Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 62 num. 4 lang. es <![CDATA[SciELO Logo]]> <![CDATA[Green light emission in aluminum oxide powders doped with different terbium concentrations]]> Abstract: Different emission intensities presented in aluminum oxide phosphors corresponding to different concentrations of doping performed with terbium are analyzed. The phosphors were synthesized by the evaporation technique and were characterized by photo and cathodoluminescence, x-ray diffraction, and EDS techniques for different incorporation percentages of terbium as dopant; they show characteristic transitions in 494, 543, 587 and 622 nm, corresponding to 5D 4 → 7F 6, 5D 4 → 7F 5, 5D 4 → 7F 4 and 5D 4 → 7F 3, respectively when they are excited with λ e x c = 380 nm wavelength at room temperature. The results of XRD show the presence of α-Al 2O 3 phases with peaks located at 2 θ = 25.78, 35.34, 37.96, 43.56, 45.8, 52.74, 57.7, 61.5, 66.74, 68.44, 77.12 and 80.94, and the δ-Al 2O 3 phase 2 θ = 32.82, 45.8, 61.36 and 66.74. These compounds were heat treated for two hours at 1100 ∘ C. EDS analyzes indicate that these compounds have close to 60% oxygen around of 40% aluminum in the presence of terbium as dopant which indicates a stoichiometry close to the expected one for alumina. <![CDATA[Numerical study of unsteady mixed convection stagnation point flow over a stretching cylinder with sinusoidal surface temperature]]> Abstract: The paper provides the analysis of unsteady two-dimensional mixed convection stagnation point flow over a vertical stretching cylinder with sinusoidal wall temperature. The governing partial differential equations are converted into dimensionless form by using suitable transformations. For the numerical solution of dimensionless partial differential equations, an implicit finite difference scheme namely Keller Box method is applied. The comparison is made to show the accuracy of our results with literature for some special cases. Graphs of velocity and temperature profiles are plotted for assisting and opposing flow cases at fixed value of time. The assisting buoyant flow augment the momentum boundary layer while opposing buoyant flow show opposite behavior. The thermal boundary layer thickness grows with the passage of time. Skin friction and Nusselt number are plotted for unsteadiness parameter and amplitude of surface temperature oscillations against time. It is apparent that as the values of surface temperature oscillations drops, the amplitude of oscillations in skin friction and Nusselt number also drops. Furthermore, isotherms are drawn to exhibit the influence of the amplitude of oscillations on curvature parameter with time. <![CDATA[Effect of neutron flux on the frequency dependencies of electrical conductivity of silicon nanoparticles]]> Abstract: It has been reviewed the frequency dependencies of electrical conductivity of nanoparticles affected by neutron flux at different times and initial state, at various constant temperatures such as 100 K, 200 K, 300 K and 400 K. Measurements have been carried out at each temperature at the different 97 values of frequency in the 1 Hz - 1 MHz range. From interdependence between real and imaginary parts of electrical conductivity it has been determined the type of conductivity. Moreover, in the work it is given the mechanism of electrical conductivity according to the obtained results. <![CDATA[Novel balanced diplexer with band design flexibility]]> Abstract: In this paper a novel differential mode diplexer is presented. This circuit allows complete design independence between both bands. It will be shown that the diplexer can be designed to provide single-ended or differential outputs while having a differential input. All circuits are suitable for 2D fabrication. <![CDATA[The Feng’s first integral method applied to the nonlinear mKdV space-time fractional partial differential equation]]> Abstract: In this paper, the fractional derivatives in the sense of the modified Riemann-Liouville derivative and the Feng’s first integral method are employed for solving the important nonlinear coupled space-time fractional mKdV partial differential equation, this approach provides new exact solutions through establishing first integrals of the mKdV equation. The present method is efficient, reliable, and it can be used as an alternative to establish new solutions of different types of fractional differential equations applied in mathematical physics. <![CDATA[Heat transfer over a stretching cylinder due to variable prandtl number influenced by internal heat generation/absorption: a numerical study]]> Abstract: An analysis is carried out to investigate the two dimensional flow of an electrically conducting Newtonian fluid over a permeable stretching cylinder. The main objective of this study is to analyze the thermo-physical properties on the axisymmetric flow with Prandtl variable in the presence of internal heat generation/absorption. Unlike typical studies, the temperature dependent physical properties are discussed in highly coupled velocity and temperature fields. The solution of the problem is computed numerically with Chebyshev Spectral Newton’s Iterative Scheme (CSNIS) and results are shown through graphs for various emerging parameters. Moreover the exchange of energy from the surface to the fluid is examined through variations in Nusselt number. To verify our results, comparisons are made with the published studies which provides authentication of our present results and numerical scheme. <![CDATA[Theoretical studies of the local structure and the epr parameters for tetragonal Cu<sup>2+</sup> center in mixed alkali cadmium phosphate glasses]]> Abstract: The local structure of the Cu2+ center in mixed alkali cadmium phosphate (10Li2O-10Na2O-20CdO-59.5P2O5) glass is theoretically investigated from the optical spectral data and high-order perturbation formulas of the electron paramagnetic resonance (EPR) parameters (g-factors g║,g┴ and the hyperfine structure constants A║,A┴ ) for a 3d9 ion in a tetragonally elongated octahedron. In these formulas, the ligand orbital and spin-orbit coupling contributions are taken into account based on the cluster approach in view of moderate covalency. The required crystal-field parameters are determined by the superposition model. According to the calculations, the impurity-ligand distances parallel and perpendicular to the C4 axis of [CuO6] 10- cluster are found to be, respectively, R ║ ≈2.051 Å and R ┴ ≈ 2.029 Å due to Jahn-Teller effect. Based on the above local structure data, the optical spectra are satisfactorily interpreted. The signs of A║ and A┴ for the Cu 2+ center are suggested in the discussion. <![CDATA[Estudio teórico sobre el cambio de índice de refracción y la absorción óptica en un punto cuántico en presencia de un campo magnético uniforme]]> Resumen Se estudia teóricamente el cambio del índice de refracción lineal y no lineal como también la absorción óptica lineal y no lineal en un punto cuántico cilíndrico (CQD) en presencia de un campo magnético uniforme, usando el formalismo de la matriz densidad y un procedimiento perturbativo. Los niveles de energía y las funciones de onda de un electrón en el CQD se obtuvieron solucionando la ecuación de Schrödinger independiente del tiempo en el marco de la aproximación de masa efectiva. Los cálculos numéricos se efectúan para un CQD de GaAs/GaAlAs. Se encontró que la posición de los picos de resonancia, se desplazan hacia regiones de bajas energías (corrimiento al rojo) cuando la intensidad del campo magnético disminuye y aumentan las dimensiones de la nano-estructura.<hr/>Abstract: We study theoretically linear and nonlinear change of refractive index as well the linear and nonlinear optical absorption in a cylindrical quantum dot (CQD) in the presence of a uniform magnetic field by using the formalism of matrix density and perturbation method. The energy levels and wave functions of an electron in the CQD were obtained by solving the Schrödinger equation independent of time within the effective mass approximation. Numerical calculations are performed for a CQD GaAs/GaAlAs. It was found that the position of the resonance peaks, shifts to regions of lower energies (red shift) when the magnetic field intensity decreases and the dimensions of the nanostructure increases. <![CDATA[Analytical calculation of radiative corrections of a THDM potential]]> Abstract: We obtain a closed form effective potential at the one-loop level of a Two Higgs Doublet Model. Through the loop expansion we reproduce the expression presented by Weinberg and Coleman, showing explicitly every step involved in the calculation. The formalism is then extended to include interaction terms between two scalar doublets and a final expression of the one-loop level contributions is presented. <![CDATA[Diffusion of chemically reactive species in stagnation point flow of a micropolar viscoelastic fluid in a porous medium over a stretching/shrinking sheet]]> Abstract: The transfer of chemically reactive species in stagnation point flow of a laminar micropolar viscoelastic fluid immersed in a porous medium over a stretching/shrinking surface is considered. The reactive species diffused into the fluid from the surface undergo a one stage isothermal and homogenous reaction. A similarity transformation is employed to transform the developed partial differential equations into a system of coupled ordinary differential equations. A convergent series solution is developed using homotopy analysis method in the whole spatial region (0 ≤ η ≤ ∞). The obtained solutions for velocity, microrotation and concentration of species are analysed for various emerging parameters through graphs and tables. <![CDATA[Family of Lamé spheroconal quadrupole harmonic current distributions on spherical surfaces as sources of magnetic induction fields with constant gradients inside and vanishing asymptotically outside]]> Abstract: Constant gradient magnetic induction fields play key roles in Magnetic Resonance Imaging and in Neutral Atom Traps. This communication reports the construction of a family of solutions of the magnetostatic Gauss and Ampere laws in their boundary condition forms, identifying the Lamé quadrupole spheroconal harmonic current distributions on spherical surfaces as sources of magnetostatic fields with constant gradients inside and vanishing asymptotically outside. The advantages of the spheroconal quadrupole sources and fields, which include the familiar spherical harmonic counterparts as special cases, are illustrated analytically and graphically.<hr/>Resumen: Los campos de inducción magnética con gradientes constantes juegan papeles claves en Imágenes por Resonancia Magnética y Trampas de Atomos Neutros. Esta comunicación reporta la construcción de una familia de soluciones de las Leyes de Gauss y Ampere de la magnetostática en sus formas de condiciones de frontera, identificando distribuciones de corriente armónicas esferoconales cuadrupolares de Lamé sobre superficies esféricas como fuentes de campos magnetostáticos de gradientes constantes en el interior y que se anulan asintóticamente en el exterior. Se ilustran analíticamente y gráficamente las ventajas de las fuentes y campos cuadrupolares esferoconales, los cuales incluyen a sus contrapartes armónicas esféricas como casos especiales. <![CDATA[On the birefringence evaluation of single-mode fibers]]> Abstract: Due to its low absorption, short lengths of single-mode fibers are modeled as homogeneous retarders and their birefringence is measured using polarimetric techniques. Usually this characterization is limited to the evaluation of intensity changes of the output polarized signal. These measurements provide information on the retardation rate, described in terms of the polarization beat length but do not provide information on the type of retarder (linear, circular or elliptical) and its polarization eigenmodes. Furthermore, most fibers present residual torsion, a contribution that breaks the periodic evolution of the polarization state associated to the polarization beat length. In this work we present and compare the techniques used to characterize the fiber birefringence emphasizing the changes introduced by the presence of residual torsion and show how to overcome this problem. <![CDATA[Synthesis and crystal structure of three new quaternary compounds in the system (Cu-III-Se<sub>2</sub>)<sub>1-</sub> <em><sub>x</sub></em> ZnSe <em><sub>x</sub></em> (III = Al, Ga, In), formed by Zn incorporation in Cu-III-Se<sub>2</sub> chalcopyrites]]> Abstract: The crystal structure of the chalcogenide alloys CuZnAlSe3, CuZnGaSe3 and CuZnInSe3, new members of the system I-II-III-VI3, were characterized using X-ray powder diffraction data. All materials crystallize in the tetragonal space group P 4 2c (N° 112) with a CuFeInSe3-type structure.