Vol. 49 No. 2 (2003): Revista Mexicana de Física.

Published: 2003-01-01

Articles

  • Optical vibronic spectra of aggregates in Eu$^{\mathbf2+}$ -doped KCl and KBr crystals

    R. Pérez-Salas, T. M, R. Aceves, R. Rodríguez -Mijangos, M. Barboza-Flores
    102-0
    Abstract:
    The vibronic structure on the optical absorption, emission, and excitation spectra of aggregates in Eu$^{2 + }$-doped KCl and KBr crystals has been obtained. The crystals were annealed at 435K. During the aggregation process, four groups of narrow bands were found in KCl:Eu$^{2 + }$ and two, in KBr:Eu$^{2 + }$, which were considered as the signature of different Eu$^{2 + }$-ion aggregated phases. The vibration frequency for each group is very similar.
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  • Concentric dual-loop RF coil for magnetic resonance imaging$^*$

    R. Hernández, A. Rodríguez, P. Salgado, F.A. Barrios
    107-0
    Abstract:
    A surface coil for MRI consisted of two concentric loops was developed to produce brain images with a commercial MR imager. Prior to build the coil prototype, the magnetic field (B\( _{1} \)) generated by the coil was numerically simulated. This field simulation is based on the Biot-Savart law for the circular- and square-shaped loop coils. From these theoretical results, we can appreciate an improvement on the B\( _{1 \)} homogeneity. Comparison of experimental SNR for a single-loop and dual concentric figure coils show that the latter has better sensitivity and uniformity. Phantom and brain images were obtained on a 1.5 Tesla clinical imager, and showed a good image quality. These brain images when compared against images obtained with a circular-shaped coil exhibit a better capability of penetration. This receiver coil can generate high quality brain images. This coil design can be used in clinical imagers and is compatible with conventional imaging sequences and protocols.
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  • Fuerza termoelectromotriz en semiconductores bipolares: nuevo punto de vista

    Yu. Gurevich, A. Ortiz
    115-0
    Abstract:
    We present a new method for calculating the thermoelectromotive force in bipolar semiconductors within the linear approximation to the theory, taking into account the non-equilibrium charge carriers generated in the sample when the temperature field is applied. For the first time it is precisely defined which are the non-equilibrium charge carriers and how to write the Poisson equation to take them into account. For the first time it is taken into account the term that is proportional to the local change in temperature produced by the gradient of temperature, in the expression for the recombination rate to calculate the thermoelectromotive force given the result that this and the semiconductor's resistance do not depend solely on traditional parameters such as electric conductivities and thermoelectric powers of electrons and holes, but do depend also on surface and bulk recombination rates as well.
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  • Caracterización física y química de pastas de cementos óseos con ZrO$_{2}$

    A. Quinto Hernández, Ma. C
    123-0
    Abstract:
    Setting times and temperature of sixteen calcium phosphate cements added with ZrO$_{2}$ were evaluated. Their behaviors were analyzed to be used like injectable formulations in surgery of bone. Two cements of calcium phosphates enriched with ZrO$_{2}$ with the best characteristics in setting times and temperature, were mechanically tested after 1 and 7 days of prepared. Density was determined using a picnometer, chemical composition was determined by X-ray diffraction and the molecular structure was determined by infrared spectroscopy.
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  • Stochastic modeling of some aspects of biofilm behavior

    R.F. Rodríguez, J.M. Zamora, E.Salinas-Rodríguez., E. Izquierdo
    132-0
    Abstract:
    A unified stochastic description of the effects of internal and external fluctuations on the thickness and roughness of a biofilm is given in terms of linear and nonlinear master equations ($ME$). In the absence of detachment the $ME$ is linear, while erosion renders it to be nonlinear. For the linear case the influence of the environment is modeled through an external noise in one of the transition probabilities per unit time and the $% ME$ is solved analytically. For the nonlinear case we only consider internal fluctuations and use van Kampen's systematic expansion to solve the $ME$. In both cases the thickness and roughness dependence on time is calculated and expressed in terms of the first two moments of the probability distribution function. An analytical expression for roughness as a function of thickness is also obtained in both cases. For both cases we compare our analytical results with reported experimental measurements of these quantities for $P$. Aeruginosa. The best fitting values of the transition probabilities and external noise parameters are determined, so that the relative error $% \delta $ between the calculated and the experimentally measured values of the thickness and roughness is minimized. We find that for the linear case the mean relative error $<\delta >$ is relatively small, 1.8 %-6.2 %, while in the presence of detachment is slightly higher, 6.7 %- 9.3 %. We close the paper by discussing the advantages, scope and limitations of our approach.
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  • Caracterización de una señal pulsada propagandose en fibra optica

    J.L. Camas-Anzueto, H.H. Cerecedo-Núñez, E.A. Kuzin, S. Mendoza-Vazquez, .
    144-0
    Abstract:
    In this paper we describe the basic principles of an optical fiber communication system and we show experimental propagation of a pulsed signal. We invest the broadening and attenuation of this pulses under adverse conditions of temperature, belts and twist. We also describe details of the mechanism of generation, propagation and reception of this pulses. For pulse generation it was used a COMBO circuit and two clocks (4.096 MHz and 8 KHz.). For transmission of this pulses it was used a 10 m multimodal optical fiber. Results of experimental analysis shows that pulse codification by this set up is safety and easy to build.
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  • Influence of surface generation velocity and field-enhanced carrier generation on the measured generation lifetime and relaxation time constant in MOS structures

    P. Pe, kov., J. Carrillo, M. Aceves
    150-0
    Abstract:
    Today's high quality semiconductor materials are characterized with generation lifetimes in the range 10$^{ - 3 }$-- 10$^{ - 2}$ sec. This requires re-examination of the influence of some factors on the correct extraction of generation lifetime with the measurement techniques used. Surface generation velocity and field-enhanced carrier generation influence on the measured generation lifetime and relaxation time constant in MOS structures. In the present work, analysis of this influence is presented. It is shown how a simple interpretation of the experimental data can introduce a large error in the determination of these parameters. The influence of all factors must be taken into account.
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  • Límite de detección de un giroscopio de fibra óptica usando una fuente de radiación superluminiscente

    G.E. S, oval-Romero., V.A. Nikolaev
    155-0
    Abstract:
    The main objective of this work is to establish the dependence of characteristics of the fiber optics gyroscope (FOG) with respect to the parameters of the superluminiscent emission source based on doped optical fiber with rare earth elements (Superluminiscent Fiber Source, SFS), argument the pumping rate election of the SFS to obtain characteristics limits of the FOG sensibility. By using this type of emission source in the FOG is recommend to use the rate when the direction of the pumping signal coincide with the superluminiscent signal. The most results are the proposition and argumentation of the SFS election as emission source to be use in the FOG of the phase type. Such a decision allow to increase the characteristics of the FOG sensibility in comparison with the use of luminiscent source of semiconductors emission which are extensively used in the present time. The use of emission source of the SFS type allow to come closer to the threshold of the obtained sensibility limit (detection limit) which is determined with the shot noise.
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  • Simulation of the motion of a sphere through a viscous fluid

    R.M. Valladares, P. Goldstein, C. Stern, A. Calles
    166-0
    Abstract:
    We have designed a friendly program to help students, in the first courses of physics and engineering, to understand the motion of objects through fluids. In this paper we present a simulation of the dynamics of a sphere of arbitrary but relatively small radius through an incompressible viscous fluid. The external forces acting on the sphere are gravity, friction, a stochastic force that simulates microscopic interactions and buoyancy. The Reynolds numbers are small enough to assure unseparated and symmetrical flow around the sphere. The numerical analysis is carried out by solving the equation of motion using the Verlet algorithm. Besides the numerical results, the program includes an interactive animation of the physical phenomenon. Although originally conceived for teaching, the program may be used in research to investigate, among other things, the motion of raindrops or pollutants in the atmosphere.
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  • Transition between quasi 2 and 3D behaviour of the binding energy of screened excitons in semiconducting quantum well structures.

    G.J. Vázquez, M.del Castillo-Mussot., J.A. Re, es., J. Lee, H.N. Spector
    175-0
    Abstract:
    We have calculated the binding energy of screened excitons in a semiconducting quantum well structure as a function of screening parameter and the width of the quantum well using variational wave functions to obtain upper bounds for the energy. The binding energy decreases with increasing values of the screening parameter and with increasing well width. However, as long as the well width is narrow enough so the electrons and holes occupy their lowest-energy subbands, the exciton remains bound even for large values of the screening parameter whenever the electron gas remains nondegenerate.
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  • Magnetic fields of spherical, cylindrical, and elipsoidal electric charge superficial distributions at rotation

    M.A. Avila
    182-0
    Abstract:
    The vector potentials $ A( r)$ produced by spherical, cylindrical, and elipsoidal uniform superficial distributions of electrical charge rotating at a constant angular velocity $ \omega$, are found. This is done by modeling such a distributions as if they were simple bobbins made of $N$ loops of a very thin coil carrying a current $I$ and calculating simply the dipolar potential $ A_{\text{dip}}( r)$ produced by them. Due that in the case of the spherical geometry the potential $ A( r)$ has already been calculated its value is used as a consistence test of the present approach, for the two other geometries the analytical calculation of the potentials is not so trivial by this reason the equalness between $ A_{\text{dip}}( r)$ and $ A( r)$ is proved trough a numerical evaluation of the complex integrals appearing in the Biot-Savart expression for $ A( r)$. The respective magnetic fields generated by these three rotating distributions have an identical structure: they are constant inside the surfaces while outside them they are dipolar-like (nearby to radiation zone). An application of the above results to quark confinement inside hadrons is proposed.
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  • What fuel for a rocket?

    E. N, a.
    191-0
    Abstract:
    Elementary concepts from general physics and thermodynamics have been used to analyze rocket propulsion. Making some reasonable assumptions, an expression for the exit velocity of the gases is found. From that expression one can conclude what are the desired properties for a rocket fuel.
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