Vol. 66 No. 1 Jan-Feb (2020): Revista Mexicana de Física

We present the relativistic treatment of the Hellmann-generalized Morse potential using Nikiforov-Uvarov(NU) method. The relativistic equations(Klein-Gordon and Dirac equation) were solved using the conventional NU method. In order to overcome the centrifugal barrier, we employed the  well-known Greene and Aldrich approximation scheme. The corresponding normalized eigenfunctions was also obtained in each case. It was shown that in the non-relativistic limits, both energy equations obtained by solving Klein-Gordon and Dirac equations, and wavefunctions reduced to the non-relativisitc energy Equation. The bound state energy eigenvalues for N2, CO, NO, CH and HCl diatomic molecules were computed for various vibrational and rotational quantum numbers. It was found that our results agree with those in literature.

CuInSe₂ I–III–VI₂ ternary semiconductor considered as one of the most promising semiconductor material which considers a very efficient solar energy conversion material. An organometallic pyrolysis method is used to prepare monodisperse CuInSe₂ nanoparticles using a mixture of oleylamine, and trioctylphosphine (TOP) as capping materials. Controlling the particle shape dot, rods or flowers occurs via varying the reaction temperatures (160, 200, 220°C) respectively. The obtained particles have been characterized to determine the shape and size of CuInSe₂ nanoparticles using HR-TEM and XRD. The optical and the electronic properties of these particles have been investigated and discussed in details. Then the different shapes of CIS nanoparticles (nanodots, nanorods, and nanoflowers) were introduced to the DSSC to study their effect on the optical switching properties. It was found that the nanoflowers provide better photovoltaic performance than the other shapes; since it reduces the settling time to 50 milliseconds after it was more than 17 second before adding CIS nanoparticles to the cells.

Atomic and magnetic force microscopy were employed in order to investigate the local magnetic state of individual cobalt clusters electrodeposited onto gold electrodes. The analysis indicated that the diameter of the clusters was similar regardless of the potential formation, or growth stage, but the height and surface coverage changed as the potential augmented. Individual examination of the clusters showed that the height determined the magnetic transition from the single to the multi domain state. By using a theoretical single-domain ferromagnetic model in terms of the clusters dimensions, the magnetic exchange constant at different potential voltages was obtained, which was higher than the cobalt bulk value. In addition, a micromagnetic simulation study was employed to validate the experimental results, and it correctly confirmed the experimental magnetic transition. In addition, a fcc crystalline structure and some insights of the clusters growth mechanism on the gold surface were inferred from the results.

The Mn₂SnSe₄ compound was synthesized by the melt and annealing technique and its structure was refined by the Rietveld method using X-ray powder diffraction data. This compound crystallizes in the olivine-type structure with unit cell parameters a = 12.9028(2) Å, b = 7.9001(1) Å, c = 6.5015(1) Å, V = 662.72(2) ų in the orthorhombic space group Pnma (Nº 62). This olivine structure can be described from a hexagonal close-packing of selenium atoms where manganese atoms occupy ½ of the octahedral sites while thin atoms lay in ⅛ of the tetrahedra.

In this work, we have used a variational technique within the effective mass approximation to study the variation of the photoionization cross section of a donor impurity in a cylindrical GaAs quantum dot with incident photon frequencies and applied uniaxial stress. We have used the dipole approximation and assumed that the barrier potential is infinite. Our results show that the photoionization cross section begins at a finite value and increases with increasing frequency until it reaches a peak and then it decreases gradually, almost exponentially, until it reaches a finite value when it is almost insensitive to any further increase in frequency. Furthermore, for a particular quantum dot length, the photoionization cross section decreases with increasing applied uniaxial stress. We have also noted that the longer the quantum well dot the larger is the photoionization cross section

A preliminary study of the magnetic relaxation in (Bi, Pb)-2223 superconducting ceramics doped with α-Al2O3 nanoparticles is presented, taking as starting point the measurements of the M(t) dependence, both in samples in the form of powder as in pellet. The relaxation of the three types of vortices that can exist inside these materials is analyzed, emphasizing on the intragranular region
where the planar defects exist and, consequently, Abrikosov-Josephson vortices emerge. Finally, interesting results of the comparison between the behavior of the samples in the form of powder and in the form of pellet are shown from the estimation of the pinning energy of the vortices.

A comparative study between the addition of Co₃O₄ micro-particles and nano-particles as densifying dopant of a SnO₂ based varistor system was conducted. The ceramic composition was (99.9-X) %SnO₂–X %Co₃O₄–0.05 %Cr₂O₃–0.05 %Nb₂O₅ where X = 0, 0.5, 1.0, 2.0 and 4.0 mol%. Two particle sizes of Co₃O₄ were used (~5 µm and ~50 nm). The addition of 0.5 mol% of Co₃O₄ nano-particles promoted an increase of grain size of sintered samples up to 7.9 µm, that is, the maximum value among all variations.  Characterization techniques such as TGA, DTA, XRD, and Rietveld analysis revealed a decrease of 16 ºC in the formation temperature of Co₂SnO₄ as well as an increase of 2.6 wt% in the amount of said phase with the use of 4.0 mol% of Co₃O₄ nano-particles in comparison with micro-particles. Statistical analysis indicated that the addition of nano-particles of Co₃O₄ yield better repeatability on densification of ceramic samples. Residual porosity also was decreased. Electrical breakdown and non-linear coefficient values correspond to a non-ohmic behavior with potential application on manufacture of high voltage varistors. The findings of this work can be used as a reference for conducting a later study to improve the electrical properties or even to lower the sintering temperature.

En el presente trabajo es aplicada una técnica numérica 2D acoplada de Dinámica de Fluidos Computacional (CFD) para reproducir, estudiar y analizar los proceso de combustión turbulenta de sprays. El método utilizado es el Promediado de los Esfuerzos de Reynolds de las Ecuaciones de Navier-Stokes (RANS) acoplado al modelo de combustión de flamas en contraflujo (flamelets) a través de una función de densidad probabilística (PDF) definida por el usuario. La correcta simulación del proceso de combustión está basada en el modelo experimental desarrollado por el NIST y los resultados experimentales reportados por Widmann y Presser [1]. Los resultados obtenidos de la simulación son comparados con los resultados experimentales de velocidad a 9.5 y 17.6 mm del inyector en la dirección axial. Estos resultados muestran correspondencias cerradas con los datos experimentales de los perfiles de velocidades radial, axial y tangencial a lo ancho de la cámara de combustión en el intervalo de 14 a 50mm. Además de la velocidad del flujo también se compara la distribución, tamaño y velocidad de las gotas de combustible, así como la composición de los gases de escape en relación a los NOx que se producen. De acuerdo a los resultados la diferencia en la distribución de gotas sobre el ángulo de dispersión del spray afecta al resto de características del spray. La diferencia en el diámetro promedio de Sauter sugiere que posiblemente la velocidad de evaporación de las gotas de mayor tamaño es sobreestimado. Finalmente se confirma que, dada la suposición sobre la velocidad de reacción rápida, el modelo de combustión predice que la combustión es prácticamente completa, aunque se tengan gotas cuya evaporación se logra más allá de la zona de reacción.

In this work, the experimental and numerical results from the study of the effects caused in a submerged jet flow by the change in the wall thickness of the circular tube from which said flow originates are presented. For small values of the Reynolds number Re (Re\approx0.11), four cases, regarding the ratio of pipe wall thickness to its radius, are considered: (I) pipe thickness is a fourth of the radius, (II) pipe thickness is a half the radius, (III) pipe thickness is equal to the radius and (IV) pipe thickness is three times the radius. The Particle Image Velocimetry (PIV) technique was used in order to obtain experimentally the velocity and streamlines distributions. A numerical code based on the finite difference method was developed to solve the motion governing equations and the numerical results were compared with the values obtained experimentally.

 

Massless fermions on scalar domain walls are considered. Two walls are established, corresponding to 5-dimensional static spacetime asymptotically Anti de-Sitter, differentiated  by the symmetry around the wall, and in each case massless chiral fermions are coupled to the wall by a Yukawa term. We identify a normalizable state associated to the migration of fermions toward the edge of the wall. This effect is generated by the competition between the Yukawa interaction and the gravitational repulsion on the matter fields, and it is independent of the $Z_2$ symmetry of the wall.

We have presented the single particle spectrum for a particle in a mean field of isotropic harmonic oscillator with l.s  coupling based on our semiclassical approach. It has been seen that this spectrum, without l.s  coupling, exactly matches with the quantum mechanical one (without nuclear constraints). In this case, periodicity conditions give only pendulating orbits coinciding with l=0 axis, which fully support the observations reported by Bohr and Mottelson [28]. The orbits with l>0 are generated by reflecting the particle from the nuclear surface, instead of infinity, which is the usual nuclear constraint. The mean field strength is fixed by virial theorem. The resulting spectrum compares reasonably with the quantum spectrum for a particle enclosed in a perfectly reflecting walls. The variation of particle number with energy help us to identify the significant quantum numbers n and l in this semiclassical method. Finally, the l.s coupling splits each level and the splitting width of these level compares well with that of nuclear splitting. Thus the complete nuclear shell model (with magic numbers) is reproduced without any fitting parameter.

This paper shows a novel design of a gas sensor system based on artificial neural networks and Floating-gate MOS Transistors (FGMOS). Two types of circuits with FGMOS transistors of minimum dimensions were designed and simulated by Simulink of Matlab; simulations and experimental measurements results were compared obtaining good expectations. The reason of using FGMOS is that ANN can also be implemented with these kinds of devices, since ANN’s based on FGMOS are able to produce pseudo Gaussian-functions. These functions give a reliable option to determine the gas concentration. A sensitive thin film can be deposited on the FGMOS’s floating gate, which produces a charge variation due to the chemical reaction between the sensitive layer and the gas species, modifying the threshold voltage thereby a correlation of drain current of the FGMOS with gas concentration can be obtained. Therefore, a generator circuit was implemented for the pseudo Gaussian signal with FGMOS. This system can be applied in environments with dangerous species such as CO₂, CO, methane, propane, among others. Simulations demonstrated that the implemented proposal has a good performance as an alternative method for sensing gas concentrations, compared with conventional sensors.

This paper presents an application of a Fractional Order Time Delay Neural Networks to chaos synchronization. The two main methodologies, on which the approach is based, are fractional order time-delay recurrent neural networks and the fractional order  inverse optimal control for nonlinear systems. The problem of trajectory tracking is studied, based on the fractional order Lyapunov-Krasovskii and Lur’e theory, that achieves the global asymptotic stability of the tracking error between a delayed recurrent neural network and a reference function is obtained. The method is illustrated for the synchronization, the analytic results we present a trajectory tracking simulation of a fractional order time-delay dynamical network and the Fractional Order Chua’s circuits

Allotropic carbon forms such as fullerenes, nanotubes and graphenes, have received a notable attention by the scientific community due to their unique configuration and interesting chemical and physical properties. In this respect, in this work, carbon nanotubes were synthesized through chemical vapour deposition. The reaction was carried out in tubular furnace by the catalytic reaction of ferrocene/ethanol solution onto an Inconel 600 surface and by the variation on the synthesis conditions for their production. The presence of double- and few-walled nanotubes grown in bundle arrays was observed. However, additionally, the presence of other carbon structures such as graphenes, graphitic onions and graphene nanoribbons were observed through electronic microscopy.

The purpose of this study is to develop a method of calculating the vibration partition function of diatomic molecules for the Morse potential energy. After a brief introduction about the eigensolutions obtained for the problem in question, Via the Euler-Maclaurin formula, we have determined the thermal properties for four diatomics such as \text{H}_{2}, HCl, LiH, and CO. Different situation has been exposed and explained by the appropriate curves of the thermal properties for these diatomics molecules in consideration. In addition, we have shown that our method exposed to calculate these thermal properties can be used to determine these thermodynamic quantities.

Rev. Mex. Fis. 66 (1) 2020

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