Vol. 52 No. 4 (2006): Revista Mexicana de Física.

Published: 2006-01-01

Articles

  • Optical conductivity the optical conductivity resonance from an exact description of the electronic states around the Fermi energy

    F. Puch, R. Baquero
    301-0
    Abstract:
    In this paper we show that the optical conductivity can be calculated to agree with experiment if the details of the electronic states around the Fermi level are taken into account with some care. More precisely, we present a calculation of the optical conductivity in YBa$_{2}$Cu$_{3}$O$_{7}$ on the basis of an exact (ab initio) three dimensional electronic band structure calculation from which we extract the information on the bands near the Fermi energy that can be associated with the CuO$_{2}$ plane-carrier states. To simulate the superconducting state, we superimpose a gap on these bands alone. On these basis, from the known Kubo-Greenwood formula we calculate the optical conductivity in the normal and in the superconducting state. Our calculation agrees with the experimental result even in the higher part of the frequency spectrum. Our way of calculating the resonance suggests a model of evolution for the bands under the effect of doping consistent with the recent experimental findings that the optical resonance can disappear while the sample remains superconducting. An important conclusion of this paper is that the resonance depends mostly on the details of the electronic band structure. It is enough to take into account the effect of the superconducting transition through a single parameter (the gap). No details on the mechanism are needed, so no mechanism can be tested on this basis. Our calculation suggests a model of evolution for the bands around the Fermi energy under doping that gives some microscopic foundations to the recent experiments that show unambiguously that the resonance cannot be the cause of superconductivity. Most importantly, it indicates how the background is built up and depends on the electronic excitations accessible through values of the energy transfer on a wider interval than the one contributing directly to the resonance. These electronic excitations are determined by the optical transitions allowed. From this point of view, it is an obvious consequence that the background is with small differences, common to all the cuprates having a CuO$_{2}$ plane. But the most important conclusion is that the background contains essentially the same physics as the resonance does, and so it does not have any detailed information on the superconducting mechanism as well, contrary to the conclusions of recent work.
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  • Optimization of an irreversible Carnot engine in finite time and finite size

    G. Aragón-González, A. Canales-Palma, A. Leó% n-Galicia, J.R. Morales-Gómez
    309-0
    Abstract:
    In this work, we consider the class of irreversible Carnot engines that results from combining the characteristics of two models found in the literature: the model in finite time and the model in finite size. The performance of the resulting model, including three irreversibilities, was doubly-optimized in finite time and finite size. The first optimization of power and efficiency, maintaining the thermal conductances fixed, was performed in finite time. Since the optimum time ratio from the first optimization, is the same for both maximum power and maximum efficiency, this means that the model can be newly optimized but now in finite size. Then, the second optimization, maintaining the overall heat transfer coefficient constant, was performed. For both optimizations, analytical expressions for the efficiency that maximizes the power and maximum efficiency were obtained. Changing the order in which partial optimizations were carried out, a remarkable optimal property was obtained: the resources of total contact time and the total area of heat transfer are proportional.
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  • Failure probabilities associated with failure regions containing the origin: application to corroded pressurized pipelines

    J.L. Alamilla, J. Oliveros, J. García-Vargas, R. Pérez
    315-0
    Abstract:
    This work develops expressions to calculate failure probabilities associated with failure regions containing the expected value of random variables in the standard space (origin). These expressions are an extension based on the classical case that calculates failure probabilities associated with non-origin-containing failure regions. A simple form is established to know whether the failure region is origin- o non-origin-containing and to calculate the failure probability associated with the region in question. It is shown through an example of corroded pressurized pipelines that such an extension may be necessary to calculate failure probabilities in practical conditions. Reliability methods analyzed are FORM and directional simulation.
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  • Designing the measurement cell of a swept-field differential aspiration condenser

    A.A. Solis, E. Sacristán
    322-0
    Abstract:
    We present the description of a small-size and low-cost sensor based on the aspiration method that can be used as an ion-mobility spectrometer: the planar swept-field first-order differential aspiration condenser. A mathematical model for the measurement cell of the condenser has been developed, and in this paper a design strategy based on the model is described. A measurement cell has been constructed following this strategy and was used in a prototype aspiration condenser device. Some collector-plate ion-current curves have been measured for gas samples with several different anesthetic gases in different concentrations in order to evaluate the model and the design of the measurement cell. The inverse transform via the truncated singular value decomposition (TSVD) has been applied to the data to obtain ion-mobility spectra. The results suggest that, although the model simplifies the actual physical behavior of the ions, thereby causing some inconsistencies in the mobility spectra, it is still useful in the aspiration condenser's design process. The proposed device is an attractive small-size, cost-effective alternative for ion-mobility gas analysis applications.
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  • Magnetic properties of Fe nanoclusters: \pmb{ ab initio} calculations of Fe$\pmb{_{N}}$, N = 9, 15, 27, 51, and 59

    G. López Laurrabaquio, M. Pérez Alvarez, J.M. Montejano-Carrizales, F. Aguilera-Granja, J.L. Morán-López
    329-0
    Abstract:
    The magnetic properties of iron clusters (Fe$_{N}$, $N=9$, 15, 27, 51, and 59 atoms) at $T= 0$~K with bcc-like structure and bulk parameters are studied using ab initio methods. In these studies we consider the spin-orbit coupling and applied external magnetic fields. The basis set includes wave functions of the s, p, d, and f valence electrons. An analysis of the spin and orbital magnetic moments for every shell of the different cluster sizes is performed. The results obtained in the present work agree with other results reported in the literature and call for additional experiments.
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  • Imaging properties of phase-shifting apodizers

    A. Sauceda, P.J. García Ramírez, L. García-González, J. Martínez-Castillo, L. Herrera-Ma, A. Castro, .
    336-0
    Abstract:
    We discuss the imaging properties of Phase Shifting Apodizers (PSAs) in terms of their Optical Transfer Function (OTF). The capability to transfer spatial frequencies from the input to the output of an optical imaging system, in the presence of focus error, is quantified by inspecting the three-dimensional Modulation Transfer Function (3D-MTF). We have found that, in the search of novel apodizers for increasing the focal depth in imaging optical systems, it is not sufficient to analyze the axial impulse response of that system. Some apodizers, even when they increase the focal depth along the optical axis, provide poor imaging properties with respect to defocus. Yet we identify specific values for some important parameters of the PSA that allow us to achieve high focal depth without severally sacrificing the optical performance of the system. We present computer simulations of the 3D-MTF of the system that incorporates these apodizers as well as the output images that can be obtained with it.
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  • Multi-wavelength images detector for micro-cathodoluminescence analysis

    E. Pérez-Tijerina, I. Gradilla, V. Garcia, R. Machorro, M. Avalos-Borja, O. Contreras
    342-0
    Abstract:
    A novel instrument capable of collecting the entire cathodoluminescence emission spectrum simultaneously with SEM scanning is presented in this work. The cathodoluminescence produced in a standard scanning electron microscope is collected and analyzed. The electronic signal used by the microscope to trigger the electron raster and produce an image is also used by the spectrograph to synchronize spectrum acquisition. At each point in the sample the complete spectrum is acquired. All the data are recorded and saved in an electronic file. With this information it is possible to reconstruct panchromatic and/or monochromatic images of the sample and correlate its optical properties with its microstructure. Instead of an elliptical mirror to collect the light in the conventional CL system, a single optical fiber is used. This new way of acquiring light permits a reduction in the work distance (down to 5 mm) in the microscope, thus achieving a better spatial resolution. Another advantage of this arrangement is that we can simultaneously use other detection modes such as backscattered electrons, x-rays etc. Examples showed here illustrate the capabilities of the instrument.
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  • Lamellar ceramics of Ca$_{2}$SiO$_{4}$ prepared by mechanical activation of powders

    A.B. Cabrera, M.E. Mendoza
    346-0
    Abstract:
    Stoichiometric mixtures of calcium carbonate and silicon oxide have been mechanically activated by milling during several hours (29 and~50) previous to the solid state reaction, in order to obtain dicalcium silicate, Ca$_{2}$SiO$_{4}$. X-ray powder diffraction and differential thermal analysis studies show that the mechanical processing of the powders induces changes in the orientational uniformity of crystallites and the diminution of the decomposition temperature of the calcium carbonate. Microlamellar particles of $\beta$-Ca$_{2}$SiO$_{4}$ were obtained when the solid state reaction was achieved at 1450$^\circ$C, whereas $\beta$-Ca$_{2}$SiO$_{4}$ and $\alpha$'$_{L}$ -Ca$_{2}$SiO$_{4}$ phases were obtained when the reaction took place at 1000$^\circ$C. By means of the powder processing described in this work, we can prepare phase $\beta$-Ca$_{2}$SiO$_{4}$ without any chemical stabilizer and with an unusual lamellar morphology.
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  • Transfección de células por medio de ondas de choque

    E. Armenta Ruiz, A. Varela Echavarría, G. Martínez de la Escalera, A.M. Loske Mehling
    352-0
    Abstract:
    Treatments for inherited and acquired diseases are pointing towards gene therapy, which depends on methods to introduce foreign genetic material into cells. Unfortunately, most of the methods available to allow incorporation of transgenes into the genome are not selective to target a particular population of cells. The present article describes the use of shock waves as a cell -- permeabilization method to load a reporter gene into cultured cells. Shock waves were generated by underwater electrical breakdown and focused on the vials containing cell suspension using an ellipsoidal reflector. Transfection was achieved due to acoustic cavitation generated inside the test vials after passage of each shock wave. The number of transfected cells was still low; nevertheless, our results show the feasibility of cell transfection by underwater shock waves.
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  • Método para la medición de frecuencia usando aproximaciones racionales

    D. Hernández-Balbuena, V.E. T, rsa.
    359-0
    Abstract:
    An alternative method for frequency measurement of highly stable oscillators based on the direct comparison of frequencies, the use of rational approximation, and the Archimedean property of the real numbers is presented. This method allows figure out a level of measurement uncertainty comparable in magnitude to the frequency uncertainty of an atomic oscillator in a very short amount of time. Simulation results based on the mathematical model of the proposed method are presented.
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  • Hydrogen desorption process in Mg$_{2}$Ni hydrides

    J.L. Iturbe-García, B.E. López-Muñoz, R. Basurto, S. Millán
    365-0
    Abstract:
    One of the most interesting intermetallic compounds investigated for energy storage purposes has been the Mg$_{2}$Ni alloy which was prepared by mechanical alloying. Hence it was used here for hydriding-dehydriding implied mechanisms studies. Hydrogenation of milling prepared Mg$_{2}$Ni alloy samples under a 1 MPa H$_{2}$ flux at 473K for 5 min allowed the formation of two Mg$_{2}$Ni-hydrides which were identified by the empirical formulae Mg$_{2}$NiH$_{4}$ and Mg$_{2}$NiH$_{0.3}$ [1]. Dehydriding behavior along the temperature range from 298 to 623K was recorded by Thermo Gravimetric Analysis (TGA). TGA spectrum exhibited two weight decrease peaks. The maximum hydrogen amount desorbed was 3.95 $\pm $ 0.01 wt%. Isotherms between 470 and 600K, every 30K, were obtained by using the same technique, showing a discontinuity which might be associated with a dehydriding process taking place in two steps. Approximately 40-50% of the total H$_{2}$ content seems to be desorbed in the first faster step. The present results might confirm that the two hydrides formed present a differentiated desorption outline.
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  • High resolution (3+1) REMPI and dissociation of acetone via 3p$ \leftarrow $n Rydberg transitions

    E. Mejía-Ospino, I. Álvarez, C. Cisneros
    368-0
    Abstract:
    Measurements of multiphoton ionization and dissociation of acetone are reported in the wavelength range 490.0-510.0 nm at 1 nm intervals. To our knowledge there are no available results of (3+1) REMPI on acetone in this region. The experiments were performed using an Nd:YAG-OPO (optical parametric oscillator) laser system coupled to a time-of-flight mass spectrometer. The experimental results show two dissociation channels of the acetone ion, leading to the products (CH$_{3}$CO$^{ + })$ and (CH$_{3}^{ + })$, the channel CH$_{3}$COCH$_{3}^{ + } \to $ CH$_{3}$CO$^{ + }$ + CH$_{3}^{ }$ being the most favored. The acetone and acetyl ions are observed in the entire wavelength range investigated. Finally, the three-photon resonance multiphoton spectrum of the acetone 3p$ \leftarrow $n Rydberg transition is also reported.
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  • Encriptador experimental retroalimentado de Lorenz con parámetros desiguales

    R. Núñez
    372-0
    Abstract:
    The Lorenz encrypter circuit is characterized and specified to show experimental confirmation that with message and random noise feedback at the transmitter it is possible to achieve a high quality message recovery, even though of transmitter and receiver Rayleigh parameters have a 10% of mismatching, and a channel random noise of the same magnitude of the message. This modified chaos-based communication scheme puts away from the Carroll and Pecora (1991) requirement that says that the circuits to synchronize must be identical. In conclusion, the encryption robustness indicates that the proposed scheme can be built with low cost components and therefore could be appropiate to academics purposes. It is need to performance more studies regarding to electrical noise and long time stability for confidentially to use the circuit in real applications for transmission of messages hidden by chaotic signals.
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  • Caracterización de componentes fotónicos utilizando reflectometría óptica de baja coherencia*

    C. Palavicini, Y. Jaouën, P. Gallion, G. Campuzano
    379-0
    Abstract:
    Optical low-coherence reflectometry has been succesfully applied to the characterization of photonic devices. This non-destructive and versatile technique permits the detection, localization and quantification of scattering discontinuities of optoelectronic devices, yielding an accurate and direct information of the optical properties of the device.
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  • A new type of analog cosine converter

    Leszek Kawecki.
    387-0
    Abstract:
    This paper presents the synthesis and the computer-aided design for a new type of analog dynamic converter which carries out the mathematical operation of the cosine function. The lab investigation results of static and dynamic precision of this converter are also included.
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