Vol. 54 No. 6 (2008): Revista Mexicana de Física.

Published: 2008-01-01

Articles

  • Electrical tuning of refraction in a two-dimensional photonic crystal infilled with a liquid crystal

    J.A. Re, es-Avendaño., P. Halevi
    407-0
    Abstract:
    We study a novel electro-optic structure with potential applications for ray steering, optical multiplexers, logic gates or switches. Our idea is based on the electrical tuning of refraction in a two-dimensional photonic crystal infilled by a liquid-crystal, with a direct-current electric field applied parallel to the cylinders. Using a two-step homogenization process, we show that, for sufficiently low frequencies, this structure can be represented by two field-dependent effective refractive indices. We demonstrate that the direction of the ordinary and extraordinary refracted rays can be sensitively tuned by varying the magnitude of the applied field.
    ⬇️ Scroll down to see the full summary

  • A parameterized family of single-double-triple-scroll chaotic oscillations

    E. Campos-Cantón, I. Campos-Cantón, J.S. González Salas, F. Cruz Ordaz
    411-0
    Abstract:
    We present a system with three equilibrium points which exhibit a single, double or triple scroll oscillation without introducing more equilibrium points. The study is based on one parameter of the nonlinear function which is the bifurcation parameter. With this bifurcation parameter it is possible to control the eigenvalues of the equilibrium points and consequently the type of oscillation.
    ⬇️ Scroll down to see the full summary

  • MBE-growth and characterization of In$_{x}$Ga$_{1-x}$As/GaAs (x=0.15) superlattice

    B. Sar{\i}kavak, M.K. Öztürk, H. Altunta\c{s}, T.S. Mammedov, \c{S}. Alt{\i}ndal, S. Özçelik
    416-0
    Abstract:
    A qualified In$_{0.15}$Ga$_{0.85}$As/GaAs superlattice was grown on an n-type GaAs(100) substrate by molecular beam epitaxy(MBE). Analysis of this structure was first carried out by X-Ray diffraction(XRD) and this structure's the interface thicknesses, roughness and x concentration determined at nanoscale. Secondly, the electrical characteristics of this sample such as the current-voltage-temperature (I-V-T), capacitance-voltage-temperature (C-V-T) and conductance-voltage temperature (G-V-T) were studied over a wide temperature range. The energy distribution of interface states was determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height. Experimental results show that the forward and reverse I-V characteristics are similar to Schottky-junction behavior. The ideality factor n, series resistance $R_{s}$, barrier height $\Phi_{B}$ and density of interface states $N_{ss}$ were found to be strong functions of temperature. According to thermionic emission (TE) theory, the zero-bias barrier height $(\phi _{Bo})$ calculated from forward bias I-V characteristics was found to increases with increasing temperature. In addition, the value of $R_{s}$ as a function of both voltage and temperature was obtained from C-V and G-V characteristics. The temperature dependent of I-V, C-V and G-V characteristics confirmed that the distribution the $R_{s}$ and $N_{ss}$ are important parameters that influence the electrical characteristics of these devices.
    ⬇️ Scroll down to see the full summary

  • Modeling of coagulation and dispersion of aerosolsin the atmosphere

    A.T. Celada, A. Salcido
    422-0
    Abstract:
    A simple model to analyze the effects of coagulation and turbulent dispersion on the behaviour of atmospheric aerosols is proposed here. This model does not use the concept of monomeric structure for the aerosol particles as proposed by Smoluchowsky to describe the coagulation. Instead, a probabilistic estimate of the production rate of the aerosol particles that are created by coagulation is carried out in the model as a function of the diameter ranges of the particles that collide. Only collisions of two particles with preservation of mass and volume are taken into account, and it is assumed that coagulation always occurs. The dispersion process of the aerosol is implemented by means of a simple Monte Carlo approach, where the mass fluxes are estimated with the mean wind field and the distribution of the turbulent fluctuations of wind velocity. Coagulation and dispersion are coupled in the mathematical formulation and numerical solution of the mass balance equations of the model. With this model we studied the steady state behaviour of an aerosol released to the atmosphere by an elevated point source. In particular, we studied the way in which the processes of coagulation and dispersion affect the distribution of the particle number, and also their effects on the concentration of the number of particles as a function of the downwind distance from the emission source. Qualitatively, some results obtained, such as a very fast consumption of the smallest aerosol particles near the source, and the accumulation of larger particles at greater distances, are in agreement with documented experimental findings.
    ⬇️ Scroll down to see the full summary

  • Optimization of the gain in non-uniform gratings in a Bi$_{12}$SiO$_{20}$ crystal considering the variation of fringe period, optical activity and polarization angles in a strong non-linear regime

    G. González, A. Zúñiga, L.F. Magaña
    433-0
    Abstract:
    We solved numerically the set of non-linear differential material rate differential equations, and using these solutions, we include the non-uniformity of the grating and of the magnitude and phase of light modulation along a sample thickness to calculate self-consistently the energy exchange in two-wave mixing. We optimize the gain, considering strong nonlinear conditions, variation of fringe period, optical activity, birefringence, absorption, polarization angle, applied fields and two crystal orientations: the grating vector parallel and perpendicular to the face [001] Under these conditions there is a complex relationship among all these parameters, and the prediction of the conditions for the optimum value of the gain is not simple We report the optimal sample thickness for different situations We obtained a maximum gain of 5.2.
    ⬇️ Scroll down to see the full summary

  • Neutron emission effects on final fragments mass and kinetic energy distribution from low energy fission of $\u234$

    M. Monto, a., J. Rojas, I. Lobato
    440-0
    Abstract:
    The standard deviation of the final kinetic energy distribution ($\sigma_{e}$) as a function of mass of final fragments ($m$) from low energy fission of $\u234$, measured with the Lohengrin spectrometer by Belhafaf \etal, presents a peak around $m=109$ and another around $m=122$. The authors attribute the first peak to the evaporation of a large number of neutrons around the corresponding mass number, i.e. there is no peak on the standard deviation of the primary kinetic energy distribution ($\sigma_{E}$) as a function of primary fragment mass ($A$). The second peak is attributed to a real peak on $\sigma_{E}(A)$. However, theoretical calculations related to primary distributions made by H.R. Faust and Z. Bao do not suggest any peak on $\sigma_{E}(A)$. In order to clarify this apparent controversy, we have made a numerical experiment in which the masses and the kinetic energy of final fragments are calculated, assuming an initial distribution of the kinetic energy without structures on the standard deviation as function of fragment mass. As a result we obtain a pronounced peak on $\sigma_{e}(m)$ curve around $m=109$, a depletion from $m=121$ to $m=129$, and an small peak around $m=122$, which is not as great as that measured by Belhafaf \etal Our simulation also reproduces the experimental results on the yield of the final mass $Y(m)$, the average number of emitted neutrons as a function of the provisional mass (calculated from the values of the final kinetic energy of the complementary fragments) and the average value of fragment kinetic energy as a function of the final mass ($\bar e$). From our results we conclude that there are no peaks on the $\sigma_{E}(A)$ curve, and the observed peaks on $\sigma_{e}(m)$ are due to the emitted neutron multiplicity and the variation of the average fragment kinetic energy as a function of primary fragment mass.
    ⬇️ Scroll down to see the full summary

  • Two step synthesis of TlBa$_{2}$Ca$_{2}$Cu$_{3}$O$_{x}$ films on Ag substrates by spray pyrolysis of metal-acetylacetonates

    L. Pérez-Arrieta, M. Aguilar-Frutis, J.L. Rosas-Mendoza, C. Falcon, M. Jergel, .
    446-0
    Abstract:
    A two-step synthesis process was performed to obtain TlBa$_{2}$Ca$_{2}$Cu$_{3}$O$_{x}$ (Tl-1223) superconductor films. The synthesized films were obtained under different thallium diffusion conditions in a two-zone furnace precursor films were deposited at 550$^\circ$C on silver substrates using the spray pyrolysis technique from acetylacetonates (2, 4-pentanedionates) as reagents. Second, a thallium diffusion process is carried out to incorporate Tl in the films. For this task Tl$_{2}$O$_{3}$ pellets, as a Tl source, were used at 750$^\circ$C. Different oxygen flow rates at atmospheric pressure were used in order to get the thallous oxide p(Tl$_{2}$O) partial pressure in the range of $6.9{\times}10^{ - 4 }$ to $6.1{\times}10^{ - 2}$ atm. The Tl-1223 phase was obtained in all cases, though for a low p(Tl$_{2}$O), the films presented the BaCuO$_{2}$ phase mixed with the Tl-1223 one. Critical temperature (T$_{C})$ values for these films were in the range of 90 to 102 K.
    ⬇️ Scroll down to see the full summary

  • SU(1,1) coherent states as Bessel-Gauss states

    S. Hac, an.
    451-0
    Abstract:
    The SU(1,1) coherent states of Barut and Girardello can be interpreted as Bessel-Gauss states that are time-dependent solutions of the Schrödinger equation. In this article, some basic properties of these states and their related operators are analyzed.
    ⬇️ Scroll down to see the full summary

  • Bound states of the hydrogen atom in parabolic coordinates

    G.F. Torres del Castillo, E. Navarro Morales
    454-0
    Abstract:
    The spectrum and separable eigenfunctions in parabolic coordinates for the bound states of the hydrogen atom, as well as their relation with the separable eigenfunctions in spherical coordinates, are obtained making use of the elementary theory of angular momentum.
    ⬇️ Scroll down to see the full summary

  • Lattices with variable and constant occupation density and q-exponential distribution

    P. Cavalcante da Silva, G. Corso, L.R. da Silva
    459-0
    Abstract:
    In this paper we test the hypothesis that $q$-exponential distribution fits better on distributions arising from lattices with a heterogeneous topology than a homogeneous topology. We compare two lattices: the first is the typical square lattice with a constant occupation density $p$ (the lattice used in standard percolation theory), and the second is a lattice constructed with a gradient of $p$. In the homogeneous lattice the occupied number of neighbors of each cell is the same (on average) for the full lattice, otherwise in the $p$-gradient lattice this number changes along the lattice. In this sense the $p$-gradient lattice shows a more complex topology than the homogeneous lattice. We fit the $q$-exponential and the stretched exponential distribution on the cluster size distribution that arises in the lattices. We observe that the $q$-exponential fits better on the $p$-gradient lattice than on a constant $p$ lattice. On the other hand, the stretched exponential distribution fits equally well on both lattices.
    ⬇️ Scroll down to see the full summary

  • Compositional analysis of aqueous solutions by laser-induced plasma spectroscopy

    J. P, R. Cabanzo, E. Mejía-Ospino
    464-0
    Abstract:
    Laser-induced plasma (LIP) was generated on the surface of an aqueous solution using a single pulse of the fundamental and second harmonic from a Q-switched Nd:YAG laser. Aqueous Ca, K, Mg, Na and Sr solutions were used to observe the presence of these elements in the plasma through the emission of their atomic species. In order to obtain the spectra of the aqueous solutions, we used a charge-coupled camera (CCD) in combination with a compact spectrometer. We also determined laser-induced plasma temperatures at 1064 and 532 nm with Boltzmann plots of hydrogen lines (656.28, 486.13 and 434.05 nm).
    ⬇️ Scroll down to see the full summary

  • Implementación electrónica de sistemas lineales en $\mathbb{R}^2$

    I. Campos-Cantón, S.Tesheira., C. Jiménez-Calvillo, E. Campos-Cantón
    468-0
    Abstract:
    A second order linear system suited for analog instrumentation is presented. Shown are the corresponding phase portrait nodes, a root focus and a central point. The experimental prototype is of easy implementation and of low cost as it is comprised only of passive elements (i.e resistors, capacitors) and operational amplifiers.
    ⬇️ Scroll down to see the full summary