Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 63 num. 6 lang. es <![CDATA[SciELO Logo]]> <![CDATA[Levitation forces between a finite rectangular superconductor and a spherical magnet]]> Abstract A model to describe the interaction between a spherical permanent magnet and a square superconductor in the mixed state was elaborated. The proposal is based on image dipole ideas as is the case of the Frozen dipole model that has been used in the literature. More precisely, a frozen dipole was put in the center of the superconducting sample; this dipole “was born" when the first critical field was reached in that position. This led to the attractive force magnet-superconductor in the penetrated-field state. In the description of the Meissner state, the demagnetizing field was not included in the calculation. Both vertical and horizontal forces are discussed. <![CDATA[Transformada discreta de ambigüedad]]> Resumen En este trabajo se presenta un algoritmo de cómputo para calcular la Transformada Discreta de Ambigüedad y mediante sus representaciones en el espacio tiempo-frecuencia caracterizar y analizar las señales temporales producidas por vibraciones mecánicas. Partiendo de la igualdad de Riemann entre la integral acotada de una función continua y el límite al infinito de la sumatoria de la misma función se obtiene una versión discreta de la transformada. El programa fue escrito para software de computo simbólico en MATHEMATICA 10 ©, que con su transformada de Fourier compilada permite reducir el tiempo de cómputo. Se caracterizan y analizan los registros de vibración del eje de las máquinas de rotación, se comparan con funciones diseñadas compuestas de sinusoides de frecuencias constantes y moduladas linealmente. La metodología seguida es útil y eficiente para conocer la composición estructural y el comportamiento de señales de vibración mecánica.<hr/>Abstract In this work, a computer algorithm is presented to calculate the Discrete Ambiguity Transform and through its representations in the time-frequency space to characterize and analyze the temporal signals produced by mechanical vibrations. Starting from the equality of Riemann between the bounded integral of a continuous function and the limit to infinity of the sum of the same function, a discrete version of the transform is obtained. The program was written for symbolic computing software in Mathematica 10 ©, which with its compiled Fourier transform allows reducing computation time. The vibration registers of the axis of rotating machines are characterized and analyzed then compared with designed functions composed of sinusoids of constant and linearly modulated frequencies. The methodology followed is useful and efficient to know the structural composition and behavior of mechanical vibration signals. <![CDATA[Synthesis of poly (3-hexylthiophene-2,5-diyl) in presence of CdS nanoparticles: microscopic and spectroscopic studies]]> Abstract In this paper, the synthesis of nanocomposites of poly(3-hexylthiophene-2,5-diyl)) (P3HT) and cadmium sulfide (CdS) nanoparticles are reported. CdS nanoparticles were first synthesized using chemical precipitation. Then P3HT was synthesized by direct oxidation of 3-hexylthiophene with FeCl3 as oxidant in presence of CdS nanoparticles. The goal of this work was to investigate the effect of the CdS nanoparticles during the synthesis of P3HT. The resulting films of P3HT/CdS nanocomposites were investigated by Fourier Transform Infrared Spectroscopy (FT-IR), Ultraviolet-Visible Spectroscopy (UV-Vis), X-Ray diffraction, Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Homogeneous distribution of CdS nanoparticles in P3HT was demonstrated by SEM, AFM and TEM. FTIR analysis showed interaction between CdS and the S atoms of the thiophene rings. This result, together with UV-Vis spectra and XRD patterns suggest a better arrangement of the polymer chains. It is possible that the CdS nanoparticles are coupled with the unpaired electrons of S atoms in the thiophene rings through the positive delocalized charge, resulting in a more ordered P3HT polymer matrix with embedded CdS nanoparticles. <![CDATA[Investigations of the spin hamiltonian parameters and local structure for Cr<sup>3+</sup> in K<sub>2</sub>PdCl<sub>4</sub> single crystal]]> Abstract The spin Hamiltonian parameters (SHPs) (g factors and zero-field splittings (ZFSs) D and E) and the local structure for Cr3+ in K2PdCl4 single crystal are theoretically investigated from the perturbation calculations of the SHPs for an orthorhombically distorted octahedral 3d3 cluster. The impurity Cr3+ is found not to occupy the conventional square planar Pd2+ site but to locate at the octahedral K+ site, associated with the relative axial compression ratio ρ( ≈ 11.1%) and the perpendicular bond length variation ratio τ( ≈ 11.7%). The impurity centre exhibits much smaller axial compression distortion and much larger perpendicular orthorhombic distortion than the host K+ site in K2PdCl4 ρ H ≈ 18.6% and τ H ≈ 0) and hence a more regular ligand octahedron, corresponding to the very small ZFSs. The contributions to g-shifts Δ g C T from the CT mechanism are opposite in sign and twenty-one times larger in magnitude compared with Δ g C F from the CF mechanism. For the ZFSs, the CT contributions DCT and ECT are opposite in sign and almost the same in magnitude with respect to the corresponding DCF and ECF. <![CDATA[Orientational relaxation for small wavevectors of rod-like Brownian particles]]> Abstract We present a theorical study of the kinetic orientational relaxation of a colloidal monodispersed system of stiff and long rods, for small wavevectors. This process is described by the dynamic structure factor. The relationship between this factor with the fluctuations of the second order parameter is provided in the limit of wavevectors going to zero. The second order parameter is computed with the Smoluchowski formalism, neglecting hydrodynamic interactions, and using the Maier-Saupe potential for the rods interaction. Considering suitable properties, the isothermal orientation susceptibility proposed is able to localize the isotropic and nematic concentrations in qualitative agreement with well known computer simulations results. At relaxation our model predicts a single value for the maximum of the orientation property proposed, which it is reached at different times, according to the concentration. <![CDATA[Electromagnetic quasinormal modes of five-dimensional topological black holes]]> Abstract We calculate exactly the QNF of the vector type and scalar type electromagnetic fields propagating on a family of five-dimensional topological black holes. To get a discrete spectrum of quasinormal frequencies for the scalar type electromagnetic field we find that it is necessary to change the boundary condition usually imposed at the asymptotic region. Furthermore for the vector type electromagnetic field we impose the usual boundary condition at the asymptotic region and we discuss the existence of unstable quasinormal modes in the five-dimensional topological black holes. <![CDATA[Glucose in aqueous solution thermal characterization by photopyroelectric techniques]]> Abstract Glucose is an important molecule, it is responsible for providing energy to cells in our body, in human glucose appears in blood, excessive glucose levels in the blood are an indicator of diabetes mellitus, therefore it is important to measure it. To test the feasibility of photothermal techniques to measure glucose, thermal properties of glucose in aqueous solution were determined in the 0.7 to 3.2 mg/ml range. Thermal effusivity diminishes as glucose concentration increase. Obtained results show that used setups can measure glucose concentrations up to 600 μg/ml. Full thermal characterization for water-glucose mixtures is presented in this work. <![CDATA[The universal optimal relations of the allocation and effectiveness of the heat exchangers for power plants with <em>n</em> Carnot-like cycles]]> Abstract A model of irreversible Carnot-like power plant with n Carnot-like cycles is optimized. The irreversibilities of each cycle are: finite rate heat transfer between the working fluid and the external heat sources, internal dissipation of the working fluid, and heat leak between reservoirs; is extended to two or more of this combined model. Applying the Bellman’ Principle, we find the optimal recurrence relations for the allocation of the heat exchangers for thes power plants. The optimal allocation or effectiveness is determined by two design rules, applied alternatively: internal thermal conductance fixed or areas fixed. The optimal recurrence relations obtained for this combined model are invariant to the power and efficiency and to the heat transfer law.<hr/>Resumen Se optimiza un modelo de planta de potencia irreversible con n ciclos tipo Carnot. Las irreversibilidades de cada uno de los ciclos son: transferencia finita de calor entre el fluido de trabajo y los depósitos de temperatura, disipación del fluido de trabajo y fuga de calor entre ambos depósitos; es extendido a dos o más de este ciclo combinado. Aplicando el Principio de Bellman, encontramos las relaciones de recurrencia óptimas para la dimensión y efectividad de los intercambiadores de calor para modelo combinado. La óptima dimensión y efectividad se determina mediante dos reglas de diseño, aplicadas alternativamente: conductancias térmicas restringidas ó área total constante. Las relaciones óptimas para la dimensión ó efectividad obtenidas para este modelo combinado son invariantes a la potencia y eficiencia y a la ley de transferencia de calor. <![CDATA[Energy on a timelike particle in dynamical and electrodynamical force fields in De-Sitter space]]> Abstract In this study, we firstly define equations of motion based on traditional Newtonian mechanics in terms of the Frenet-Serret frame adapted to the worldline of the moving particle in De-Sitter space. Then, we compute energy on the moving timelike particle in resultant force field using geometrical description of the curvature and the torsion of the worldline belonging to the particle in the space. We also investigate the relation between energy on the moving timelike particle in different force fields and energy on the moving timelike particle in the Frenet-Serret vector fields. <![CDATA[Effect of annealing atmosphere on optic-electric properties of ZnO thin films]]> Abstract In this work the study of the structural, morphologic characteristics, optical and electrical properties of the thin films of ZnO in temperatures and annealing atmospheres different was realized. The films were obtained by the sol-gel method, utilizing zinc acetate dihydrate as the precursor, monoethanolamine (MEA) as a stabilizing agent and 2-methoxyethanol as a solvent and deposited by spin-coating. The films were crystallized at 600, 800 and 1000 ∘ C in oxygen and nitrogen atmospheres. The results obtained by XRD, SEM, photoluminescence and Hall effects of the ZnO films were related and depend strongly on the temperature and atmosphere annealing. <![CDATA[Distribution of charge particles confined between three interfacial surfaces]]> Abstract We present a model of a charged membrane where the charge density is distributed in a region of thickness d m . The model consists of three flat regions having the same dielectric constant were charged particles can be distributed with cylindrical symmetry. The concentration profile of particles and their pair correlation functions were calculated for various parameters of the model (distance and charge density). The particles profiles, at the limit of large distances and small charge densities, are equal to those found in the solution of the Poisson-Boltzmann equation. For high charge density, the contact profiles show a significant structure, and they are different to those found by the Poisson-Boltzmann solution and for a model of stiff membranes. These results indicate that a model of membrane with thickness d m (internal structure) may be necessary to study the effects of pressure between the surfaces. <![CDATA[The 1-dimensional confined harmonic oscillator revisited]]> Abstract We study the size effect on the energy levels of the 1-dimensional harmonic oscillator confined within a box of with impenetrable walls and large L. We use the particle in a box basis set to diagonalize the Hamiltonian of the confined harmonic oscillator. In this way we obtain the energy eigenvalues and eigenfunctions as a functions of L. We compare our numerical results with those reported in literature finding good agreement with the exact ones. <![CDATA[Entropía de discos rígidos auto-impulsados como parámetro de orden para peatones en contraflujo]]> Resumen En este trabajo se estudia el comportamiento colectivo de un conglomerado de peatones en contraflujo sobre un corredor. Los peatones individuales se modelan como discos rígidos auto-impulsados con interacciones locales de evasión. Se consideran tres preferencias de caminado de los peatones: estándares, relajados y apresurados para incluir heterogeneidad en el flujo promedio de caminado del conglomerado. Hemos encontrado que los patrones espaciales muestran dos etapas hacia la formación de veredas. Una etapa corresponde a la segregación de peatones por contraflujo con la subsecuente formación de veredas globales denominadas patrones de auto-organización. La otra etapa se presenta como un segundo proceso de segregación de peatones debido a las diferentes preferencias de caminado. Se considera la evolución temporal de la entropía como una medida de la auto-organización del sistema con relación a una competencia orden-desorden. Esta entropía se evalúa a partir de la función de distribución de velocidades de caminado de los peatones individuales considerando un intervalo alrededor de la densidad media de ocupación del corredor. La entropía de los conglomerados heterogéneos resulta ser mayor que cualquiera de los conglomerados homogéneos.<hr/>Abstract In this paper we study the collective behavior of a counter-flow pedestrian crowd on a corridor. Individual pedestrians are modeled as self-propelled hard disks with local evasive interactions. Three walking preferences for pedestrians are considered: hurry, standard and relaxed in order to include heterogeneity into the average flow of the crowd. We have found that spatial patterns show two stages towards the walking line formation. One stage corresponds to the segregation of pedestrians due to counter-flow conditions with the subsequent formation of global lines, the so called self-organization patterns. The other stage is presented as a second process of pedestrian segregation due to different walking preferences. The temporal evolution of entropy is introduced as a measure of self-organization of the system according to an order-disorder relation. This entropy is obtained from the velocity distribution function of walking of the individual pedestrians considering an interval around the medium density of the corridor occupation. <![CDATA[The fundamental absorption edge of CuGa<sub>3</sub>Te<sub>5</sub> ordered defect semiconducting compound]]> Abstract The analysis of the temperature variation of the optical absorption spectra of the ordered defect compound CuGa3Te5, a semiconducting material which crystallizes in a chalcopyrite-related structure with space group P 4 ¯2c, is made. It has been established that this compound has a direct-allowed band gap between parabolic bands which varies from 1.187 to 1.090 eV in the temperature range from 10 to 300 K. The mean temperature of the phonon involved in the direct band-to-band transition is θ ≈ 125 K. This is comparable with 3/4 θ D ≈ 156K, θ D being the Debye temperature of the compound. <![CDATA[Family of Lamé spheroconal quadrupole harmonic current distributions on spherical surfaces of magnetic induction fields with constant gradients inside and vanishing asymptotically outside [<em>Rev. Mex. Fis.</em> 62 (2016) 362-368]]]> Abstract The analysis of the temperature variation of the optical absorption spectra of the ordered defect compound CuGa3Te5, a semiconducting material which crystallizes in a chalcopyrite-related structure with space group P 4 ¯2c, is made. It has been established that this compound has a direct-allowed band gap between parabolic bands which varies from 1.187 to 1.090 eV in the temperature range from 10 to 300 K. The mean temperature of the phonon involved in the direct band-to-band transition is θ ≈ 125 K. This is comparable with 3/4 θ D ≈ 156K, θ D being the Debye temperature of the compound.