Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 65 num. 3 lang. en <![CDATA[SciELO Logo]]> <![CDATA[String percolation in AA and p+p collisions]]> Abstract A brief review of the string percolation model and its results are presented together with the comparison to experimental data. First, an introduction to the quark-gluon phase diagram and the lattice results concerning the confinement and the percolation center domains is done. The interaction of the strings produced in nucleus-nucleus and proton-proton collisions is studied, showing how the string percolation arises. The main consequences of the string percolation, concerning the dependence on the energy and centrality, on the multiplicities and the mean transverse momentum, are obtained comparing with experimental data. the non-abelian character of the color field of the strings forming the cluster is emphasized to reproduce the rise of the transverse momentum with multiplicity and the relative suppression of multiplicities. It is also studied different observables like multiplicity and transverse momentum distributions, dependence with multiplicity and transverse momentum correlations, forward-backward correlations, the strength of the Bose-Einstein correlations, dependence on the multiplicity of J/Ψ production and its possible suppression in p+p collisions at high multiplicity, strangeness enhancement, elliptic flow, and ridge structure are also studied. The comparison with the data shows an overall agreement. The thermodynamical properties of the extended cluster formed in the collision are discussed computing its energy and entropy density, shear viscosity over entropy density ratio, bulk viscosity, sound speed and trace anomaly as a function of temperature, showing a remarkable agreement with lattice QCD evaluations. The string percolation can be regarded as the initial frame able to describe the collective behavior produced in AA and p+p collisions. <![CDATA[Theoretical study of the electron correlation and excitation effects on energy distribution in photon impact ionization]]> Abstract We performed a detailed theoretical study of the electron correlation and core excitation effects on the energy distribution of the ejected electrons in the process of photon impact tunnel ionization. We used the Landau-Dykhne approach to obtain analytical formulas for the transition rate and the energy distribution with included these effects. We have limited ourselves to a non-relativistic domain, in which the rate and distribution are determined by the electrical component of the laser field, while the influence of magnetic one can be neglected. We observed helium and helium like atoms. We have shown that the tunneling ionization mechanism may be understood as the combination of mentioned processes. We considered the case of a monochromatic wave with an elliptically polarized laser field. We compared our results with experimental and shown that ellipticity plays an important role and that inclusion of additional processes significantly influences the transition rate, as well as the energy distribution of the ejected photoelectrons. <![CDATA[Effect of magnetic field and impurities in InAs/GaAs and GaN/AlN self-assembled quantum dots]]> Abstract A theoretical study on the effect of a magnetic field or impurities on the carrier states of self-assembled quantum dots is presented. The magnetic field is applied along the growth direction of the dots and for comparison two systems are considered; InAs embedded in GaAs and GaN in AlN. The electronic states and energy are calculated in the framework of the k ⋅ p theory in 8 bands including the strain and piezoelectric effects. Zeeman splitting and anticrossings are observed in InAs/GaAs, while the field introduces small changes in the nitrides. It is also included a study about hidrogen-like impurities, which may be negative or positive. It is noted that, depending on the type of impurity, the energy of carriers is changed and the distribution of the probability density of the carriers is affected too. <![CDATA[Analysis of the oscillatory liquid metal flow in an alternate MHD generator]]> Abstract The zero-mean oscillatory flow of a liquid metal in an alternate magnetohydrodynamic electric generator is explored analytically. The flow, confined in a two-dimensional insulating wall duct under a transverse magnetic field, is driven by an externally imposed oscillatory pressure gradient. The flow behaviour is analyzed in two different regions. First, asymptotic solutions for low and high oscillating frequencies in the uniform magnetic field region far from the magnet edges, are used to explore the phase lag produced by the Lorentz force between the velocity and the axial pressure gradient. In addition, the entrance flow region where the oscillatory fluid motion interacts with the non-uniform magnetic field is studied. A perturbation analysis of the boundary layer flow in this region reveals that non-linear effects lead to the appearance of steady streaming vortices superimposed on the harmonic flow. The influence of these vortices on the performance of the generator is analyzed. <![CDATA[Generalities on finite element discretization for fractional pressure diffusion equation in the fractal continuum]]> Abstract In this study we explore the application of the novel Fractional Calculus in Fractal Continuum (FCFC), together with the Finite Element Method (FEM), in order to analize explicitly how these differential operators act in the process of discretizing the generalized fractional pressure diffusion equation for a three-dimensional anisotropic continuous fractal flow. The Master Finite Element Equation for arbitrary interpolation functions is obtained. As an example, MFEE for the case of a generic linear tetrahedron in ℝ 3 is shown. Analytic solution for the spatial variables is determined over a canonical tetrahedral finite element in global coordinates. <![CDATA[Congruence kinematics in conformal gravity]]> Abtract In this paper, we calculate the kinematical quantities of the Raychaudhuri equations, to characterize a congruence of time-like integral curves, according to the vacuum radial solution of Weyl theory of gravity. Also, the corresponding flows are plotted for definite values of constants. <![CDATA[Near-field analysis and field transformation applied to a parabolic profile at 5 GHz]]> Abstract We propose a method of near-field analysis and field transformation that is applied to a 1.5 m diameter parabolic reflector at a frequency of 5 GHz. An antenna of such dimensions requires at least a 75 m region to obtain its radiation pattern, and this represents a problem, here arises the necessity to make field transformations like the one presented in this work. Near field is modeled by means of Finite Difference Time Domain Method (FDTD) and current distribution is obtained using the discrete Pocklington equation. Radiation pattern is calculated applying the array factor for parabolic profiles. Results are compared with those obtained by CST Microwave Studio with a very good agreement.<hr/>Resumen Proponemos un método para el análisis en campo cercano y transformación de campo, que es aplicado a un reflector parabólico de 1.5 m de diámetro, a una frecuencia de 5 GHz, una antena de tales dimensiones requiere una región de al menos 75 m para obtener su diagrama de radiación, lo cual representa un problema, aquí es donde surge la necesidad de realizar transformaciones de campo como la presentada en este artículo. El campo cercano es modelado mediante el método de Diferencias Finitas en el Dominio del Tiempo (MDFDT) y la distribución de corriente es obtenida usando la ecuación discreta de Pocklington. El diagrama de radiación es calculado aplicando el factor de arreglo para perfiles parabólicos. Nuestros resultados del análisis en campo cercano y transformación de campo, son comparados contra el modelado de la estructura en CST Microwave Studio, encontrando muy buenas coincidencias entre ambos métodos. <![CDATA[Design and implementation of an experimental Raman spectrometer]]> Abstract Non-invasive medical diagnosis has become popular due to the possibility of detecting illnesses in vivo and in real time this technique, often referred to as “optical biopsy” it comprises several optical techniques such as thermography, diffuse reflectance spectroscopy, optical coherence tomography and Raman spectroscopy among others. Particularly Raman spectroscopy is an optical technique based on the inelastic scattering of light that can detect disease markers, this technique has been successfully used to detect several types of diseases, however the high price of a Raman spectrometer makes it difficult for the medical community to adopt its use as a common diagnostic procedure. In this work, an experimental Raman spectrometer was designed and fabricated from low-cost readily available components. The system was characterized and the Raman spectra obtained was compared to commercial systems. Results show that it is possible to fabricate an experimental Raman system with the desired optical configuration and with some improvements in the selected commercial components, it could be useful for non-invasive medical diagnosis. <![CDATA[A study on the negative ion beam production in the ININ sputtering ion source]]> Abstract To improve the beam brightness produced in the 12 MeV Van de Graaff linear accelerator at Instituto Nacional de Investigaciones Nucleares we studied the beam generation inside the ion source. New 3D particle tracking simulations have been compared with measurements, and the agreement is better than using the traditional approach of only accounting for the primary beam. Among the results, it was observed that the main limit in the generation of intense beams is the suppression of the Cesium production due to space charge on the surface of the ionizers. In addition, the beam dynamics variation due to the erosion of the target inside the cathode has been determined. All these results allow us to find the optimal combinations for beam extraction. <![CDATA[Analytical solution of the bioheat equation for thermal response induced by any electrode array in anisotropic tissues with arbitrary shapes containing multiple-tumor nodules]]> Abstract The Pennes bioheat transfer equation is the most used model to calculate the temperature induced in a tumor when physical therapies like electrochemical treatment, electrochemotherapy and/or radiofrequency are applied. In this work, a modification of the Pennes bioheat equation to study the temperature distribution induced by any electrode array in an anisotropic tissue containing several nodules (primary or metastatic) with arbitrary shape is proposed. For this, the Green functions approach is generalized to include boundaries among two or more media. The analytical solution we obtain in a very compact way, under quite general assumptions, allows calculating the temperature distributions in the tumor volumes and their surfaces, in terms of heat sources, initial temperature and calorific sources at the boundary of tumors. <![CDATA[Numerical analysis of the cathodic material influence on the arc plasma jet]]> Abstract The cathodic arc discharge is a deposition technique widely used to synthesize hard coatings and thin films. The structure of the plasma generated by the electrical discharge and its interaction with neutral particles was studied using numerical simulations. Typical plasma parameters were characterized considering their spatial and temporal dependence, as well as several cathode materials that are commonly used in these systems. For the evolution of the ion density, it was observed the formation of Knudsen layer, and also a dependence of pressure gradients in the global behavior. With respect to the kinetic energy, it was found a deceleration of ions, which is represented by a shock front produced in the plasma−neutrals interaction. On the other hand, the energy releasing was generated due to the heat transference between electrons and ions. The plasma potential follows a behavior, which is similar to that of the ion density, and it is caused by the dynamics of charged particles which is directly affected by the concentration of neutrals and ions. In general, the physical quantities are directly affected by electrical and thermal conductivity of the cathode material. Our results can be applied to understand the plasma phenomena produced in a cathodic arc discharge. <![CDATA[Modelo matemático de difracción en región convergente y divergente de una lente esférica]]> Resumen Usando el método de propagación del espectro angular, se determinaron los modelos matemáticos de difracción óptica asociados a la distribución de amplitud del campo difractado por dos aberturas circulares de diámetros diferentes. Se establece la existencia de un patrón de difracción derecho y otro izquierdo, así como el desfasamiento del campo al propagarse en las regiones de Fresnel (convergente y divergente) de una lente esférica. Se muestran resultados experimentales, así como los generados mediante simulación.<hr/>Abstract The mathematical models of optical diffraction were determined using spectral angular propagation, which are associated with the amplitude distribution of the field diffracted by two circular apertures with different diameters. The existence of a right and left diffraction pattern is established and also the field offset as it propagates in the Fresnel zones (convergent and divergent) of a spherical lens. Herein, experimental and the simulation resoults are shown.