Vol. 57 No. 1 (2011): Revista Mexicana de Física.

Published: 2011-01-01

Articles

  • Nano-channels filling flow of arbitrary cross-sections

    M. Pliego, G.J. Gutiérrez, A. Medina
    1-0
    Abstract:
    The filling kinetics of liquids in straight nano-channels of arbitrary cross-sections is discussed. We assume that the capillary force induces a fluid flow in channels characterized by their compactness, $C=P^{2}/A$, where $P$ is the perimeter and $A$ is the cross-sectional area. Analytical expressions for the distance between the capillary meniscus and the capillary inlet, $l$, as a function of the elapsed time, $t$, are given for several typical cross-sections. The comparison between our theoretical predictions and a set of reported data for a complex nano-channel [A. Han, J. Colloid Interface. Sci. 293 (2006) 151] allows us to conclude that the model describes well the filling kinetics.
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  • Diffraction of beams by infinite or finite amplitude-phase gratings

    S. Cruz-Arreola, O. Mata-Mendez
    6-0
    Abstract:
    In this paper a theory for the diffraction of beams by thin amplitude-phase gratings in the scalar diffraction regime is given. The grating can be strictly periodic and therefore of infinite spatial extent (infinite grating) or can be a grating with a finite number of periods (finite grating). The main result of this paper is that we can write down mathematical expressions for the diffraction of beams by these kinds of gratings. General expressions for the diffraction patterns at the far-field region are derived. As a numerical application of the theory presented in this paper the diffraction of Hermite-Gaussian and distorted beams by a Ronchi ruling (infinite and finite) is studied.
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  • Effects of elastic energy on the spinodal decomposition of InNAsP/InP heterostructures

    S.F. Díaz Albarrán, V.A. El, ukhin., P.R. Peralta
    17-0
    Abstract:
    Spinodal decomposition of the $\mbox{InN}_x \mbox{As}_y \mbox{P}_{\mbox{1 - x - y}} $ quaternary alloys lattice-matched to the InP as the result of the internal deformation and elastic energies is described. The alloys are represented as quasiternary regular solutions. The internal deformation energy is obtained from the interaction parameters between the constituent compounds estimated by the valence force field model. Ranges of spinodal decomposition of the $\mbox{InN}_x \mbox{As}_y \mbox{P}_{\mbox{1 - x - y}} $ alloys up to $\mbox{y} \le \mbox{0.109}$ with and without elastic energy are demonstrated. Our results suggest that such energy prevents spinodal decomposition.
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  • Scattering of nucleons from nuclei with couplings to particle-unstable excited states

    P.R. Fraser, K. Amos, L. Canton, S. Karataglidis, J.P. Svenne, D. van der Knijff
    20-0
    Abstract:
    The physics of radioactive ion beams implies the description of weakly-bound nuclear systems. One key aspect concerns the coupling to low-lying collective-type excited states, which for these systems might not be stable levels, but particle emitting resonances. In this work we describe how the scattering cross section and compound spectra change when the colliding fragments have such collective excitations featuring particle emission. We explore this question in the framework of a multi-channel algebraic scattering method of determining nucleon-nucleus cross sections at low energies. For a range of light-mass, particle-unstable nuclear targets, scattering cross sections as well as the spectra of the compound nuclei formed have been determined from calculations that do and do not consider particle emission widths for nuclear states. Assuming a resonance character for target states markedly varies evaluated cross sections from those obtained assuming the target spectrum to have entirely discrete states.
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  • Propiedades electrónicas de la hoja de carburo de germanio tipo grafeno

    E. Chigo Anota, G. Murrieta Hernández
    30-0
    Abstract:
    Through first principles calculations based on the Density Functional Theory (DFT) at the level of LDA (PWC) the electronic properties of un-doped and nitrogen doped germanium carbide sheets (GeC) were studied. The effect of structural vacancies on the electronic properties of the proposed models was investigated. It was found that the optimal geometry of the GeC sheet is planar, same is true for the nitrogen doped model. On the other hand, it was concluded that the presence of a germanium vacancy leads to structural instability of the Ge$_{11}$C$_{12}$N$_{12}$ system. A semiconductor-metal transition was detected when N is incorporated in the Ge$_{12}$C$_{12}$H$_{12}$ system; band gap energies of 2.34 and 0.17 eV were estimated for un-doped an N doped models, respectively. Furthermore, the incorporation of a carbon antisite (N$_{C})$ leads to a strong increase of polarity, changing form ionic to covalent.
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  • Dependencia con la presión del índice de refracción del AgGaS$_{2}$

    Ch. Power, E. Calderón, J. González, J.C. Chervin
    35-0
    Abstract:
    In this work, we study the pressure behavior of the optical absorption spectrum of a single crystal AgGaS$_{2}$, taking measurements in the infrared energy range from 0.30 up to 0.70 eV for pressures values P below 4GPa and room temperature T, using a diamond anvil cell in combination with infrared micro spectroscopy technique [1]. With this study, we determine the refraction index $n$ variations in terms of pressure within the stability range of the chalcopyrite structure [2-6] as well as the changes under pressure of both the static ($\varepsilon _{0})$ and high frequency ($\varepsilon _{\infty })$ dielectric constants. These results can be compared with the experimental values reported by Boyd et al. [7].
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  • Coupled spinors orthonormalization criterion in multiband systems

    A. Mendoza-Alvarez, J.J. Flores-Godo, G. Fernández-Ana, a., L. Diago-Cisneros, .
    40-0
    Abstract:
    Some fundamental physical quantities are determined by solving the eigenvalue problem that comes from a system of N coupled second order linear differential equations. An uncommon scenario evolves from the second order derivatives that appear in most multiband Hamiltonians, which leads to wave function spaces with non orthogonal axes. This notorious property has often been ignored by many authors. In this paper we discuss a possible criterion for the orthonormalization of eigenspinors (N$\times$1) derived from the eigenvalue quadratic problem associated to the differential equation system. Such eigenspinors are taken as the basis on which the propagating wave modes system is built. When the norm of the new space is reformulated, the non-standard character of the weighted internal product comes to the forefront. This scheme has been successfully applied to the study of hole tunneling as it is described by the (4$\times$4) Kohn Lüttinger model.
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  • Energy eigenvalues for free and confined triple-well potentials

    N. Aquino, J. Garza, G. Campo, A. Vela, .
    46-0
    Abstract:
    Some confined and unconfined (free) one--dimensional triple--well potentials are analyzed with two different numerical approaches. Confinement is achieved by enclosing the potential between two impenetrable walls. The unconfined (free) system is recovered as the positions of the walls move to infinity. The numerical solutions of the Schrödinger equation for the symmetric and asymmetric potentials without confinement, are comparable in precision with those obtained anaylitically. For the symmetric triple--well potentials, $V(x)=\alpha x^2-\beta x^4+x^6$, it is found that there are sets of two or three quasi-degenerate eigenvalues depending on the parameters $\alpha$ and $\beta$. A heuristic analysis shows that if the conditions $\alpha=(\beta^2/4) \pm 1$ (with $\alpha >0$ and $\beta>0$) are satisfied, then there are sets of three eigenvalues with similar energy. An interesting behavior is found when one impenetrable wall is fixed and the other is moved to different positions. In summary, the number of local minima that the potential has in the confined region determines a two- or three-fold degeneracy.
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  • Relativistic charged particle in a uniform electromagnetic field

    G.F. Torres del Castillo, C. Sosa Sánchez
    53-0
    Abstract:
    The equations of motion for a relativistic charged particle in a uniform electromagnetic field are solved in a covariant form by calculating the exponential of the matrix corresponding to the electromagnetic field tensor. It is shown that owing to the antisymmetry of the electromagnetic field tensor, the exponential mentioned above can be easily calculated. Some results are then applied to study the algebraic properties of the energy-momentum tensor of the electromagnetic field and the orthogonal transformations in spaces of dimension 4.
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  • On the phenomenology underlying Taylor's hypothesis in atmospheric turbulence

    J.J. Castro, A.A. Carsteanu, J.D. Fuentes
    60-0
    Abstract:
    G.I. Taylor's hypothesis of transposition of turbulent statistics from the spatial to the temporal domain (and vice-versa) is usually explained in terms of smaller features being advected by a large-scale transport velocity, while intrinsic temporal velocity fluctuations are slower than the corresponding inertial terms, and turbulent velocity fluctuations remain small in comparison with the transport velocity. This formulation, widely known as ``frozen turbulence'', is undoubtedly correct in laboratory experiments where the stated conditions are being fulfilled, and perhaps in many natural settings. However, temporal structure functions of measured velocities in the atmospheric boundary layer during periods of higher transport velocities (tropical day time), when compared with periods of low activity (night time), show a very similar behavior, hereby raising the question whether the space--time similarity of turbulent fluctuations in terms of statistical moments is really due only to transport-like advection, or there might exist a different underlying phenomenology leading to the same result, and accounting for the behavior during low-advection periods. Based on the multifractality observed in the structure functions, the alternative explanation of a 4-D space-time multifractal field is suggested.
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  • The hydrogen atom with an origin centred singularity

    N. Aquino
    65-0
    Abstract:
    We study the problem of a hydrogen atom with an infinitely repulsive singularity at the origin, in two cases: in the first, the electron is free to move throughout space, while in the other, the system is confined in a spherical box of impenetrable walls centred at the nucleus. We show that spherically symmetric $s$ states cannot exist in these systems.
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  • Multiphoton ionization of naphthalene clusters at 266 nm

    J.C. Poveda, I. Álvarez, C. Cisneros
    68-0
    Abstract:
    In this work we analyze the photoionization of Naphthalene clusters: dimmer, trimmer and tetramers when radiation of 266 nm from Nd:YAG laser at intensities up to 10$^{9}$ W$ \cdot $cm$^{ - 2}$ was used, in the range from 0.5 to 20.0 mJ per pulse. The clusters were formed by thermal desorption of a sample at 373 K and translational and vibrationally cooled in a jet by adiabatic expansion. At these experimental conditions the ionization processes were accomplished in the multiphoton regime by two photon absorption. A complete identification of fragment ions arising from the clusters was carried out, using a linear time of flight mass analyzer, ToF-MA. Evidence of proton and carbon atoms migration between molecular units in the clusters was observed, and the ions C$_{10}$H$_{8}$-H$^{ + }$ and (C$_{10}$H$_{8})_{2}$-H$^{ + }$ were detected. The analysis of ion yield of clusters as a function of the energy per pulse showed that the dimmer stability is higher than trimmer and tetramer structures. The efficiency of clusters formation, and ionization processes were analyzed as a function of different carrier gases, CG: He, Ne, Ar, Kr and Xe. With He and Ne as CG, the ion yield decreases comparatively when Ar, Kr, Xe or not CG were used.
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  • Design of an optimal control for an autonomous mobile robot

    E.M. Gutiérrez-Arias, J.E. Flores-Mena, M.M. Morin-Castillo, H. Suárez-Ramírez
    75-0
    Abstract:
    In this article, we present an autonomous mobile robot that is provided with two active wheels and passive one, as well as two control algorithms for the stabilization of the programmed paths. The dynamic programming constitute the bases for the determination of both control laws. The first law of optimal control is obtained by solving the Ricatti matricial differential equation. The second is deduced taking into account the work done by Kalman, which makes possible the reduction of a matricial differential equation into an algebraic matricial equation. The simulation of both algorithms is made when the programmed path is a straight line and this makes possible to observe the optimal control law, which represents the principal goal of this paper, and which presents an improved quality for the stabilization that the control law obtained following the work of Kalman.
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  • Evaluando las sincronizaciones sin y con retroalimentación en circuitos de Lorenz

    R. Núñez
    84-0
    Abstract:
    Since the synchronization is essential for sending private information through chaotic encryption and parametric commutation, the paper describes an experimental study, on two synchronization schemes namely: without and with feedback, is performed using as a chaotic source the Lorenz circuit. The idea is to know the most efficient and robust for application in chaotic encryption. Transmitter and receiver circuits are implemented in a very similar way and worked in almost identical conditions of operation. It is considered a linear variation of the Rayleigh parameter to find the errors in the synchronization and the chaotic dynamics alterations of the circuits. To do this, we use phase planes, spectral analysis and coherence. Finally, synchronization with feedback has a higher synchronization error, decreases the chaotic dynamics and encryption capabilities and information signal retrieval are very committed; together requiring a physical connection more. It is less efficient than the synchronization without feedback, although for the latter further experimentation is needed on the behavior regarding to electrical noise, and instabilities of the components to come in real applications of chaotic encryption.
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  • Espectroscopía gamma con LaBr$_{3}$ y LYSO para su aplicación en medicina nuclear

    M. Rodríguez-Villafuerte, H. Alva-Sánchez, T. Murrieta, A. Martínez-Dávalos, .
    91-0
    Abstract:
    Image formation in nuclear medicine is based on the detection of radiation emitted from a small quantity of a radiopharmaceutical administered to patients. This requires high detection efficiency, good spatial resolution and the ability to identify the energy of the incident radiation. These and other properties of detector materials for gamma radiation are reviewed. Standard detector calibration and characterization procedures are described, and the results from gamma spectroscopy measurements for sodium iodide (NaI:Tl), conventional scintillator widely used in nuclear medicine applications, and two other novel scintillation crystals, lanthanum bromide (LaBr$_{3})$ and lutetium-yttrium oxyorthosilicate (LYSO) are presented. Both materials showed a linear energy response from 80 to 1332 keV, and within this energy interval, LaBr$_{3}$ had a superior energy resolution compared to LYSO and NaI:Tl. These and other new materials are expected to replace those used in current detectors for nuclear medicine scanners.
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  • Errata

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