Vol. 57 No. 4 (2011): Revista Mexicana de Física.

Published: 2011-01-01

Articles

  • Revisión de la teoría de perturbaciones en Relatividad General

    Adolfo De Unánue.
    276-0
    Abstract:
    This work presents a review of the different approaches for perturbation theory in General Relativity: the Gauge Invariant formalism, the 1+3 Covariant Gauge Invariant theory and the traditional gauge fixing method. In particular, this review focuses in the Gauge Invariant formalism, due to it has a broader applicability (it applies not only to General Relativity but, to any theory that must fulfill the principle of general covariance) than the other two formalisms and because it has an algorithmic method for calculate the invariant variables to perturbation orders larger than the linear one. The article includes too, a brief discussion about the root of the problem of gauge-invariance in the perturbation theory in General Relativity. To help the reader, this last approach is applied to the cosmological scenario.
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  • Aluminum doping of CdTe polycrystalline films starting from the heterostructure CdTe/Al

    M. Becerril, O. Vigil-Galán, G. Contreras-Puente, O. Zela, a-Angel.
    304-0
    Abstract:
    Aluminum doped CdTe polycrystalline films were obtained from the heterostructure CdTe/Al/Corning glass. The aluminum was deposited by thermal vacuum evaporation and the CdTe by sputtering of a CdTe target. The aluminum was introduced into the lattice of the CdTe from a thermal annealed to the CdTe/Al/Corning glas heterostructure. The electrical, structural, and optical properties were analyzed as a function of the Al concentrations. It found that when Al is incorporated, the electrical resistivity drops and the carrier concentration increases. In both cases the changes are several orders of magnitude. From the results, we conclude that, using this deposition techniques, $n$-type Al doped CdTe polycrystalline films can be produced.
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  • Influencia de la geometría y dopaje en las propiedades electrónicas de la hoja SiC

    Ernesto Chigo Anota.
    309-0
    Abstract:
    In this work it is presented a study concerning the effect of geometry and nitrogen doping on the electronic properties of the silicon carbide (SiC) sheet by means of the Density Functional Theory (DFT) at the level of Local Density Approximation (LDA). Two different basic geometries were used to model the SiC system: rectangular (arcmchair type; Si$_{12}$C$_{12}$H$_{12})$ and triangular (zig-zag type; Si$_{12}$C$_{12}$H$_{10})$. It was found that both systems are stable. Although a semiconductor behavior was recognized for both models, the gap energy shows strong dependence on the geometry. While the calculated gap energy for the rectangular cluster was 1.15 eV, for the triangular geometry it was twice as large (2.75 eV). A transition from covalent to ionic character by controlling the geometry of the SiC cluster is reported. The calculated dipolar moment for the triangular and rectangular cluster was 12$\times $10$^{ - 3}$ and 5271.7$\times $10$^{ - 3}$ Debye, respectively. Finally the effect of nitrogen doping on the structural stability and electronic properties of the systems are discussed.
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  • Amplitude and phase recovering from a micro-digital hologram using angular spectrum

    M. León, R. Rodríguez-Vera, J.A. Ra, as., S. Calixto
    315-0
    Abstract:
    In this paper we present a method to get the complex amplitude of a digital hologram obtained through phase shifting transmission digital holographic microscopy. This is based on the angular spectrum method as a reconstruction algorithm employing an in-line experimental setup. The reconstruction technique automatically eliminates the zero order and the twin images from the digital hologram. By means of a reference hologram reconstruction method, a temporal averaging procedure and a focus distance averaging of the reconstructed complex amplitude, we extend an already efficient methodology to obtain high precision phase images with a reduced amount of aberrations caused by the microscope objective, and at the same time, we reduce the optical noise of the reconstructed complex amplitude. Using a homemade micro-thin film step surface, our system was calibrated and made traceable to an atomic force microscope (AFM). It is shown that our experimental results, compared to those given by the AFM, reach an axial accuracy of 9.7 nm from a typical phase sample.
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  • Decay width $Z_1 \to l\bar l$: effects of the little Higgs model

    A. Gutiérrez-Rodríguez
    322-0
    Abstract:
    In the framework of the Little Higgs Model (LHM), we calculate the decay widths $\Gamma(Z_1 \to l\bar l)$ and $\Gamma_{inv}(Z_1 \to \nu\bar \nu)$ with corrections of QED and QCD. We analyze this with recent data from LEP and compute the contribution of the model. We find that the deviations of the decay width of reactions $Z_1 \to l\bar l$ and $Z_1 \to \nu\bar \nu$ from its SM value are relatively large in the parameter space preferred by the electroweak precision data. Furthermore, with reasonable free parameter values, the absolute value of the relative correction parameter $\delta\Gamma/\Gamma_{SM}$ is of $15%-50%$. The experimental measurement values could generate possible constraints on the free parameters of the LHM.
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  • Formas de equilibrio de gotas que emergen lentamente de cilindros de pared gruesa

    A. López-Villa, A. Pérez-Terrazo, G.J. Gutierrez, C.A. Vargas
    330-0
    Abstract:
    The equilibrium shapes of the drops that emerge slowly outside of vertical, thick-walled tubes were calculated. For low flow rates it is possible to calculate the shape of drops by using the balance equation between the hydrostatic and the capillary pressure. Asymptotic solutions of the balance equation were found in terms the drop shapes just before of their rupture. Other equilibrium shapes are also calculated through the numerical solution of the Young-Laplace equation. The qualitative agreement between experiments and theoretical results validate this work. As a result, it is found that the sizes and shapes of drops depend on the Bond number, the injection pressure and the value of the contact angle.
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  • Estudio de la alta correlación electrónica en el ZnO:Eu

    A. Blanca-Romero, J.M. Hern, ez-Alcántara., M.A. Ocaña-Bribiesca, J.F. Rivas-Silva
    338-0
    Abstract:
    A doped material can be obtained by substitution of one of its atoms with an impurity. In this work we propose four structural models which simulate a ZnO system doped with Eu and study the influence of the electron correlation in the system with the density of states (DOS). In particular, we observe that the lanthanide impurities exhibit peculiar optical and electronic properties due to the \emph{4f} subshell, which produce a Kondo-like resonance at the Fermi level.
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  • Percolación discreta en redes tridimensionales

    W. Lebrecht, M.I. González
    344-0
    Abstract:
    Bond and site percolation on a three-dimensional lattice is studied. A bond (site) is occupied or empty with probability $p$ or $1-p$ respectively, for any size $N$. Through an exact numerical analysis, the different percolating trajectories are obtained as a function of its length $L$ for each three-dimensional cell. A polynomial function $f(p,N)$ associated to bond and site percolation. On each cell is determined, where symmetrical and asymmetrical cells are included in order to calculate the percolation thresholds and the critical exponent $\nu$, $\beta$ and $\gamma$ for each cell. Applying the finite size scaling techniques, these parameters are obtained in the thermodynamic limit. These results are in a good agreement with the similar ones obtained by means of other procedures and techniques described in literature for three-dimensional lattices.
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  • Sinergia entre la corrosión erosión del acero 1045 recubierto por multicapas de TiN/TiAlN

    W. Aperador, C. Ramírez-Martin, J. Bautista Ruiz
    350-0
    Abstract:
    This paper presents the results of the evaluation to the resistance to dynamic corrosion, erosion and corrosion-erosion at an impact angle of 90$^{\circ}$ of thin films in the form of multilayer TiN / TiAlN deposited with periods of 2, 6, 12 and 24 bilayers on substrates of AISI 1045 steel, through a system with rf reactive magnetron sputtering from Ti and Al targets of high purity (99.99%) in an atmosphere composed of Ar / N. The electrochemical characterization was carried out by using Tafel polarization curves and microstructural characterization by scanning electron microscopy. There was a significant decrease in the corrosion rate for the coated samples in terms of dynamic corrosion and corrosion-erosion. The sample covered by 24 bilayers had the best behavior in both situations, confirming a good performance of the bilayers to the resistance to corrosion and erosion corrosion. The results of scanning electron microscopy showed a good sealing of the bilayers, which confirms the protective effect given by the substrate coatings.
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  • Electrostatic interaction of oppositely charged double layers

    A. Flores-Amado, M. Hernández-Contreras
    356-0
    Abstract:
    We determined with anisotropic hypernetted chain theory the effective pressure and counterion excess of two oppositely charged surfaces immersed in an electrolyte solution. These thermodynamic properties are enhanced in surfaces with evenly smeared surface charges with respect to those containing confined mobile charges. For low surface charges and moderate reservoir salt concentration ions are expelled from the slit between the planes. Depletion arises from ionic charge positional correlations. Its effect was determined through the number of excess of counterions which displays a negative value in the range of low surface charge density. However, a Poisson-Boltzmann calculation lead to positive definite counterion's excess due to its lack of ionic correlations.
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  • Hall effect: the role of nonequilibrium charge carriers

    S. Molina Valdovinos, Yu. G
    368-0
    Abstract:
    A new model of the Hall effect in the case of a bipolar semiconductor is present. Taking into account the nonequilibrium carriers, thermal generation and recombination processes assisted by traps (Shockley-Read model), the expressions for the electrochemical potential of electrons and holes, Hall field and Hall constant $R_ {\rm H}$ are obtained. The dependence of these expressions of the distribution of the carriers along the direction of the Hall field in the case of intrinsic and extrinsic semiconductors is studied.
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  • Obtención de los parámetros ópticos de la piel usando algoritmos genéticos y MCML

    B. Morales Cruzado, S. Vázquez, Montiel.
    375-0
    Abstract:
    In this work we present the results obtained by means of a program developed to calculate the optical parameters of the skin: the absorption coefficient, the scattering coefficient and the scattering anisotropy factor starting from value experimentally measured (the total transmittance, the collimated transmittance and the diffuse reflectance) of a sample of biological tissue. The program bases its operation on genetic algorithms and in the method \emph{Monte Carlo Multi-Layered} (MCML), that it is of the public domain. Finally, we compare our results with the simplified Kubelka-Munk's model and with some results published in the literature. Also, we discuss its importance inside the medical applications.
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  • Non-equivalence of the microcanonical and canonical ensembles in a bosonic Josephson junction

    L.A. González-García, R.A. Zamora-Zamora, R. Paredes
    382-0
    Abstract:
    We investigate the thermodynamic properties of a bosonic Josephson junction in the full quantum approach and, in particular, we concentrate in studying the thermal averages of one- and two-body properties below and above the transition from delocalized to self-trapped regimes. This temperature dependence is determined by using the fact that at equilibrium the microcanonical and canonical ensembles should be equivalent. To establish the robustness of the equilibrium state, we first study a one body property and show numerically that any arbitrary state localized in energy, when evolved, reaches a stationary or equilibrium state. Comparison among averages of one- and two-body properties in the microcanonical and canonical ensembles reveals discrepances, thus leading to non-equivalence among these ensembles. Such averages differences can be attributed to the fact that the Hilbert space of the system scales as its size $N$, and consequently, the entropy does not scale as $N$. We further find as a natural consequence of studying the finite bosonic Josephson junction in the two-mode Bose Hubbard context, that positive and negative temperatures are obtained. This result can be generalized for any finite optical lattice.
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