Vol. 50 No. 3 (2004): Revista Mexicana de Física.

Published: 2004-01-01

Articles

  • Relation between the field quadratures and the characteristic function of a mirror

    B.M. Rodríguez-Lara, H. Mo, a-Cessa.
    213-0
    Abstract:
    We analyze the possibility of measuring the state of a movable mirror by using its interaction with a quantum field. We show that measuring the field quadratures allows us to reconstruct the characteristic function corresponding to the mirror state.
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  • Influence of baking on the photoluminescence spectra of In$\pmb{_{1-x}}$Ga$\pmb{_{x}}$As$\pmb{_{y}}$P$\pmb{_{1-y}}$ solid solutions grown on Inp substrates

    V.A. Mishurn, i., A. Yu, F. De Anda, J. Nieto-Navarro
    216-0
    Abstract:
    The influence of thermal treatments on the photoluminescence spectra of In$_{1-x}$Ga$_{x}$As$_{y}$P$_{1-y}$epitaxial layers of various compositions grown by LPE on InP substrates has been studied. To prevent the epitaxial layers from degradation, due to phosphor evaporation during the baking, their surface was covered by spin-on SiO$_{2}$ layers. The photoluminescence spectra did not change for solid solutions whose compositions were near InP and InGaAs. For compositions in the middle of the lattice-matched region, the variations were very noticeable because the appearance of additional peaks in the luminescence spectra. This could be related to the decomposition of those solid solutions whose compositions lie inside a theoretically predicted miscibility gap.
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  • Spectral and thermodynamical properties of systems with noncanonical commutation rules: semiclassical approach

    J.C. Flores, S. Montecinos
    221-0
    Abstract:
    We study different quantum one dimensional systems with noncanonical commutation rule $[x,p]=i\hbar (1+sH),$ where $H$ is the one particle Hamiltonian and $s$ a parameter. This is carried-out using semiclassical arguments and the surmise $\hbar \rightarrow \hbar (1+sE),$ where $E$ is the energy. We compute the spectrum of the potential box, the harmonic oscillator, and a more general power-law potential $\left| x\right| ^{\nu }$. With the above surmise, and changing the size of the elementary cell in the phase space, we obtain an expression for the partition function of these systems. We calculate the first order correction in $s$ for the internal energy and heat capacity. We apply our technique to the ideal gas, the phonon gas, and to $N$ non-interacting particles with external potential like $\left| x\right| ^{\nu }$.
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  • Transmisión de pulsos luminosos ultracortos que inciden oblicuamente sobre un medio absorbente y/o dispersivo

    H. Ramírez Díaz, E.A. Martí Panameño, M.M. Méndez Otero, C.A. Alejo Armenta
    225-0
    Abstract:
    In this work is studied the transmission of linearly polarized optical pulses, which arrive from vacuum into a material medium at arbitrary angle of incidence. The considered geometry take into account that the electric field has parallel and perpendicular components to the plane of incidence. Analitical expressions are obtained for electrical and magnetic fields inside the material medium, the functional form of the expressions allow to relate the imaginary and real parts of the wave vector with the loss of the pulse spectral amplitud and with its propagation direction, respectively. It is found a generalized integral relation for the Poynting vector, in the time domain, which is valid for dispersive and absorbent media.
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  • Novel tunable acceptor doped BST thin films for high quality tunable microwave devices

    M.W. Cole, R.G. Ge, er.
    232-0
    Abstract:
    The materials properties of undoped and low concentration Mg doped Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$ (BST) thin films are reported. The films were fabricated on single crystal (100) MgO and Pt coated Si substrates via the metalorganic solution deposition (MOSD) technique using carboxylate-alkoxide precursors and post-deposition annealed at 800$^\circ$C (film/MgO substrates) and 750$^\circ$C (film/Pt-Si substrates). The dielectric properties were measured at 10 GHz using unpatterned/non-metallized films via a tuned coupled/split dielectric resonator system and at 100~kHz using metal-insulator-metal capacitors. The structure, microstructure, surface morphology and film/substrate compositional quality were analyzed and correlated to the films dielectric and insulating properties. The Mg doped BST films exhibited improved dielectric loss and insulating characteristics compared to the undoped BST thin films. The improved dielectric properties, low leakage current, and good tunability of the low level Mg doped BST thin films merits strong potential for utilization in microwave tunable devices.
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  • Decay $\pmb{t\rightarrow b W Z}$ within the context of the left-right mirror model

    P. Fernández de Córdoba, R. Gaitán Lozano, A. Hernández-Galeana, J.M. Rivera-Rebolledo
    239-0
    Abstract:
    In this paper the left-right mirror model is applied to the decay $t\rightarrow b W Z$, according to the Feynman rules given by the model. We write the corresponding width in compact form in terms of the Standard Model width by assuming the contribution to the $WZW$ vertex being of the same order of magnitude as that of the $tZt$ and $bZb$ vertices. The width has to be compared with recent experimental data in order to get preliminary values for the parameters of the model, since these quantities have not been measured yet. With the appropriate rules given by the model we can deal with other related decays and improve results.
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  • Estudio del número de pares creados por un campo eléctrico*

    J. Haro
    244-0
    Abstract:
    In this work we study the number of produced pairs in an electric field using the semiclassical aproach. We see that the stochastic process $N(t)$=``net number of produced pairs at time $t$'', is an stochastic Poisson process with expected value $(\x \mathcal E(t))/(64 mc^2)$, where $\x$ is the fine structure constant and ${\mathcal E}(t)$ is the energy of the electric field at time $t$.
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  • Theoretical and numerical study of diffraction on electromagnetic optics VI. Obliquely incident t.e.-polarized gaussian beams on a finite grating with conducting substrate

    O. Mata-Mendez, F. Chavez-Rivas, J. Suma, a-Martinez.
    255-0
    Abstract:
    Diffraction of an obliquely incident TE-polarized Gaussian beams by N equally spaced slits (finite grating) with conducting substrate is treated. The substrate can be either vacuum or conductor. The diffracted and scattered patterns, the transmission and reflection coefficients, and the normally diffracted energy are analyzed as a function of several optogeometrical parameters. Particularly, the coupling between slits and the influence of the substrate is considered. We have found that, when the substrate is a conductor, the grating equation in reflection predicts with good precision the angular positions of the orders of a finite grating; the angular positions of these orders are independents of the beam width, the spot position on the finite grating, and the conductivity of the substrate. Besides, the envelope of the reflected energy is conserved constant when the position of the spot is changed.
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  • Simultaneous description of elastic, fusion and total reaction cross sections for the $\pmb{^{6}He+^{209}Bi}$ system for energies around the coulomb barrier

    A. Gómez Camacho, E.F. Aguilera
    265-0
    Abstract:
    A systematic and simultaneous description of elastic scattering, reaction and fusion cross sections for the system $^{6}He+^{209}Bi$ at energies below the Coulomb barrier is given within the direct reaction theory. Woods-Saxon optical potentials are used in the calculations where a cut-off parameter $% R_{F}$ is defined to separate the fusion and direct reaction absorption processes. A $\chi ^{2}-$search for energy dependent forms for the diffuseness parameter $a_{I}$ and the radial parameter $R_{I}$ of the imaginary potential $W_{\alpha }$ that best fits the experimental data is carried out. Good fits to the data are found. A relationship between the large values found for $a_{I}$ and $R_{I}$ with the large spatial extension of the halo nucleus $^{6}He$ wave function is also looked for.
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  • Principles of magnetic resonance imaging

    A.O. Rodríguez
    272-0
    Abstract:
    The concepts of magnetic resonance imaging are reviewed and its application to medical and biological systems is described. The magnetic resonance phenomenon can be described by both classical and quantum mechanical approaches. Magnetic resonance imaging is based on the techniques of nuclear magnetic resonance. The scanner first aligns the nuclear spins of hydrogen atoms in the patient and starts rotating them in a perfect concert. The nuclei emit maximum-strength electromagnetic waves at the start, but over time the rotating spins get out of synch, simply due to small differences in local magnetic fields. The unsynchronized spins cause the combined electromagnetic signal to decay with time, a phenomenon called relaxation. A slice is selected applying a gradient in a particular direction (\emph{X}, \emph{Y} or \emph{Z}). Magnetic resonance signals are then formed by means of the application of magnetic field gradients along three different directions. Finally, the signals are acquired and Fourier transformed to form a two-dimensional or three-dimensional image. Important parameters determining the image quality such as signal-to-noise ratio, contrast and resolution are discussed too. A review of the most widely utilised imaging techniques is given including ultra-fast sequences.
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  • Photo-oxidation of water sensitized by TiO$_2$ and WO$_3$ in presence of different electron acceptors

    A. Mills, M.A. Valenzuela
    287-0
    Abstract:
    The photo-oxidation of water is studied in presence of UV-light ($\lambda <$ 400 nm) using titanium dioxide (TiO$_{2})$ and tungsten oxide (WO$_{3, }$micro- and nano-crystalline) semiconductors in presence of different sacrificial electron acceptors (SEA): Fe(NO$_{3})_{2}$, Na$_{2}$S$_{2}$O$_{8}$, Ce(SO$_{4})_{2}$,\linebreak Co[(NH$_{3})_{5}$Cl]Cl$_{2}$, AgNO$_{3}$, HgCl$_{2}$ and Cu(NO$_{3})_{2}$. TiO$_{2}$ is 5 to 10 times more photoactive than WO$_{3}$ with reference to oxygen evolution. Ag ions are the best of the SEAs, for all the semiconductors tested in the photo-redox process. No oxygen evolution is observed when Hg$^{2 + }$ or Cu$^{2 + }$ ions are used as SEAs. The effect of high (10$^{ - 2}$ mol dm$^{ - 3})$ and low (10$^{ - 3}$ mol dm$^{ - 3})$ SEA concentrations is also studied but no common trend is observed. Instead, each system (i.e., SEA+H$_{2}$O+Semiconductor) exhibits a different behaviour and the results are rationalised in terms of the spectral and redox potential features of the system.
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  • An all-optical 4-bit register based on a four-order scattering of light by coherent acoustic phonons in single crystals

    A.S. Shcherbakov, E. Tepichin Rodriguez, A. Aguirre Lopez
    297-0
    Abstract:
    A specific case of a four-order non-collinear light scattering in anisotropic media is presented. Compared to our previous studies, an innovation lies in the fact that now we consider passing just the quartet of incident light waves through a single crystal that is perturbed by a coherent stream of acoustic phonons. The exact and closed analytical model for describing this strongly nonlinear phenomenon is developed. In fact, a specially designed regime of a four-order light scattering, when transitions of four input light beams into four output light modes are allowed and electronically controlled, is examined. The feasibility of applying such an effect to perform an all-optical switching is analyzed. An opportunity for arranging the digital 4-bit register is revealed and algorithmically analyzed.
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  • Schrödinger--Pauli equation for spin-3/2 particles

    G.F. Torres del Castillo, J. Velázquez Castro
    306-0
    Abstract:
    A non-relativistic equation for spin-3/2 particles is proposed and the gyromagnetic ratio for charged spin-3/2 particles is determined.
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  • Estudio de películas amorfas de TiAlN preparadas por erosión catódica reactiva por radiofrecuencias

    L. García-González, J. Morales-Hernández, J.P. Bartolo-Pérez, O. Ceh-Soberanis, J. Muñoz-Saldaña, F.J. Espinoza-Beltrán
    311-0
    Abstract:
    Using the reactive magnetron rf sputtering technique, we prepared TiAlN films with amorphous structure on Corning glass and steel substrates in a reactive atmosphere of nitrogen and argon using a target of Ti-Al (40/60 wt.%). The average temperature of the substrates was about 25$^\circ$C, with the purpose of obtaining amorphous films. The ratio of partial pressure of nitrogen to argon, P$_{N}$/P$_{Ar}$, was varied according to these values: 0.14, 0.28, and 0.43; fixing these values during whole the evaporation. Further on, films were prepared introducing nitrogen in periodic pulses with maximum values of P$_{N}$/P$_{Ar} \approx $ 4.7 during 45 seconds, with fixed periods of 10, 15 and 20 minutes. In all cases amorphous films were obtained, according to X-ray Diffraction. The chemical composition of the samples was measured by electron dispersive spectroscopy, showing a clear dependence with the evaporation conditions. In spite of the amorphous structure of the material, atomic force microscopy measurements showed a surface morphology dependent on the nitrogen content. Additionally, measurements of electronic spectroscopy for chemical analysis and Raman scattering spectroscopy for identification of chemical bonds were carried out. Measurements of mechanical properties of the samples were carried out using nanoindentation and micro-hardness Vicker's tests.
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  • L X-ray anisotropy and L$_{3}$-subshell alignment in rare earths induced by $^{12}$C ion impact

    O.G. de Lucio, J. Mir, a.
    319-0
    Abstract:
    The alignment parameter $A_{20}$ of the L$_{3}$-subshell of Ce, Nd, Eu, Dy, and Yb has been calculated by measuring the anisotropic emission of the L$% _{l}$ X-ray line induced by $^{12}$C ion impact at 9 MeV bombardment energy. A new Coster-Kronig transition probabilities database is used, as well as more recent theories for ionization cross sections. These experimental results are in better agreement with the theory than previously published data.
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  • Detección de pistón por polarimetría

    J. Salinas-Luna, E. Luna-Aguilar, A. Cornejo-Rodríguez
    323-0
    Abstract:
    We report a qualitative method to detect piston errors in an optical segmented surface. The test consists of illuminating a segmented surface, with a spherical wavefront coming from a linearly polarized beam. The behavior of the vectors of polarization of the light in the reflected beam are analyzed doing the experimental measurement of the Stokes parameters U and Q, rotating the analyzer for four positions of 0, 45, 90 and 135 degrees. The mirror used for this test has 20 and 120 centimeters for the diameter and the paraxial radius of curvature, respectively.
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