Vol. 56 No. 3 (2010): Revista Mexicana de Física.

View Vol. 56 No. 3 (2010): Revista Mexicana de Física.
Published: 2010-01-01

Articles

  • Ab initio calculation of structural and electronic properties of Al$_x$Ga$_{1-x}$N and In$_x$Ga$_{1-x}$N alloys

    E. López-Apreza, J. Arriaga, D. Olguín
    183-0
    Abstract:
    Using the density functional theory (DFT) with the generalized gradient approximation (GGA), we calculated the structural and electronic properties of wurtzite AlN, GaN, InN, and their related alloys, Al$_x$Ga$_{1-x}$N and In$_x$Ga$_{1-x}$N. We have performed accurate ab initio total energy calculations using the full--potential linearized augmented plane wave (FP--LAPW) method to investigate their structural and electronic properties. We found that in both alloys the fundamental parameters do not follow Vergard's law. The lattice parameters, $a, c,$ and $u$, for the Al$_x$Ga$_{1-x}$N alloy are found to exhibit positive bowing parameters, while for In$_x$Ga$_{1-x}$N there is a negative bowing for the $a$ and $c$ parameters and a positive bowing for the internal parameter, $u$. We calculated as well the nearest neighbor and next nearest neighbor distances, as a function of the concentration, and we obtained a good agreement with experimental results. Furthermore, we found that for both alloys, the band gap does not follows the Vegard law. As a by--product of our electronic band structure calculations, the effective masses of the binary compounds and their alloys were calculated. All the calculated properties show good agreement with most of the previously reported results. Finally, using the frozen phonon approach, the A$_1(TO)$ mode for the different systems studied in this work was calculated. Our calculations show good agreement with experimental values reported for the binary compounds. For the ternary alloys, our calculations reproduce experimental values for Al$_x$Ga$_{1-x}$N as well as theoretical predictions for In$_x$Ga$_{1-x}$N.
    ⬇️ Scroll down to see the full summary

  • New coordinates for the four-body problem

    E. Piña
    195-0
    Abstract:
    A new coordinate system is defined to study the physical Four-Body dynamical problem with general masses, with the origin the of coordinates at the center of mass. The transformation from the frame of inertial coordinates involves a combination of a rotation to the system of principal axis of inertia, followed by three changes of scale modifying the principal moments of inertia yield to a body with three equal moments of inertia, and finally a second rotation that leaves unaltered the equal moments of inertia. These three transformation steps yield a mass-dependent, rigid, orthocentric tetrahedron of constant volume in the baricentric inertial coordinates. Each of those three linear transformations is a function of three coordinates that produce the nine degrees of freedom of the Physical Four-Body problem, in a coordinate system with the center of mass as origin. The relation between the well-known equilateral tetrahedron solution to the gravitational Four-Body problem and the new coordinates is exhibited, and the planar case of central configurations with four different masses is computed numerically in these coordinates.
    ⬇️ Scroll down to see the full summary

  • Analytical study of the obsidian hydration process

    Y. Kudriavtsev, S. Gallardo, A. Villegas, G. Ramirez, M. Luna, M. Aguilar, R. Asomoza
    204-0
    Abstract:
    Depth profiling analysis of some elements in a set of naturally hydrated obsidian samples was performed by using Secondary Ion Mass Spectrometry. We have investigated the mechanism of water penetration into obsidian and have revealed that water penetrates into the top surface layer alone. From this layer atomic hydrogen penetrates into the obsidian bulk through a still unknown mechanism; the penetration cannot be described by Fick's law with a constant diffusion coefficient.
    ⬇️ Scroll down to see the full summary

  • Fiber optic sensing of relative humidity using a twin low coherence interferometer

    A.V. Khomenko, J. Tapia-Mercado, M.A. García-Zarate
    208-0
    Abstract:
    A fiber-optic implementation of psychrometer is reported. It consists of two Sagnac interferometers with twisted highly birefringent fiber that is used as a temperature sensitive element. One interferometer is used for dry-bulb temperature measurement and the other is for wet-bulb temperature monitoring. The interrogation technique for low coherence interferometric sensor with signal processing in spectral domain is described. The measurement uncertainties for temperature and relative humidity measurements no worse than $0.01^{\circ }$C and $4%$, respectively, have been achieved experimentally.
    ⬇️ Scroll down to see the full summary

  • Carrier heating effects on transport phenomena in intrinsic semiconductor thin films

    G. Gonzalez de la Cruz, Yu G. Gurevich
    211-0
    Abstract:
    The excess of nonequilibrium charge carriers due to heating by electric fields influences substantially the electron heat-diffusion and the carrier current density in thin film semiconductors. With the assumption of hole and phonon thermal equilibrium, the current density for electrons and holes and electron heat flux in the semiconductor thin films are calculated analytically taking into account the contribution of the nonequilibrium of carriers and the electron temperature. By using the continuity equations for the carrier densities and energy balance equation with appropriate boundary conditions at the surfaces of the sample, we find that the current density and electron heat flux depend substantially on the size of the sample.
    ⬇️ Scroll down to see the full summary

  • Absorción óptica a altas presiones del TLGaSe$_{2}$

    Ch. Power, I. Molina, L. Chacón, J. González, A. Jagui Pérez-Kuroki, J.C. Chervin
    217-0
    Abstract:
    In this paper the spectrum of optical normal transmission not polarized of TlGaSe$_{2}$ is measured as a function of pressure up to 27.6 GPa at room temperature. Our results show that in the range of the visible exist the contributions of two direct gaps of energy, which present a linear dependence with the pressure. The weak transition E$_{A}(\Gamma _2^v \to \Gamma _{3,1}^c )$ assigned to the fundamental gap of energy, decreases up to 1.5 GPa with a linear coefficient of -5.31$\times$10$^{ - 2}$ eVGPa$^{ - 1}$ and the transition E$_{B}(\Gamma _2^v \to \Gamma _{3,2}^c )$ shows a coefficient of -9.95$\times$10$^{ - 2}$ eVGPa$^{ - 1}$ up to 5.3 GPa (limit of pressure in the visible spectrum). The results in the infrared do not show the presence of the transition E$_{A}$ allowing to see only the behavior of the second transition rightly up to the pressure of metallization 24.6 GPa. In the range of pressure studied from 0.0 to 27.6 GPa, the transition E$_{B}$ shows a not linear behavior with the pressure of quadratic coefficient 1.83$\times$10$^{ - 3}$ eVGPa$^{ - 2}$.
    ⬇️ Scroll down to see the full summary

  • BEC transition of a weakly interacting ultracold Bose gas in a linear quadrupolar trap

    N. S, oval-Figueroa., V. Romero-Rochín
    223-0
    Abstract:
    We discuss the thermal gas to Bose-Einstein Condensation (BEC) transition of an ultracold Bose gas in a linear quadrupolar trap with contact interatomic interactions within the Hartree-Fock (HF) approximation. We briefly review the theoretical framework of the thermodynamics of a gas confined by a non-uniform potential to show how mechanical equilibria must be formulated in term of generalized volume and pressure variables ${\cal V}$ and ${\cal P}$ that replace the usual volume and hydrostatic pressure of a uniform system. We solve for the density profile within HF as a function of temperature $T$ and molar volume $v = {\cal V}/N$. With this information, we are able to calculate all the thermodynamic properties of the system and analyze the behavior of the gas through the BEC (superfluid) transition. We find that the transition is completely smooth, showing no sign of critical behavior. We emphasize how these predictions can be readily measured with the current experimental setups.
    ⬇️ Scroll down to see the full summary

  • Maximum efficiency of an irreversible heat engine with a distributed working fluid and linear phenomenological heat transfer law

    Lingen Chen., Shaojun Xia., Fengrui Sun.
    231-0
    Abstract:
    Maximum efficiency of an irreversible heat engine with a distributed working fluid, in which the heat transfers between the working fluid and the heat reservoirs obey the linear phenomenological heat transfer law [$q \propto \Delta (T^{ - 1})$], is studied in this paper by using finite-time thermodynamics based on Orlov and Berry's work$^{i}$. Two kinds of efficiencies are defined, and the problems are divided into three cases. Optimal control theory is used to determine the upper bounds of efficiencies of the heat engines for various cases. Numerical examples of the two efficiencies for the irreversible heat engine with lumped-parameter model working between variable temperature reservoirs are provided, and the effects of changes of the reservoir's temperature on the maximum efficiency of the heat engine are analyzed. The obtained results are also compared with those obtained by Orlov and Berry$^{ii}$ with Newtonian heat transfer law [$q \propto \Delta (T)$].
    ⬇️ Scroll down to see the full summary

  • A note on a traveling wave on an extensible capsule membrane --with bending rigidity-- in poiseuille flow

    M.A.H. Re, es., F. Méndez-Lavielle, E. Geffro, .
    239-0
    Abstract:
    We consider the effects of the bending rigidity of a capsule membrane convected by Poiseuille flow. We show numerically how the transition from a ``discoidal shape'' to ``V shape or cordial form'' capsules is mediated by travelling waves responsible for the required change in curvature. We show numerically that the wave velocity is an increasing function of both the rigidity of the membrane and the pressure gradient. A qualitative explanation of the observation is given in terms of a Burger's type equation for the curvature.
    ⬇️ Scroll down to see the full summary

  • Constructal complex-objective optimization of electromagnet based on magnetic induction and maximum temperature difference

    Shuhuan Wei., Lingen Chen., Fengrui Sun.
    245-0
    Abstract:
    The good performance of an electromagnet requires high magnetic induction and a low temperature. A new complex-objective function reflected magnetic induction and maximum temperature difference is set up, and the electromagnet is optimized using the new complex-objective function. The optimization results show that the performance of the electromagnet is improved as the number of high thermal conductivity cooling discs inserted increases. When the performance of the electromagnet achieves its best level, the solenoid becomes longer and thinner as the number of high thermal conductivity cooling discs increases. Simultaneously, the magnetic induction becomes higher and the maximum temperature difference becomes lower. The optimized performance of the electromagnet also improves as the volume of solenoid increases; simultaneously, the magnetic induction first increases and then decreases, and the maximum temperature difference decreases all along.
    ⬇️ Scroll down to see the full summary

  • Structural study of colombian coal by fourier transform infrared spectroscopy coupled to attenuated total reflectance (FTIR-ATR)

    J.A. Orrego, R. Cabanzo Hernández, E. Mejía-Ospino
    251-0
    Abstract:
    The most important Colombian coals were studied using Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) Spectroscopy. The ATR-FTIR spectra were used to obtain structural parameters of coal and correlating these structural parameters with the rank of the coal. The structural parameters, methylene-methyl ratio (CH$_{2}$/CH$_{3})$, aromaticity factor (f$_{a})$, aliphatic-aromatic ratio (H$_{al}$/H$_{ar})$ were determined directly from the ATR-FTIR spectra and f$_{a}$ was evaluated from spectra data and an elemental analysis in coals studied here. The aliphatic and aromatic regions were considered between 3000 - 2800 cm$^{ - 1}$ and 912 - 670 cm$^{ - 1}$, respectively.
    ⬇️ Scroll down to see the full summary

  • Instrumentación y diseño de una fuente de alto voltaje para fabricar rejillas de fibra óptica mediante la técnica de arco eléctrico

    J.C. Hernández-García, J.M. Estudillo-A, ala., R. Rojas-Laguna, R.I. Mata Chávez, A. Martínez-Ríos, J.G. Gutiérrez, M. Trejo-Durán, E. Vargas-Rodríguez, J.A. Andrade-Lucio, E. Alvarado-Méndez
    255-0
    Abstract:
    In this work we present the instrumentation and construction of a source of high voltage, the source is controlled by the communication port Input/Output of a card of acquisition of data, we used programming language LabView\textregistered, is a tool for the design of acquisition systems, instrumentation and control. The source was utilize for to fabricate long period grating of optical fibers by means of inducing an electric arc with the point by point procedure, with this source is have low cost in the fabrication, this is 80% down of technique with others devices. The instrument has a panel of virtual control to modify the time of duration of the arch, the voltage and the electric current applied to the optic fiber by means of two electrodes. The gratings were fabricated with SMF-28. These gratings can be used with optical fiber lasers as optical filters and in the implementation of optical sensors.
    ⬇️ Scroll down to see the full summary

  • Medición de la evolución temporal de los campos dinámicos de desplazamiento mediante interferometría de moteado para una probeta de latón

    A. Martínez, J.A. Ra, as., J.C. Madrid Gara, .
    262-0
    Abstract:
    In this work, we have used the speckle interferometry technique and 3-steps-phase shifting to evaluate the displacement fields induced on a sample by mechanical load. The specimen was a brass sheet metal, 3 mm thick and an area of 210 mm$\times$3 mm. The sample was tested on an Instron machine working in tension at a constant speed. The mechanical load was controlled by using of a computer. Additional to the displacement fields, stress fields and Young's modulus were obtained.
    ⬇️ Scroll down to see the full summary