Vol. 49 No. 4 (2003): Revista Mexicana de Física.

Published: 2003-01-01

Articles

  • Conserved quantities in the variational equations

    C.M. Arizmendi, J. Delgado, H.N. Núñez-Yépez, A.L. Salas-Brito
    298-0
    Abstract:
    Noether's theorem relating continuous symmetries of a Lagrangian system to the existence of conserved quantities is shown to be valid at the level of the variational equations of the system. This result can be helpful in the study of perturbations and of integrability in various areas of current interest. As examples, we derive conserved quatities in linearized general relativity and obtain conserved quantities valid in perturbed classical dynamics.
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  • A CMOS transistor-only $\mathbf{2^{nd}}$ order low-pass $\mathbf{\Sigma \Delta}$ modulator

    F. S, oval-Ibarra., R. Rodríguez-Calderón
    303-0
    Abstract:
    A simple procedure for designing a switched-current second-order $\Sigma \Delta $ modulator is presented. Since this design assumes a white noise spectrum due the quantization process, and by sizing the needed current branches according the proposed equivalent MOS transistor signal-to-noise ratio, a 2$^{nd}$ order $\Sigma \Delta $ modulator suitable to develop an analog-to-digital converter for audio applications has been obtained. The experimental results verify in high percentage the utility of the proposed model. However, additional noise effects have to be taken into account to increase the performance of the modulator.
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  • Information swapping scheme in cavity QED

    L.M. Arévalo Aguilar, H. Mo, a-Cessa.
    307-0
    Abstract:
    We present a method to swap information between atomic states by manipulating the interaction time between a quantum cavity field and two two-level atoms. We show that quantum information carried by one atom can be written onto a 'blank' state of the second atom, and the information contained in atom one is completely erased.
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  • Morphology of patterned semiconductor III-V surfaces prepared by spontaneous anisotropic chemical etching

    José-Guadalupe Bañuelos., Elena V. Basiuk, José-Manuel Saniger-Blesa.
    310-0
    Abstract:
    In the present paper we report on scanning electron microscopy and atomic force microscopy study of different microreliefs obtained through a spontaneous anisotropic etching (that is without the use of masking, photochemical and photoelectrochemical techniques) of the surfaces of monocrystalline A$^{III}$B$^{V}$-type semiconductors: InP(100) doped with S and Fe, GaP(100), GaSb(100), InSb(100) and GaAs(100). The microrelief morphology (star-like, pyramides, grooves, etc.) depends on acidic etchant employed. Estimation of the activation energy demonstrates that the etching with microrelief formation occurs in the kinetic region. The most interesting InP microrelief is the two-dimensional groove-shaped one, which might be suitable to produce antireflection surfaces for solar cells. The conditions have been optimized to fabricate this microrelief with a given groove period of 0.6 to 3.7 $\mu $m. Morphology of different textured surfaces of other A$^{III}$B$^{V}$ semiconductors is also discussed.
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  • Magnetism in Rh monolayers

    L. M, R. Baquero
    317-0
    Abstract:
    We have studied the magnetic behavior of a 4$d$ transition metal Rh monolayer (ML) on different substrates (Ag, Au, Cu) and orientations. We find the Rh monolayer to be magnetic in the (001) orientation on all the substrates, in the (111) direction for Au and Ag alone ($\mu \approx 0.6$ $% \mu _{B}$) for both, in the (110) one we do not find any magnetic activity. We analyze the parameters that have been suggested as general criteria for magnetism in the monolayer-substrate system. In particular, we study several possible geometric parameters and the bandwidth as possible clues to predict the magnetic behavior. A detailed analysis shows that the physics underlying the switching on of magnetism in a Rh-ML on noble metal substrates, is determined by an intraband transfer of $d$-electronic states (as compared to the Rh surface), from lower and higher energies to the Fermi level. This effect enhances the density of states at that energy, in an important way and shrinks the $d$-bandwidth. It seems to depend strongly on specificities of the interaction between the ML and the substrate that are hardly taken into account by a single parameter, in a general way. We impose charge neutrality in our calculations but deal explicitly with the changes to be expected if charge transfer would actually occur.
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  • Determination of optical constants of Zn$\pmb{_{x}}$In$\pmb{_{1 - x}}$Se thin films deposited by evaporation

    G. Gordillo, C. Calderón, F. Rojas
    329-0
    Abstract:
    Polycrystalline Zn$_{x}$In$_{1 - x}$Se thin films with Se contents varying between x=0 (InSe) and x=1 (ZnSe), deposited on glass substrates, were optically characterized. These samples were grown by coevaporation of the ZnSe and In$_{2}$Se$_{3}$ compounds using a crucible constituted by two coaxial chambers. The optical constants (refractive index n, absorption coefficient $\alpha$ and optical gap Eg) and the film thickness d, were determined using the transmission spectrum and simple calculations based on a theoretical model including interference effect induced by multiple internal reflections in the substrate/film system. The reliability of the results was tested by comparing the experimental transmittance spectra with the theoretically ones, using values of n, $\alpha$ and d obtained experimentally.
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  • Band structure for the cladding of a hollow core photonic crystal fibre

    J. Arriaga, B. Meneses
    335-0
    Abstract:
    We calculate the photonic band structure of a two dimensional (2D) photonic crystal (PC) formed by a hexagonal array of circular rings embedded in air. This crystal has been used as the cladding of a photonic crystal fibre whose core is composed of air. We solve the Maxwell equations using the plane wave method to expand the fields and the dielectric constant. Our calculations reveal that for propagation in the plane of periodicity, the PC possesses no complete photonic band gaps. However we demonstrate the existence of complete photonic band gaps for propagation out of the plane of periodicity.
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  • Heat transfer in asymmetric convective cooling and optimized entropy generation rate

    G. Ibáñez, S. Cuevas, M. López de Haro
    338-0
    Abstract:
    The steady viscous flow between two infinite parallel planes, is used to illustrate the possibility of minimizing the global entropy generation rate by cooling the external surfaces convectively in an asymmetric way. The flow is generated by both an axial pressure gradient and the uniform motion of the upper surface (generalized Couette flow). The temperature field is determined using boundary conditions of the third kind. The analytic expressions for the velocity and temperature fields of the fluid are used to calculate the global entropy generation rate explicitly. In dimensionless terms, this function depends on the dimensionless ratio of the two possible velocity scales (characterized by the magnitudes of the pressure gradient and the upper surface velocity), the dimensionless ambient temperature and the convective heat transfer coefficients (Biot numbers) of each surface which, in general, are not assumed to be the same. When the Biot numbers for each surface are equal, the entropy generation rate shows a monotonic increase. However, when the Biot numbers are different this function displays a minimum for specific cooling conditions. Besides, we calculate the local Nusselt number at the upper wall for minimum entropy generation conditions.
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  • Hamiltonian structures for the acoustic field

    G.F. Torres del Castillo, E. Galindo Linares
    344-0
    Abstract:
    It is shown that the Hamiltonian structures for the acoustic field obtained by means of the canonical formalism using as field variables the components of the molecular displacements or the variation of the gas density are different and lead to different Poisson brackets. It is shown that by requiring that the values of the Hamiltonians coincide, the Poisson brackets also coincide.
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  • Visualization of the flow around a bubble moving in a low viscosity liquid

    R. Lima-Ochoterena, R. Zenit
    348-0
    Abstract:
    A new technique to visualize the flow around a bubble rising in a low viscosity fluid is presented. With this technique it is possible to observe streak lines of the flow as well as the shape and position of the bubble. The visualization of the streak lines is obtained by open-diaphragm photography of laser-sheet illuminated micro-tracers. The shape and position of the bubble is obtained, in the same photo plate, by simultaneously illuminating the flow with a stroboscopic light. The experiments were performed in a closed acrilic tank of $50\times50\times50 \mbox{ cm}^3$, in which bubbles were injected using a capillary tube. Filtered water was used as the working fluid. Pure Nitrogen was used to form the bubbles. Experimental results were obtained for a range of bubble sizes. The size of the injected bubbles was controlled by a fixed volume switch valve. We have identified a change of the bubble trajectory, from rectilinear to zig-zaging, as its volume increases, in accordance with previously reported studies. A characteristic change of the velocity field around the bubble is observed when the trajectory instability appears. We conclude that the point of inflection in the velocity-volume plot is directly related to the appearance of the trajectory instability.
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  • Measurements of the thermal effusivity of transparent liquids by means of a photopyroelectric technique

    J.A. Balderas-López
    353-0
    Abstract:
    A new photopyroelectric methodology for thermal effusivity measurements in transparent liquids is presented. This methodology involves the photopyroelectric technique in the standard front-surface configuration. A signal normalization procedure, which avoids the conventional requirement for transfer function determination is implemented. The thermal effusivity of three liquids was measured by means of this device and very good agreement was found with the corresponding values reported in the literature.
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  • One-parameter isospectral special functions

    M.A. Re, es., D. Jimenez., H.C. Rosu.
    358-0
    Abstract:
    Using a combination of the ladder operators of Piña [1] and the parametric operators of Mielnik [2] we introduce second order linear differential equations whose eigenfunctions are isospectral to the special functions of the mathematical physics and illustrate the method with several key examples.
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  • Principio de invariancia de norma y rompimiento espontáneo de simetría en una partícula clásica

    J. Mahecha G., L.A. Sánchez D.
    364-0
    Abstract:
    Due to the fact that only matter fields have phase, frequently is believed that the gauge principle can induce gauge fields only in quantum systems. But this is not necessary. This paper, of pedagogical scope, presents a classical system constituted by a particle in a classical potential, which is used as a model to illustrate the gauge principle and the spontaneous symmetry breaking. Those concepts appear in the study of second order phase transitions. Ferroelectricity, ferromagnetism, superconductivity, plasmons in a free electron gas, and the mass of vector bosons in the gauge field Yang-Mills theories, are some of the phenomena in which these transitions occur.
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  • Student evaluation through membership functions in CAT systems

    J. Suárez
    371-0
    Abstract:
    Generally speaking, the process of student evaluation is based on a procedure where we assume that the student belongs just to one set in a completely specified way, for example the set of excellent students, or the set of regular students. In this paper we use fuzzy sets concepts just to propose a different procedure which can be useful to manage the student's performance in a variation of a computerized adaptive testing administration process. This can be made by assuming that, for a given student, a membership function $\mu_{A}$ is assigned. This membership function gives the membership degree of the student to the fuzzy set $A$, which can be the set of excellent students, or the set of regular students, or the set of poor performance students. Furthermore, we assume that the item bank contains items belonging, with certain degree, to fuzzy sets describing the complexity of the given items. For example, we can talk about the fuzzy set of difficult questions or the fuzzy set of easy questions. By considering the evaluation process as a problem in the field of control theory, we establish a proper metaphor with a very simple, and very well studied, physical system with behavior described by variables such as the position, velocity and acceleration. Based in this model, we propose fuzzy rules just to control the item administration process as a function of the ability of the student and the item complexity.
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  • A tangent magnetometer to measure the earth magnetic field

    M. Sosa, J. Bernal-Alvarado, J.L. González-Solís, G. Gutiérrez-Juárez, M. Vargas-Luna, M. Durán-Santamaría, S.P. Preciado-Galván, J. Ríos, A. Ruiz-Velasco, V.D. Trujillo-García
    379-0
    Abstract:
    A simple and low cost experiment is proposed for measuring the earth magnetic field. Results obtained with the system described here are compared to measurements performed using a commercial magnetometer. The value for the earth magnetic field north-south component, \mbox{$B_Z=24.6\pm0.3~\mu$T}, obtained with our system is in good agreement with measurements recorded by the commercial magnetometer, which vary around $30\pm5~\mu$T. This experiment is of easy implementation and great educational value in an undergraduate laboratory course of electricity and magnetism, because it provides an application of the fundamental physics to the measurement of an ubiquous and important phenomenon.
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  • Ondas gravitatorias

    R. Lazkoz, J.A. Valiente Kroon
    384-0
    Abstract:
    The following article provides an elementary introduction to the phenomena of gravitational waves. It does not requires a prior knowledge of the theory of General Relativity. The article starts with an elementary introduction to Einstein's theory of gravity. Along these lines, gravitational waves are introduced as an effect of the propagation of perturbations of spacetime. These perturbations give rise to variations in the tidal forces. Gravitational waves differ from their electromagnetic counterparts in several ways: they are non-linear and require certain asymmetry in the sources in order to be produced ---there are for example, no spherical gravitational waves---. Gravitational waves provide a very peculiar information about the sources. Thus, a great deal of effort has been invested in order to detect them. It is to be expected that the first detections will take place within the next ten years, and that they will open a new window for Astronomy.
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  • ?`Qué forma adquiere una membrana elástica circular al aplicarle una diferencia de presión?

    A.I. Oliva, D.C. Valdés, E. Le, -Koo., H.G. Riveros
    391-0
    Abstract:
    An elastic circular membrane of uniform density and thickness and subjected to a constant pressure difference is studied theoretically and experimentally. Two solutions of the deformation of an elastic circular membrane and their comparisons with the numerical values of the measured deformations of a surgical rubber glove membrane. The first solution is an analogical calculations using the geometry of soap bubbles, and the second one is the analytical solution of the differential equation of the elastic deformation in the approximation of constant elastic modulus. Both solutions predict a spherical shape which is confirmed by the experimental data. Comparisons with the solution of a deformed plate and with the paraboloidal shape desirable for solar mirrors are also included.
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