Vol. 52 No. 2 (2006): Revista Mexicana de Física.

Published: 2006-01-01

Articles

  • Diffusion of fluids in disordered pore matrices

    Abstract:
    Diffusion of different fluids was studied in disordered pore matrices by computer simulations. The matrices were formed using zeolite parameters with two distinct methods. In the first method, the matrix was prepared from a set of particles with a fixed density and, after equilibration, the matrix structure was taken from the last configuration of the particles. In the second method, the porous matrix was prepared from a binary mixture where the first and the second components are particles of a template material and the matrix, respectively. The final matrix structure was obtained by removing the template particles from the equilibrated mixture. Therefore we were able to investigate the behavior of the diffusion of Ar, Kr, and $\rm CO_{2}$ fluids as a function of the matrix preparation method. The results show that, when matrices were prepared without a template, the diffusion is higher than in matrices prepared with a template. This can be explained in terms of the cavities formed in the matrices when they were prepared.
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    J.E. Alarcon, H. Dominguez
    93-0

  • Gelatina dicromatada modificada para incrementar su resistencia a la humedad

    Abstract:
    It is presented a comparison between the diffraction efficiency in holographic phase gratings induced in films prepared with two different mixtures based on dichromated gelatin. The traditional mixture (DCG) is modified incorporating glycerol (DCG-G) before deposition. Both kinds of films were dried under different humidity conditions. It is seen that optical recording on DCG-G films is independent on the moisture settings. Also, the induced refractive index modulation is larger than the one obtained with DGC.
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    V. Romero-Arellano, C. Solano, G. Martínez-Ponce
    99-0

  • Crack effects on the propagation of elastic waves in structural elements

    Abstract:
    Behavior of the Dynamic Stress Intensity Factor (DSIF) in cracked plates and tubular elements is analyzed. The finite Element Method was used to validate the procedure used by Chen to determine DSIF in a centrally cracked plate loaded with a step function. Once the validation is done, length and orientation of the crack are varied to determine their effect on wave propagation and DSIF values. To expand the study, the analysis is also applied to cracked tubular elements. In all cases, DSIF variation is interpreted as a function of the stresses produced by the interaction of the elastic waves with the boundaries of the structural element studied. Dependence of DSIF values on dilatational, transversal and Rayleigh waves is seen. These elastic waves and their interaction with the structural element boundaries and crack surfaces determine load and unload cycles at the crack tip affecting the stress field and DSIF values.
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    A. Rodríguez-Castellanos, J.E. Rodríguez-Sánchez, J. Núñez-Farfán, R.E. Olivera-Villaseñor
    104-0

  • Three-dimensional numerical simulation of the natural convection in an open tilted cubic cavity

    Abstract:
    In this work the numerical results of the heat transfer by natural convection in a tilted open cubic cavity are presented. The most important assumptions in the mathematical formulation are two: the flow is laminar, and the Boussinesq approximation is valid. The conservation equations in primitive variables are solved using the finite volume method and the SIMPLEC algorithm. The advective terms are approximated by the SMART scheme, and the diffusive terms are approximated using the central differencing scheme. The results in the steady state are obtained for a Rayleigh range from 10$^{4}$ to 10$^{7}$ and for a range of 0-180$^{\circ}$ for the inclination angles of the cavity. The results show that for high Rayleigh numbers, the Nusselt number changes substantially with the inclination angle of the cavity. The numerical model predicted Nusselt number oscillations for low angles and high Rayleigh numbers.
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    J.F. Hinojosa, G. Alvarez, C.A. Estrada
    111-0

  • Guided-wave modes in graded-index optical fibers by two-point quasi-rational approximants

    Abstract:
    Approximated analytic solutions for wave propagation in graded-index optical fiber have been found in the case of a parabolic profile. Approximants with high accuracy are presented that are much better than those found by other authors. A two-point quasi-rational method and two-point Padé approximants are used in this work. The approximants are explicitly determined for the azimuthal eigenvalues $ l = 0, 1, 2$ and the mth mode numbers $ m = 0, 1, 2, 3$.
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    E. Castro, P. Martín, J. Puerta, C. Cereceda
    120-0

  • A numerical study of stiffness effects on some high order splitting methods

    Abstract:
    In this paper we compare operator splitting methods of first, second, third and fourth orders that are applied to problems with stiff matrices. In order to efficiently solve the resultant subproblems is necessary to use implicit Runge-Kutta methods. It is known that, in this context, the precision order of operator splitting schemes is reduced. Furthermore, we propose a fifth order operator splitting method that is obtained by applying Richardson extrapolation to a fourth order method. All methods are tested with a model problem with matrices such that its condition number is taken up to 20,000. Our conclusion is that order reduction is more severe for low order operator splitting methods.
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    J. Salcedo-Ruíz, F.J. Sánchez-Bernabe
    129-0

  • Photochromic properties of azo dye polymer thin films: evidence of an additional reversible mechanism of molecular behaviour

    Abstract:
    Photoinduced refractive index and absorption anisotropies in thin films of an azo dye polymer have been carried out by pump-probe experiments. A refractive index change of $\Delta $n=-0.04 was obtained by independently measuring the two indices, parallel and perpendicular to pump light polarization. A long-term stability of the induced birefringence and a reversible photoinduced memory effect have been observed. The experimental results cannot be completely explained by the proposed photoisomerization models to describe the process involved.
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    A. Rodríguez, G. Vitrant, P.A. Chollet, F. Kajzar
    135-0

  • An unphysical result for the Landau-Lifshitz equation of motion for a charged particle

    Abstract:
    An unphysical result for the Landau-Lifshitz equation of motion for a charged particle is presented. The similarity with the Lorentz-Dirac equation is discussed. Indeed the reaction force obtained for the uniform electric field vanishes when the motion is parallel to it in both cases. A discussion of this unphysical result is given and the need for of an expression for the radiation rate of energy for the Landau-Lifshitz theory is emphasized.
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    G. Ares de Parga, R. Mares, S. Domínguez
    139-0

  • Density structure in low-order harmonic emission from laser-plasma interactions

    Abstract:
    Multiple harmonic generation from intense laser light incident on dense plasma targets is well-established. Both the number of harmonics generated and the corresponding power in each is strongly dependent on the intensity of the incident light. In this paper, attention is focused on an aspect of harmonic generation that has hitherto been largely overlooked, namely its dependence on plasma density. In particular we show that the conversion efficiencies of odd harmonics generated for normally incident light are strongly resonant in character, the number of resonances increasing with harmonic order. In addition to this resonant behaviour, the reflected power in low-order harmonics is further complicated by plasma emission at plasma resonant densities. Only at densities well above the resonances does the radiated harmonic power decay monotonically with density. The density dependence predicted by theory for low-order harmonics is confirmed by particle-in-cell simulations.
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    T.J.M. Bo, d., R. Ondarza-Rovira
    143-0

  • Cu halide nanoparticle formation by diffusion of copper in alkali halide crystals

    Abstract:
    Copper atoms have been introduced by diffusion in NaCl, KCl and KBr crystals at 500$^\circ$C. The crystals have been optically analyzed with photoluminescence measurements and by scanning electron microscopy. The emission and excitation spectra measured at low temperatures show the exciton confinement effect, indicating the formation of CuX (X=Cl, Br) nanoparticles, which has been confirmed by electron microscopy images. This is proposed as an alternative method to obtain CuX nanoparticles in alkali halides crystals.
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    A. Pérez-Rodríguez, M. Flores-Acosta, R. Rodríguez-Mijangos, R. Pérez-Salas
    151-0

  • Wavelet analysis of chaotic time series

    Abstract:
    In this work we analyzed experimental chaotic time series data from three known chaotic systems using the orthogonal wavelet transform. The experimental electronic implementation of the chaotic systems was used to analyze them. The wavelet analysis of the experimental chaotic time series, with a simple statistical approach, gives us useful information of such systems through the energy concentration at specific wavelet levels.
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    J.S. Murguía, E. Campos-Cantón
    155-0

  • Force constants and dispersion relations for the zincblende and diamond structures revisited

    Abstract:
    The bulk atomic equations of motion are revisited in order to show explicitly, for high symmetry directions, the transformation of this three-dimensional problem into decoupled one-dimensional problems. The force constants of the corresponding one-dimensional equations are related to a larger number of force constants of the bulk problem. We illustrate how the three-dimensional force constants (and consequently the whole dynamical matrix) can be estimated from a few either experimental or theoretical points for semiconductors in the zincblende and diamond structures.
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    D.G. Santiago-Pérez, F. de León-Pérez, R. Pérez-Álvarez
    163-0

  • Bridge bond between potential energy surfaces and fuel cells on Pt-H$_{2}$ interaction

    Abstract:
    Reaction mechanisms between catalyst (Pt) and one of the fuels (H$_{2}$) are found by means of DFT-B3LYP calculation, which represents a bridge bond between the theoretical calculations and the processes of fuel transformation in the fuel cells. All the closed shell energies involved in the process are obtained, starting when the platinum and the hydrogen are completely separated and ending until the hydrogen atoms separate and stay bound to the platinum, forming a new product. The equilibrium geometry of the latter system is reached with a broken H-H bond at an HPtH angle of about 90$^\circ$. The latter was obtained using the DFT-B3LYP method. The confidence in all of these results is based on the fact that the potential energy well of the Pt-H$_{2}$ interaction calculated using the DFT-B3LYP method is located between those calculated using Moller-Plesset and PSHONDO-IJKL-FOCK-CIPSI ab initio methods.
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    J.H. Pacheco, I.P. Zaragoza, L.A. García, A. Bravo, S. Castillo, O. Novaro
    172-0

  • El ión molecular exótico $H_4^{3+}$ en un campo magnético intenso

    Abstract:
    Using the variational method, a detailed study of the lowest $m=0,-1$ electronic states of the exotic molecular ión $H_4^{3+}$ in a strong magnetic field, in the linear symmetric configuration parallel to the direction of the magnetic field is carried out. A extended study of the $1\sigma_g$ ground state (J.C. López and A.Turbiner, Phys. Rev. \bf A 62, 022510, 2000) was performed obtaining that the potential energy curve displays a sufficiently deep minimum for finite internuclear distances, indicating the possible existence of the molecular ión $H_4^{3+}$, for magnetic fields of strength $B\gtrsim 3\times10^{13}$\,G. It is demonstrated that the excited state $1\pi_u$ can exist for a magnetic field $B=4\textrm{.}414\times10^{13}$\,G corresponding to the limit of applicability of the non-relativistic theory.
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    H.Olivares Pilón.
    178-0

  • Fuente sonora omni-direccional

    Abstract:
    The design, construction and evaluation of an omni-directional sound source is presented. It was developed in our laboratory for acoustic applications. The evaluation shows that its characteristics fulfill the requirements demanded by international standards.
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    A. Pérez López, E. Palacios González, S.J. Pérez Ruiz
    185-0