Vol. 49 No. 1 (2003): Revista Mexicana de Física.

Published: 2003-01-01

Articles

  • Electronic band structure of (001)--semiconductor surfaces: the frontier--induced semi--infinite--medium states.

    D. Olguín, R. Baquero
    1-0
    Abstract:
    In previous work we have discussed the valence band electronic structure of the (001) oriented semi--infinite medium of the II--VI wide band gap zinc--blende semiconductor compounds. We have found three characteristic surface resonances besides the known bulk bands (heavy hole, light hole and spin--orbit bands). Two of these resonances correspond to the anion--terminated surface and the third one to the cation terminated surface. Furthermore, we have shown that three non dispersive (001)--surface--induced bulk states, in the $\Gamma-K$ direction of the 2D Brillouin zone, are also characteristic of these systems. To identify these states, from other known surface states, we have called them frontier--induced semi--infinite--medium states. In order to continue with the description of these systems, we review the main characteristics of the electronic structure of the (001)--surfaces and we present a detailed theoretical discussion of the frontier--induced semi--infinite--medium states. We use tight binding Hamiltonians and the well--known Surface Green's Function Matching method to calculate the electronic surface band structure.
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  • Elastic cross sections for intermediate-energy e$^-$ - CH$_4$ collisions

    J.L.S. Lino
    6-0
    Abstract:
    We report an application of the Schwinger variational principle with plane waves as trial basis set. Elastic differential cross sections (DCS) for the scattering of electrons by CH$_4$ in the 10 to 100~eV energy range are available and our differential cross sections are found to be in reasonable agreement with existing measurements.
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  • Estudio estructural de los semiconductores AlP, GaAs y AlAs con estructura wurzita

    A. Bautista-Hernández, L. Pérez-Arrieta, U. Pal, J.F. Rivas-Silva
    9-0
    Abstract:
    In this work we present ab initio calculations of optimization geometries, lattice constant and electronic structure for semiconductors wurtzite-type, like AlN, CdS, ZnS, ZnSe, GaN and GaAs. For this, we used the CASTEP program of CERUIS with LDA and GGA approximations, in the framework of Functional Density Theory. The used pseudopotentials are available in that program and were generated using the optimization scheme of Troullier-Martins. With the lattice constant just optimized, we calculate then the X-ray spectra for studied semiconductors. We analyzed the effect of used pseudopotentials on function of the results obtained. Finally, we predicted the geometry and X-ray pattern for AlP, AlAs and GaAs with wurtzite structure, giving evidence about the semiconductor character of this materials.
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  • Grain growth in polycrystalline Ba$\mathbf{_{0,5}}$Sr$\mathbf{_{0,5}}$TiO$\mathbf{_{3}}$ ceramics prepared at different sintering times

    S. García, J. Portelles, F. Martínez, R. Font, J.R. Quiñones, J.M. Siqueiros
    15-0
    Abstract:
    A grain size study based on the measurements of the maximum permittivity and the microstructure of samples of Sr$_{0,5}$Ba$_{0,5}$TiO$_{3}$ sintered at different time lengths at a fixed temperature is reported. A linear law of grain growth is found from the experimental results and properly described by the Johnson-Mehl-Avrami theory. The model predicts a zero nucleation rate.
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  • Efecto de la quiralidad sobre solitones polarizados en un medio anisótropo

    M. Zamorano L., H. Torres-Silva
    20-0
    Abstract:
    Simulation of nonlinear Schrödinger equation is presented, for a dispersive chiral media, with Kerr-type non linearity. Spatial chirality effects is considered through the Born-Fedorov formalism. Results obtained, by means of Fourier method, show the chirality effects over circular and mixed polarized solitons pulses in an anisotropic cubic media. Numerical simulation results show that right hand circulary polarized solitons are more stable that left hand polarized, when the chirality factor increases. For this later case, a mode conversion is observed for N=1 to N=2 if the chirality factor has an adequate value. For coupled modes the chirality factor allows the existence of elliptically polarized modes together with fundamental modes.
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  • Remarks on noncommutative solitons

    H. García-Compeán, J. Moreno
    28-0
    Abstract:
    In the first part of this work we consider an unstable non-BPS $Dp-\bar{Dp}-$brane pair in Type II superstring theory. Turning on a background NS-NS $B-$field (constant and nonzero along two spatial directions), we show that the tachyon responsible for the unstability has a complex GMS solitonic solution, which is interpreted as the low energy remnant of the resulting $D(p-2)-$brane. In the second part, we apply these results to construct the noncommutative soliton analogous of Witten's superconducting string. This is done by considering the complex GMS soliton arising from the $D3-\bar{D3}-$brane annihilation in Type IIB superstring theory. In the presence of left-handed fermions, we apply the Weyl-Wigner-Moyal correspondence and the bosonization technique to show that this object behaves like a superconducting wire.
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  • Vane rheometry of an aqueous solution of worm-like micelles

    V.H. Rolón-Garrido, J. Pérez-González, L.A. Vega Acosta Montalban
    40-0
    Abstract:
    The non-linear viscoelastic behavior of worm-like micelles in aqueous solution was analyzed in this work under non-homogeneous flow with a rotational vane rheometer. The system studied was a solution of cetylpyridinium chloride 100 mM/sodium salicylate 60 mM (CPyCl/NaSal) in triple distilled water. The use of the vane rheometer enables to reach higher shear rates than a cone and plate or Couette one because ejection of the sample and slip at the rotating surface are restricted. Thus, following this experimental procedure we show in this work, the complete flow curve for a micellar solution, including a hysteresis cycle and the upturn at high shear rates predicted by theories on constitutive instabilities and shear banded flow. We show in addition, the influence of time on the onset of flow instabilities and an experimental analysis of the flow stability in the low and high shear rate branches.
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  • Wigner functions of free ``Schrödinger cat'' states

    E. Colavita, S. Hac, an.
    45-0
    Abstract:
    We study the evolution of a free ``Schrödinger cat'' state (a superposition of two coherent correlated states moving in opposite directions) using the formalism of the Wigner function. Two possible mechanisms to spatially separate the two states are considered: a ``quantum sling'' and a Paul trap that produces unstable motion. The numerical analysis shows how the two superposed states move away of each other, while keeping an interference term between them that is typical of quantum phenomena.
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  • Recurrence relations of special functions and group representations

    G.F. Torres del Castillo
    53-0
    Abstract:
    It is shown that the recurrence relations satisfied by several special functions can be related to representations of Lie algebras of dimension three or four. It is also shown that in some cases these recurrence relations can be related to the isometries of constant-curvature two-dimensional manifolds.
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  • ?`Ayuda la teoría de errores a diseñar experimentos?

    J.L. Haza
    57-0
    Abstract:
    The theory of errors, applied to physical measurements, is one of the useful tools to aid scientists to solve the reality-model contradiction during the realization of experiments as part of the constant process of production of new knowledge. The school as a simulated process of knowledge construction must use this tool, in the most adequate conditions, for the design of properly conceived experiments in the classroom, which would favor the conviction in the student of the veracity of the theoretical constructions. In school practice, the ``theory of errors'' applied to experiments in physics, usually constitutes formal practice and is reduced to a long series of calculations to finally obtain a number that would go with the calculated value of the magnitude to express the result. The present article offers an analysis of the criterion of quality in the physical measurements, derived from relative error, taking as example the experimental task that consists in the determination of the value of acceleration of gravity using the Atwood machine. This would influence the correct design of the experiment and determine the relative value interval. It would also influence the correct design of the experiment and determine the interval of values that magnitudes should take in the experience, in order to guarantee its informative force.
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  • La obra de Galileo y la conformación del experimento en la física

    J.L. Álvarez G., Y. Posadas V.
    62-0
    Abstract:
    It is very frequent to find comments and references to Galileo's work suggesting that he based his affirmations on a logic thought and not on observations. In this paper we present an analysis of some experiments that he realized and were unknown in the XVI and XVII centuries; in they we find a clear description of the methodology that Galileo follows in order to reach the results that he presents in his formal work, particularly in Discorsi. In contrast with the Aristotelian philosophy, in these manuscripts Galileo adopt a methodology with which he obtain great contributions for the modern conformation of the experimental method, founding so a methodology for the study of the movement. We use this analysis as an example of the difficulties that are present in the conformation of the modern experimentation and we point out the necessity to stress the importance of the scientific methodology in the teaching of physics.
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  • Modelo simple para la permeabilidad dinámica de fluidos viscoelásticos.

    J.A. del R{\'\i}o, J. R
    75-0
    Abstract:
    In the present work a simple form to obtain analytical expression for the dynamic permeability of Maxwellian fluids is presented. This expression gives the frequency dependent form of this dynamic permeability. In particular case, the analytic expression for the sinusoidal pressure pump fluid is illustrated in the configuration space. As an example of the feasibility of this expression the flow of human blood in a tube is presented finding that the human heart frequency has the same order that the frequencies where the dynamic permeability shows resonances. In order to make clear the above aspect of the dynamic permeability a model of pulsing pressure drops (gaussian like) are analyzed.
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  • Efficiency of a Curzon and Ahlborn engine with Dulong-Petit heat transfer law

    D. Ladino-Luna
    87-0
    Abstract:
    Using the maximization of the power output per cycle, the optimization of a thermal engine performing a Carnot-type cycle is considered. It is assumed that the heat transfer between the reservoirs and the engine occurs according to the Dulong and Petit's heat transfer law. It is found that the efficiency obtained with this heat transfer law can be written as a power series in the parameter $\lambda\sim 1/(\ln{V_{max}}-\ln{V_{min}})$, where $V_{max}$ and $V_{min}$ are the maximum volume and minimum volume spanned by the cycle, respectively. It is also shown that the calculated efficiency verifies the semi-sum property of the ecological efficiency.
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  • Two--photon absorption

    I. Pérez-Arjona, G.J. de Valcárcel, Eugenio Roldán.
    92-0
    Abstract:
    Two--photon absorption is theoretically analyzed within the semiclassical formalism of radiation--matter interaction. We consider an ensemble of inhomogeneously broadened three--level atoms subjected to the action of two counterpropagating fields of the same frequency. By concentrating in the limit of large detuning in one--photon transitions, we solve perturbatively the Bloch equations in a non-usual way. In this way we derive an analytical expression for the width of the two--photon resonance that makes evident sub-Doppler two--photon spectroscopy. We also derive an analytical expression for the Stark shift of the two--photon resonance.
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