Vol. 62 No. 6 Nov-Dec (2016): Revista Mexicana de Física

Published: 2018-06-29

Articles

  • Flow between two coaxial cylinders simulated by multiparticle collision dynamics

    A. Ayala-Hernandez, H. Híjar
    500-0
    Abstract:
    We carry out simulations of multiparticle collision dynamics fluids confined in the region between two coaxial cylinders. In order to confine these fluids in this geometry, we apply the method based on explicit fluid-solid forces introduced in a recent publication~\cite{ayala001}. We consider two independent situations in which, first, flow is produced by a uniform pressure gradient that points along the axial direction and, second, azimuthal flow is induced by a centrifugal force field. We generalize the methodology introduced in Ref.~\cite{ayala001} and show, through a large number of numerical experiments, that the correct flow expected from hydrodynamics can be obtained in the two previous cases, with conditions at the fluid-solid interaction regions that can be adjusted from partial-slip to no-slip boundary conditions.
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  • A computational model of an Einstein-Solid model to study gas sorption in solid surfaces: effects on the solid wall structure

    M. Lara-Peña, H. Domínguez
    510-0
    Abstract:
    Sorption of carbon dioxide (CO$_2$) on solid surfaces is studied by a Reactive Monte Carlo (RxMC) method. A simple model of the\linebreak A+B $\rightleftharpoons$ C reaction is used to mimic the experimental reactions CO$_2$ + Li$_2$O $\rightleftharpoons$ Li$_2$CO$_3$. Two different solid surfaces were constructed to study sorption of the gas, a face centered cubic (FCC) and a disordered walls. In each case the solids were composed of particles with two different models, the first one consisted of rigid particles and the second model considered particles which were allowed to vibrate inside the solid with a given spring constant, i.e. a solid of Einstein. Density profiles analysis showed that not only physisorption but also chemisorption was observed. Comparisons of gas absorption of the two walls with a cubic simple solid structure, reported previously, were carried out and it was observed that the disordered and the cubic simple walls present similar values. However, the FCC walls produced higher absorption than the others, in particular at low temperatures.
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  • Hidden features in (Bi,Pb)-2223 granular superconductors

    M. Hernández-Wolpez, E. Martínez-Guerra, R.F. Jardim, P. Muné
    515-0
    Abstract:
    The aim of this review is to give a summary of some hidden features related to penetration, trapping, and relaxation of the magnetic flux in (Bi,Pb)-2223 granular superconductors. The discussion is based on experimental and theoretical results obtained in samples prepared under different conditions and by different research groups. Fundamental concepts such as granularity and grain boundaries (GB), superconducting levels, types of vortices, and microstructural characteristics of (Bi,Pb)-Sr-Ca-Cu-O superconductors are invoked for supporting the discussion. As conclusions we focus on the main insufficiencies in the treatment and understanding of the phenomena mentioned above. We also propose some experiments for improving the understanding of the general physical features of granular superconductors.
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  • Calculation of the lattice energy and the energy gap of the magnetic semiconductor MnGa$_2$Se$_4$ using Hartree-Fock and density functional theory methods

    L. Romero, J. Pacheco, R. Cadenas
    526-0
    Abstract:
    The calculation of physics parameters of semiconductor compounds is of great interest in order to evaluate their potential use in a wide class of optoelectronics devices. Different methods have been developed for these calculations, based on the Hartree-Fock Hamiltonian and on the Density Functional ones. Here, a theoretical study by using Crystal 06 software and ab initio Density Functional Theory and Hartree-Focks method with different basis functions was carried out in order to calculate the lattice energy and the energy gap of the MnGa$_2$Se$_4$ magnetic semiconductor compound. It is shown that it is possible to calculate physics parameters of MnGa$_2$Se$_4$ with the mentioned software but due to the multiple values reported in literature for the same parameter, it is not possible to determinate which of the bases used is more effective.
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  • Nanocrystals of Ni doped ZnO semiconductor by Sol-Gel combustion method

    A. Carrero, V. Sagredo, J. Larionova
    530-0
    Abstract:
    Nanoparticles of the system Zn$_{0.95}$Ni$_{0.05}$O were prepared by sol gel self- combustion method and a study of their structural, optical and magnetic properties were conducted. X-ray diffraction study shows a hexagonal wurtzite structure for the nanocompound. The formation of the wurtzite structure in Ni doped ZnO was further confirmed by Fourier transform infra-red spectrometry. Transmission electron microscopy revealed an average size of 31 nm for the particles. Optical absorption spectra shows that the band energy of Zn$_{0.95}$Ni$_{0.05}$O powders is about 2.54 eV at room temperature. A study of the magnetic properties of the nanopowders of ZnO: Ni, reveals paramagnetic behavior, with interaction ferromagnetic between particles.
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  • Solutions of the Schrödinger equation given by solutions of the Hamilton--Jacobi equation

    G.F. Torres del Castillo, C. Sosa Sánchez
    534-0
    Abstract:
    We find the form of the potential depending on the coordinates and the time such that a solution, $S$, of the Hamilton--Jacobi equation yields an exact solution, $\exp ({\rm i} S/\hbar)$, of the corresponding Schrödinger equation.
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  • Comparison of variational solutions of the Thomas-Fermi model in terms of the ionization energy

    D. Sierra Porta, M. Chirinos, M.J. Stock
    538-0
    Abstract:
    A modified trial solution of Wu's function depending on three parameters and satisfying the boundary conditions, is used as a solution to the Thomas-Fermi equation using variational methods. The found solution is used to calculate the total ionization energy of atoms, taking into account a correction to this equation. Our results show that the proposed generalization is much better than Wu's one, being in agreement with other solutions found in literature. Our solution, reproduces conveniently the corresponding numerical solution.
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  • Influence of ferroelectric layer on artificial multiferroic LSMO/BTO bilayers deposited by DC and RF sputtering

    J.E. Ordoñez, M.E. Gómez, W. Lopera
    543-0
    Abstract:
    La$_{2/3}$Sr$_{1/3}$MnO$_3$ (LSMO)/BaTiO$_3$ (BTO) bilayers were deposited on (001) SrTiO$_3$ substrates via DC and RF sputtering at pure oxygen atmosphere at a substrate temperature of 830$^{\circ}$C. We studied the structural, electrical and magnetic properties on LSMO/BTO bilayers, when LSMO thickness is fixed at 40 nm and BTO thickness is varied from 20 to 100 nm. Reciprocal Space Maps in LSMO show a strained growth for all samples, while BTO layers are always relaxed. Magnetization and electrical measurements indicate the influence of the ferroelectric layer, due to saturation magnetization increases from 500 to 590 emu/cm$^3$ and coercive field decreases from 178 to 82 Oe with BTO thickness. Mean Field mechanism is identified on all samples with critical exponent $ \beta $ between 0.42 and 0.54. Resistivity measurements show electron-electron and magnon- magnon scattering conduction mechanisms. The influence on magnetic and electrical properties of bilayers with BTO thickness is attributed to crystallographic strains at the interface and the corresponding relaxation with increasing BTO layer thickness.
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  • Frequency response of the nonclassicality and its correspondence to the classical dynamics

    Shihui Zhang., Yingmin Cui.
    549-0
    Abstract:
    The time evolution of the nonclassicality is found to have strong dependence on the driving frequency in a Duffing-type driven nonlinear system with the Gaussian state as the initial Wigner function. Its frequency response curve has a remarkable peak, near which the nonclassicality of the state is enhanced. Furthermore, good correspondences are found between the frequency response curves of the time-average of the nonclassicality indicator and those of the classical mean square amplitude. Such correspondences can also be observed, when using a superposition of two coherent states as the initial Wigner function.
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  • Synthesis of self-assembled Ge nanocrystals employing reactive RF sputtering

    A. Hernández-Hernández, L.A. Hernández-Hernández, B. Marel Monroy, J. Santoyo-Salazar, G. Santana-Rodríguez, A. Márquez-Herrera, S. Gallardo-Hernández, P.G. Mani-González, M. Meléndez-Lira
    558-0
    Abstract:
    This work presents the results of a simple methodology able to control crystal size, dispersion and spatial distribution of germanium nanocrystals (Ge-NCs). It takes advantage of a self-assembled process taken place during the deposit of the system SiO$_2$/Ge/SiO$_2$ by reactive RF sputtering. Nanoparticles formation is controlled mainly by the roughness of the first SiO$_2$ layer but the ulterior interaction of the interlayer with the top layer also play a role. Structural quality of germanium nanocrystals increases with roughness and the interlayer thickness. The tetragonal phase of germanium is produced and its crystallographic quality improves with interlayer thickness and oxygen partial pressure. Room temperature photoluminescence emission without a post growth thermal annealing process indicates that our methodology produces a low density of non-radiative traps.
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  • Experimental results: spectrum output analysis of an EDFA with yterrbium-oped fiber using a superluminescent diode source

    M.A. Casas-Ramos, G.E. Sandoval-Romero, V. Argueta-Diaz, E.F. Pinzon-Escobar
    565-0
    Abstract:
    In this work we present the experimental results of the output spectra of an erbium-doped fiber amplifier when it's coupled with an ytterbium-doped fiber. The use of a superluminescent source in the proposed cascade configuration provides an improvement on 61% on SNR, moreover, in the output signal causes a decreases in spectral width, and filters the pumping signal.
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  • Entropy geometric construction of a pure substance with normal, superfluid and supersolid phases

    M. Mendoza-López, V. Romero-Rochín
    586-0
    Abstract:
    Using the laws of thermodynamics together with empirical data, we present a qualitative geometric construction of the fundamental relation of a pure substance $S = S(E,N,V)$, with $S$ entropy, $E$ energy, $N$ number of particles and $V$ volume. We analyze two very general type of substances, a ``normal" and a ``quantum" one, the main difference between them being that the latter presents superfluid phases. It is found that the constant entropy level curves are completely different in both cases, in the normal substances being obtuse while acute in quantum ones. A concomitant signature of the previous result is that the chemical potential can be both positive and negative in quantum substances, but only negative in normal ones. Our results suggest the existence of a region in the quantum substances that may be identified as a supersolid phase. We also make emphasis on the relevance of the present study within the context of superfluidity in ultracold gases.
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  • 2D naked singularity in general relativity

    J.M. García-Islas
    570-0
    Abstract:
    In the physical sense it is very helpful to understand in simple terms what naked singularities are and the properties they may have. We present a novel example of a 2-dimensional space-time naked singularity. The solution has a gravity singularity and no-horizon. This example is only a toy model and as such its motivation must be considered as mathematical only.
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  • Effects of the velocity slip on a viscous dissipation of mhd flow and heat transfer over a thin liquid film on an unsteady stretching sheet

    B. Alkahtani, M. Subhas Abel, E.H.~Aly.
    576-0
    Abstract:
    This study deals with the numerical solution of MHD flow and heat transfer to a laminar liquid film from a horizontal stretching surface. Similarity transformations were used to convert unsteady boundary layer equations to a system of non--linear ordinary differential equations. The resulting non--linear differential equations were numerically solved, using efficient shooting technique with fourth order Runge--Kutta method. % The effect of Prandtl number, Eckert number, magnetic parameter and heat source/sink parameter and the momentum slip parameter on various flow and heat transfer characteristics, were graphically shown. % It was found that, for high values of unsteadiness parameter, it reduces the surface temperature which is well in agreement with the earlier published works under some limiting cases. In addition, heat absorption is one better suited for effective cooling of the sheet. Further, it was noticed that heat generation enhance the temperature in the boundary layer.
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  • Scaling of plane-wave born cross sections for positron-impact excitations of H$_2$

    J.L.S. Lino
    596-0
    Abstract:
    We report an application of the scaling method to plane-wave Born cross sections (Y.K. Kim, Phys. Rev. A 64 (2001) 032713) using positron-impact excitation of molecules. Cross sections are obtained for positron-H$_2$ ($X^1 \Sigma_{g}^{+}$ \raisebox{-.3ex}[0cm][0cm]{-}$> B^1 \Sigma_u^+$, $ C^1 \Pi_u $) and the results are found to be in reasonable agreement with results experimental.
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  • Caracterización de capacitores MOS basados en peliculas de oxido de hafnio obtenidas a 150$^{\circ}$C

    S. Cerón, J. Hernandez, M.A. Dominguez
    600-0
    Abstract:
    The fabrication and characterization of MOS capacitors with Hafnium Oxide as dielectric obtained at 150$^\circ$C is described. The Hafnium Oxide thin films were obtained by spin-coating. The MOS capacitors were characterized employing measurements of capacitance vs. voltage, capacitance vs. frequency and current vs. voltage. The results demonstrate the feasibility of Hafnium Oxide film as a dielectric in electronic devices.
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