solve using the mean value theorem for integrals. The resulting function reproduces the behavior of the partition function in the region of validity of perfect gases; it is also analytic and compact.
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  • Comparison of NO$_{2}$ and NH$_{3}$ gas adsorption on semiconductor polyaniline thin films

    J. Elizalde-Torres, H. Hu, J.M. Saniger
    482-0
    Abstract:
    Semiconductor polyaniline (PANI) thin films have proven to be an optically sensitive element for detecting gaseous NH$_{3}$ and NO$_{2}$ at room temperature. Although the absorption of either of them on PANI films results in the color change of the films, from green to blue, the mechanism of the interaction of each gas with the polymer is different. Ultra-violet (UV)-Visible (VIS) spectra of the NO$_{2}$ saturated PANI thin films give an increasing blue shift of the polaron band (1.5 eV) as a function of the gas concentration. The NH$_{3}$ gas saturated PANI samples, however, show a small red shift of the same absorption band and a slight increase of the relative intensity of the 2.0 eV band. It is suggested that NO$_{2}$ oxidizes the semiconductor PANI just as this can be oxidized by an electrochemical potential in an aqueous acid solution, whereas NH$_{3}$ deprotonates the polymer without taking out any electrons from it. The presence of NH$_{4}^{+}$ and of NO$_{2}^{-}$ species in the gas saturated PANI samples is shown by the analysis of their Fourier Transform Infrared (FT-IR) spectra.
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  • Multi-pass confocal ultra-short pulse amplifier

    A. Ruiz-de-la-Cruz, R. Rangel-Rojo
    488-0
    Abstract:
    We present the results obtained in the design and construction of an amplifier system for ultrashort optical pulses. The system is based on the chirped pulse amplification technique (CPA). In this technique, the pulse to be amplified is first stretched in time to lower its peak power; in the next step, the pulse is amplified and it is finally compressed in time to approximately its original duration. Our amplifier stage is very simple, using only 2 spherical mirrors in a confocal configuration to make 8 passes of the signal pulse through the amplifying medium, a 5mm long Ti:Sapphire rod. We report obtaining output pulses with an energy of several $\mu$J.
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  • Características dosimétricas de fuentes isotópicas de neutrones

    H.R. Vega-Carrillo, E. Manzanares Acuña, V.M. Hernández Dávila, G.A. Mercado Sánchez E, A. Lorente Fillol
    494-0
    Abstract:
    Using Monte Carlo methods, the neutron spectra produced by $^{252}$Cf, $^{252}$Cf/D$_{2}$O, $^{241}$AmBe, $^{239}$PuBe, $^{140}$LaBe, $^{239}$Pu$^{18}$O$_{2}$ and $^{226}$RaBe were calculated. The mean energy and the dosimetric features for each source were determined. The calculated dose were H, H*(10), H$_{p,slab}$(10,0$^\circ)$, $E_{AP}$ y $E_{ISO}$. During calculations the sources were modeled as point-like and the dosimetric features were determined in vacuum. For $^{239}$PuBe and $^{241}$AmBe neutron sources the Monte Carlo calculations were carried out using the actual dimensions and their dosimetric characteristics were evaluated inside of an air-filled region. These results were compared with experimental data.
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  • Thermal evolution of porous anodic aluminas: a comparative study

    M.E. Mata-Zamora, J.M. Saniger
    502-0
    Abstract:
    The comparative study presented in this work concerning the thermal evolution of sulfuric, oxalic and phosphoric porous anodic aluminas, points out some differences among their physico-chemical and structural properties which were not previously reported. Empirical formulas calculated from the thermal analysis of all the anodic aluminas under study indicate that sulfuric aluminas have a significant higher content of dopant anionic species, oxygen excess, and hydroxyl groups than the oxalic and phosphoric aluminas, indicating that porous sulfuric anodic aluminas should have a higher structural disorder and hydrophilic character than its counterparts. For all samples, transition alumina phases are formed around 900$^{\circ}$C and $\alpha $-alumina above 1200$^{\circ}$C, but in these transformations sulfuric and oxalic aluminas follow a different evolution from phosphoric alumina. In the former case, the formation of transition aluminas occurs with the almost simultaneous thermal decomposition of sulfates and oxalates and finally a pure $\alpha $-Al$_{2}$O$_{3}$ phase is formed; while for the phosphoric alumina, the phosphate does not decompose even at 1400$^{\circ}$C, when a $\alpha $-Al$_{2}$O$_{3 }$phase unpurified with AlPO$_{4}$ is observed. Infrared spectroscopy studies show that the coordination modes of the sulfuric and oxalic dopant species start to change well before their thermal decomposition, while, in the case of the phosphoric alumina, aluminum phosphate starts to form at the same time as the transition alumina phases.
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  • A simple method for changing the state of polarization from elliptical into circular

    M. Monto, a., G. Paez, D. Malacara-Hernández, J. García-Márquez
    510-0
    Abstract:
    Changes of polarization occur as a consequence of the interaction of light and the various optical elements through which it passes. A circularly polarized light beam may change its state to slightly elliptically polarized for many reasons. To correct this is not always easy but we show a very simple method for correcting circular polarization that has changed slightly into elliptic polarization. In this paper we propose to restore the circular state of polarization of an elliptically polarized light beam back to circular by means of a glass plate properly oriented while polarization is being measured. The basic idea is to modulate the transmittances of the electric field in both the major and minor axes of the ellipse of polarization. It is done by means of glass plates at non-normal incidence. Experimental results are consistent with theory.
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  • Transition from quantum to classical behavior for some simple model systems

    D. Schuch, M. Moshinsk, .
    516-0
    Abstract:
    There is an increasing interest in the question of why typical quantum mechanical properties, such as those connected with the superposition of states or diffraction patterns for material systems, are not observed on the classical macroscopic level. By discussing two simple model problems connected via the free-particle propagator, we show under what circumstances typical quantum effects that show up in these systems can attain significant magnitudes so as to have a chance to be observable. The influence of the interaction with a dissipative environment will also be considered, and the time scale where the effects reach their maximum and how they decay afterwards will be discussed. Furthermore, a comparison with recent scattering experiments will be given.
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  • Determination of the point and space groups for hydroxyapatite by computer simulation of CBED electron diffraction patterns

    E. Sánchez-Pastenes, J. Re, es-Gasga.
    525-0
    Abstract:
    The structure of natural hydroxyapatite $n$HAP (i.e. the hydroxyapatite found in teeth and bones) has not been completely characterized experimentally until now. This involves the study of the structural characteristics of synthetic hydroxyapatite sHAP (i.e. one whose stoichiometric formula is Ca$_{10}$ (PO$_{4})_{6}$ (OH)$_{2})$ using many techniques, in particular electron diffraction computer simulation. Thus, any variation presented in its structure will be easily detected. In this work we comment on the crystallographic elements presented in simulated convergent beam electron diffraction (CBED) patterns for $s$HAP, in its versions of Laue Zones of Zero Order (ZOLZ), First Order (FOLZ), Second Order (SOLZ) and Higher Order (HOLZ), and the deduction of its space group P6$_{3}$/m. These results are compared with those reported experimentally.
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  • Diseño de sistemas ópticos afocales con microlentes de silicio

    A.G. Rojas H., F. Renero C., W. Calleja, R. Pérez R.
    530-0
    Abstract:
    Design of afocal systems, for $1.5 \mu $m wavelength, are presented. They are based on the Galilean Telescope, composed of a silicon negative microlens ($305~\mu$m diameter) and a positive lens chosen between 10 different materials, according to the best optical performance. The spherical aberration and the Strehl ratio are used to compute the best optical performance. It is shown that the afocal system, that uses the ZnSe as the positive lens, has the best optical performance.
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  • Síntesis y caracterización de materiales moleculares de ftalocianinas metálicas en el módulo electroquímico del microscopio de fuerza atómica

    M.E. Sánchez Vergara, M.A. Ruíz Farfán, A. Ortiz, M. Rivera, C. Alvarez Toledano
    535-0
    Abstract:
    Recently, molecular materials have been developed because of their electrical properties which give conductor, semiconductor, or superconductor behavior. This study is focused on the Molecular Materials preparation by means of two different electrosynthesis techniques: electrocrystallization in conventional cells, and electrosynthesis in the electrochemical module of an Atomic Force Microscope (AFM). The latter represents an alternative method in which is possible to monitor and observe the new material as it is synthesized. Moreover, this second method presents huge advantages concerning cyclic voltammetry studies, time of electrosynthesis and preparation of the Molecular Material films for their later characterization and electrical properties evaluation. From Lead (II) phthalocyanine and 1,8 dihidroxianthraquinone compounds, new materials were synthesized by both methods. The characterization of the materials was accomplished using several analysis techniques such as IR, UV-Vis, SEM and EDS Spectroscopy. Finally, electrical conductivity was evaluated, suggesting that the materials can be classified as semiconductor molecular materials.
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  • Diseño y construcción de un magnetizador de pulsos

    M. Mirabal García, F.J. García González, E. Calderón Avalos, G.G. López Rocha, J.L. Sánchez Llamazares
    542-0
    Abstract:
    In the present paper we present the design and construction of a pulsed field magnetizer that has been set up at the IF-UASLP. The system is based on the discharge of the energy stored in a capacitor bank (C$_{total}$ = 23.1 mF; U = $1/2 $CV$^{2 } \approx $2300 J ) into a resistive double-layer copper solenoid supported by reinforcing glass fibers. This magnetizer is capable of generating pulsed magnetic field strengths up to 9.0 T, in a cylindrical volume of 17 mm diameter at the center of the solenoid, with a pulse duration of about 2.0 ms. The principle of operation of the magnetizer as well as the design of the field solenoid are described. A pick-up coil system combined with an electronic integrator is also available in the apparatus to monitor the shape of the field pulse, peak field, and pulse duration.
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