Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 61 num. 6 lang. es <![CDATA[SciELO Logo]]> <![CDATA[<b>Time evolution of spinor perturbations in regular black holes</b>]]> In this report we present results concerning the study of complete time evolution of massless fermion perturbations propagating in several four dimensional regular black hole space-times. The black hole solution arises from coupling gravity with external matter sources, like nonlinear electrodynamics or some phantom fields. After numerical integration we obtain the time evolution profiles for fermion fields in this space-times. By fitting the numerical data, we compute the quasinormal frequencies that characterize the test field evolution at intermediary times, and for late times, we find power law tails, in strong similarity with usual results for non regular black holes. Quasinormal modes was also investigated using a WKB approach at six order beyond the eikonal limit, obtaining a good correspondence with the numerical calculated frequencies. <![CDATA[<b>Phenomenological and microscopic model analysis of elastic scattering reactions</b> <b>of <sup>18</sup>O by <sup>24</sup>Mg, <sup>28</sup>Si, <sup>58</sup>Ni, <sup>64</sup>Zn, <sup>90</sup>Zr, <sup>120</sup>Sn, and <sup>208</sup>Pb target nuclei</b>]]> In the present study, the optical potentials are obtained to describe the interactions of 18O at different incident energies. With this goal, the elastic scattering angular distribution data measured for many systems, ranging from 24Mg to 208Pb are analyzed by using the phenomeno-logical model (PM) and the double folding model (DFM) within the framework of optical model (OM). It is presented that the theoretical results with the PM and the DFM are in very good agreement with both the experimental data and the results of previous studies. A new global set of imaginary potential of the double folding calculations is derived to describe the interactions of 18O at low energies. Also, the volume integrals of potentials, the cross sections and x²/N values obtained by means of the theoretical calculations for each system are given. <![CDATA[<b>A polynomial model of purely affine gravity</b>]]> We present a purely affine gravitational model in four dimensions built up entirely on the bases of full diffeomorphism invariance, and power-counting renormalizability. We show that its non-relativistic limit around a homogeneous and isotropic spacetime yields to a Newtonian gravity. <![CDATA[<b>SNOM characterization of a potential low cost thin gold coated micro-structured</b> <b>grating using a commercial CD substrate</b>]]> In this work near-field optical measurements of a corrugated grating coated with a 30 nm thick gold film are presented. The grating was made using the polycarbonate corrugated substrate of a commercially available recordable CD as template. This has been proved to be a versatile and low cost technique in producing large 1.6 μm period gratings. The study was carried out using a Scanning Near-Field Optical Microscope (SNOM) working in both collection and reflection modes at two different wavelengths, 532 nm and 633 nm. The results illustrate that the intensity patterns of near-field images are strongly polarization-dependent, even showing different periodicity of the localized fields for orthogonal polarization states. When electric field of the light is polarized parallel to the grooves, the periodicity of the SNOM images is coincident with the grating period, whereas when the light is polarized perpendicular to the grooves the SNOM pattern shows a periodicity twice that of the corresponding topography of the grating. Numerical simulations of the SNOM data based on a two-dimensional Finite Difference Time-Domain (2D-FDTD) model have been realized. The results of the simulations are in good agreement with the experimental data, emphasizing the need of performing numerical simulation for the correct interpretation of SNOM data. <![CDATA[<b>Refinement of magnetic domains in FeTbGe<sub>2</sub>O<sub>7</sub></b>]]> Antiferromagnetic polycrystalline FeTbGe2O7 compound presents an ordered magnetic phase when its temperature decreases below the Neel temperature T N = 42 K. By mean of Rietveld refinement of collected neutron diffraction data, the broadened part of the magnetic peaks was modeled in order to analyze the effect of average size of magnetic domains on the evolution of magnetic structure with temperature. The rise of the magnetic structure was found to be sensitive to the shortening of distances along the c-axis between the magnetic atoms located in the bc-sheets of the layered structure. When the magnetic structure is generated, chains along the b-axis of Tb3+-Tb3+ atoms are antiferromagnetically coupled. At the same time, ferromagnetic coupling along the c-axis between Tb3+-Fe3+ atoms located in contiguous chains, couples all the chains along a sheet in the layered structure. The three-dimensional magnetic structure is reached by the ferromagnetic coupling between the set of sheets parallel to bc in the layered structure. A correlation between the size ofthe magnetic domain and the reach of the saturation value for the magnetic moment for Fe3+ is suggested. The magnetic reflections appear below 42 K and were modelled independently from those reflections coming from the crystal structure ignoring the effect of magnetostriction.<hr/>El compuesto policristalino FeTbGe2O7 antiferromagnético presenta una estructura magnética cuando su temperatura desciende por debajo de la temperatura de Neel T N = 42 K. Por medio del refinamiento Rietveld de los datos de difracción de neutrones, se modeló el ensanchamiento de las reflexiones magnéticas con el fin de analizar el efecto del tamaño medio de los dominios magnéticos y la evolución de la estructura magnética con la temperatura. Se encontró que el desarrollo de la estructura magnética es sensible al acortamiento de distancias a lo largo del eje b de átomos magnéticos situados sobre planos paralelos a bc en la estructura laminar. Cuando se genera la estructura magnética, se forman cadenas de átomos Tb3+-Tb3+ a lo largo del eje b que se acoplan antiferromagnéticamente. A su vez, se establece un acoplamiento ferromagnético a lo largo del eje c entre átomos Tb3+-Fe3+ que se sitúan en cadenas contiguas en todas las cadenas que forman una hoja en la estructura laminar. La estructura magnética tridimensional se logra por el acoplamiento ferromagnético entre el conjunto de hojas paralelas a bc en la estructura en capas. Se sugiere una correlación entre el tamaño del dominio magnético y el alcance del valor de saturación para el momento magnético de Fe3+. Las reflexiones magnéticas que aparecen por debajo de 42 K pudieron modelarse independientemente de las reflexiones procedentes de la estructura cristalina, ignorando el efecto de la magnetostricción. <![CDATA[<b>Raman scattering from Ge<sub>1-<i>x</i></sub>Sn<i><sub>x</sub></i> (<i>x</i> ≤ 0.14) alloys</b>]]> Ge1-xSn x alloys with x concentration up to 0.14 were grown on Ge(001) and GaAs(001) substrates in a conventional R. F. Magnetron Sputtering system at low substrate temperatures. The structural characteristics of these alloys were studied for different Sn concentrations between 1 to 14 % by high resolution X ray diffraction, and Raman spectroscopy. Contrasting characteristics of the grown layers are observed if the Sn concentration is larger or smaller than 6 % as revealed by X-ray diffraction and Raman spectroscopy. <![CDATA[<b>A conjecture for the algorithmic decomposition of paths over an <i>SU</i>(3) <i>ADE</i> graph</b>]]> Through a geometric understanding of the creation, cap, annihilation and cup operators for ADE graphs in SU (3) we propose the first steps towards an algorithm that would allow one to write an arbitrary elementary path as an ordered combination of creation and cap operators acting upon an essential path. We propose a sketch of a proof and use our proposal for some examples for the A2 and E5 graphs of the SU (3) family. Attaining this decomposition is an important step in obtaining the path formulation of the quantum Algebra of a modular invariant RCFT. <![CDATA[<b>MHD effects on natural convection laminar flow from a horizontal circular cylinder in presence of radiation</b>]]> In this study, the effect of magnetoliydrodynamic (MHD) on natural convection flow from a horizontal circular cylinder in the presence of radiation has been investigated. The governing boundary layer equations are converted into non-dimensional partial differential equations by using the suitable transformation and then solved numerically by employing an accurate implicit finite difference scheme known as Keller-box method. We presented the influence of emerging non-dimensional parameters namely the MHD parameter M with combination of surface heating parameter θw and radiation-conduction parameter Rd on velocity and temperature profiles, skin friction coefficient and Nusselt number through graphs and tables. It is observed that the Lorentz force reduces the velocity, skin friction coefficient and Nusselt number. Moreover temperature increases in the presence of MHD effect. The streamlines and isotherms reflect some attractive flow patterns which show that magnetic parameter M and radiation parameter Rd have deep influence on these fluid and heat flow patterns. <![CDATA[<b>Dynamics of the population grating formation in erbium-doped fibers</b>]]> Experimental results on formation dynamics of population gratings in erbium-doped fiber (EDF) investigated via two-wave mixing (TWM) are reported. It is shown that at all three utilized essentially different recording wavelengths 1492, 1526 and 1568nm the transient TWM response in EDF with low erbium concentration (≈ 640 ppm) has the characteristic rate significantly larger than that of the population of the Er3+ meta-stable level evaluated from the fluorescence dynamics. Additionally in EDF with high erbium concentration (≈ 5600 ppm) the TWM response is faster (even at low recording power) and the relative TWM amplitude is lower than that in EDF with low concentration. General increase of the TWM response rate is explained by contribution of a transient decrease of the fiber average absorption level, while above-mentioned differences observed in heavily doped EDF - by spatial migration of the excited states. The spectral dependence of the later effect is attributed to the difference in the absorption/emission cross-section ratio of the Er3+ ions at the utilized wavelengths. <![CDATA[<b>Supersymmetric features of the Error and Dawson's functions</b>]]> Following a letter by Bassett, we show first that it is possible to find an analytical approximation to the error function in terms of a finite series of hyperbolic tangents from the supersymmetric (SUSY) solution of the Poschl-Teller eigenvalue problem in quantum mechanics (QM). Afterwards, we show that the second order differential equation for the derivatives of Dawson's function can be found in another SUSY related eigenvalue problem, where the factorization of the simple harmonic oscillator Hamiltonian renders the wrong-sign Hermite differential equation, and that Dawson's second order differential equation possess a singular SUSY type relation to this equation. <![CDATA[<b>Observation of Cosmic Ray at the top of the Sierra Negra volcano in Mexico with the SciCRT prototype</b>]]> We are currently in the process of calibration of a new cosmic ray detector called SciBar Cosmic Ray Telescope (SciCRT) located at the top of the Sierra Negra volcano at 4,600 m.a.s.l., in Eastern Mexico. The SciCRT will work mainly as a Solar Neutron and Muon Telescope, with a high angular resolution (~ 1°), but it will also serve as a gamma ray and hadron shower detector. The mini-SciCR is a prototype of the SciCRT, it uses the same scintillator bars and recording hardware, the size of the mini-SciCR is 1/1568 compared with the SciCRT. In this paper we will report the main results obtained with the mini-SciCR that was operating at the top of the Sierra Negra volcano from October 2010 to July 2012. Our main aims were to show the appropriate performance of all the detector systems and to develop a tecnique to separate the flux of soft and hard secondary cosmic rays with the help of a Monte Carlo simulation, our energy range of interest is from 100 MeV to a few GeV. Aditionally we will report results with a modification of the detector setup that helped to confirm the correct identification of the particle species.