Vol. 65 No. 3 May-Jun (2019): Revista Mexicana de Física

A brief review of the string percolation model and its results are presented together with the comparison to experimental data. First, it is done an introduction to the quark-gluon phase diagram and the lattice results concerning the connement and the percolation of center domains. It is studied the interaction of the strings produced in nucleus-nucleus and proton-proton collisions 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. It is emphasized the non-abelian character of the color eld of the strings forming the cluster to reproduce the rise of the transverse momentum with multiplicity and the relative suppression of multiplicities. It is also studied dierent 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 pp collisions at high multiplicity, strangeness enhancement, elliptic ow, and ridge structure. 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 pp collisions.

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 electrical component of the laser field while the influence of magnetic 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.

A theoeritical study on the effect of a magnetic field or impurities on the carries 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 embeded 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 confinement energy of carriers is changed, and the distribution of the probability density of the carriers is affected  too.

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.

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 (MFEE) for arbitrary interpolation functions is obtained. As an example, MFEE for the case of a generic linear tetrahedron in $\mathbb{R}^3$ is shown. Analytic solution for the spatial variables is determined over a canonical tetrahedral finite element in global coordinates.

In this paper we calculate the kinematical quantities possessed by 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 denfinite values of constants.

We proposed a method of analysis and field transformation is applied to a 1.5 m diameter parabolic reflector at a frequency of 5 GHz, an antenna of these 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.

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", 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 a Raman spectroscopy system 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 a custom Raman system with the desired optical configuration for non-invasive optical diagnosis at low costs and portable size.

To improve the beam brightness produced by a Source of Negative Ions by Cesium Sputtering we studied the beam generation in the 12~MeV Vandergraff linear accelerator at Instituto Nacional de Investigaciones Nucleares. Results of 3D particle tracking simulations of the ion source and beamline have been compared with measurements, with better agreement than traditional codes that only take into account the negative beam, and they determine a suppression in the Cs$^{+}$ production due to space charge, which in turn explains the intensity limits for negative beam production in both ionizers, and the best way to overpass them. Also, the beam dynamics variation due to the erosion of the target inside the cathode has been determined, helping to prevent beam losses and enhance the beam brightness.

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 suppositions, 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.

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

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. It establishes the existence of a right and a left diffraction
pattern, as well as the out of phase field as it propagates in the (convergent and divergent) Fresnel zones of a spherical lens. Herein,
experimental and the simulation resoults are shown.