Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 63 num. 5 lang. en <![CDATA[SciELO Logo]]> <![CDATA[On the decay of an accelerated proton]]> Abstract We compute the decay rate width of the strong decay p → n + π + for a linearly accelerated proton in both the inertial frame and in the coaccelerated proton frame. In this last reference system we use the Unruh effect, where the proton sees a bath of thermal particles at the temperature T = a / 2 π, where a is proton’s acceleration. Analytical results agree, thus giving a simpler example where the Unruh effect is necessary to keep the consistency between inertial and Rindler frame calculations of a physical observable. <![CDATA[Electromagnetic transmittance in alternating material-metamaterial layered structures]]> Abstract Using the transfer matrix method we examine the parametric behavior of the transmittance of TE and TM electromagnetic plane waves propagating in frequency range which are far from the absorption bands of a periodic multilayered system. We focus on the dependence of the transmittance on the frequency and angle of incidence of the electromagnetic wave for the case in which the periodic structure comprises alternating material-metamaterial layers of various permittivities and permeabilities. A specific example of high transmittance at any angle of incidence in the visible region of the spectrum is identified.<hr/>Resumen Usando el método de la matriz de transferencia examinamos la dependencia paramétrica de la transmitancia de las ondas planas electromagnéticas TE y TM en rangos de frecuencia lejanos de las bandas de absorción de un sistema multicapas periodico. Nos enfocamos en la dependencia de la transmitancia de la frecuencia y el ángulo de incidencia de la onda electromagnética para el caso en el cual la estructura periodica contiene una alternancia de capas de material y metamaterial de varias permitividades y permeabilidades. Un ejemplo especifico de transmitancia alta para cualquier ángulo es identificado en la región del espectro visible. <![CDATA[Stable bound states of <em>N</em>’s, <mml:math> <mml:mi mathvariant="bold-italic">Λ</mml:mi> </mml:math>’s and <mml:math> <mml:mi mathvariant="bold-italic">Ξ</mml:mi> </mml:math>’s]]> Abstract We review our recent work about the stability of strange few-body systems containing N’s, Λ’s, and Ξ’s. We make use of local central Yukawa-type Malfliet-Tjon interactions reproducing the low-energy parameters and phase shifts of the nucleon-nucleon system and the latest updates of the hyperon-nucleon and hyperon-hyperon ESC08c Nijmegen potentials. We solve the three- and four-body bound-state problems by means of Faddeev equations and a generalized Gaussian variational method, respectively. The hypertriton, Λ n p ( I ) J P = ( 1 / 2 ) 1 / 2 +, is bound by 144 keV; the recently discussed Λ n n ( I ) J P = ( 1 / 2 ) 1 / 2 + system is unbound, as well as the Λ Λ n n ( I ) J P = ( 1 ) 0 + system, being just above threshold. Our results indicate that the Ξ N N, Ξ Ξ N and Ξ Ξ N N systems with maximal isospin might be bound. <![CDATA[Energy density profile inspired by noncommutativity]]> Abstract An important consequence which comes from noncommutativity (NC) is undoubtedly the energy density characterized by a microscopic free parameter; indeed a Trans-Planckian parameter. However, its functional form is an interesting and useful equation which can be analyzed in astrophysical scenarios giving now astrophysical constraints. In this sense, this paper is devoted to explore the astrophysical consequences of an energy density with the same functional form of NC; mainly in stellar dynamics and rotation curves of galaxies. We start exploring toy models of stars with incompressible and polytropic fluids respectively, with the addition and coexistence with this new fluid. In both cases, we propose an appropriate constriction based on the difference between a correct and an anomalous behavior. As a complement, we explore the rotation curves of galaxies assuming that the halo is a fluid with the same characteristic of a NC equation, obtaining the range of values for the free parameter through the analysis of eighteen LSB galaxies. Our results are compared with traditional models studied in literature like Pseudoisothermal (PISO), Navarro-Frenk-White (NFW), Burkert and WaveDM dark matter models. Finally, we have computed the surface density, ρ i r i for each dark matter model, where i is for PISO, NFW, Burkert, WaveDM and NC macroscopic version. In the later case, following the results found using SPARC galaxy catalog, we have found a theoretical value of 116.97 M ⊙ pc-2 while the data analysis gives us a value of 144.21 M ⊙ pc-2. The values of the surface density ρ i r i are roughly constant and their mean values depend on the dark matter model. Also we have computed the mass of each dark matter model within 300 pc and found that there is a common mass for spiral galaxies of the order of 107 M ⊙, that is in agreement with the results for dSph Milky Way satellites. This would give a central density for the halo of ~0.1 M ⊙ pc-2 independent of the dark matter model. <![CDATA[Transparent conductive carbon nanotube films]]> Abstract This work presents the results of a study of the fabrication of multiwall carbon nanotubes (MWCNTs) by chemical vapour deposition (CVD), the purification and the development of transparent conductors films (TCFs) using these nanotubes. The as produced MWCNTs had some impurities due the CVD method including small amounts of the iron catalyst, amorphous carbon and hydrocarbon residues from the organic precursors. These impurities have been reported to be detrimental to both the electrical and optical properties of the TCFs. For this reason the purification of the nanotubes was studied using 3 different techniques. The three methods involved; A) selective oxidation in air of the MWCNTs followed by a hydrochloric acid solution etch to remove the iron, B) a selective oxidation using H2O2 followed by the hydrochloric acid etch treatment, and C) an oxidation-etch procedure using a mixture of H2O2 and hydrochloric acid. The best result results were obtained using the combined H2O2 and hydrochloric acid treatment. With this method MWCNTs were obtained with a purity of up to 94%. The purified MWCNTs were dispersed in solutions of different solvents without surfactant. Thin films of MWCNT were fabricated using a combined spray and spin coating technique, which produced a uniform deposit of controllable thickness. Either a transparent nitrocellulose-based varnish or acrylic was used to fix the film to the glass substrate. Both the optical transparency and the electrical square resistivity of the samples were measured by spectrophotometry and standard electrical techniques, respectively. Our results were similar to those of other groups in that the transparency was almost inversely proportional to the resistivity. <![CDATA[On the kinetic arrest of martensitic transformation in Ni-Mn-In melt-spun ribbons]]> Abstract Kinetic arrest (KA) of martensitic transformation (MT) has been observed in as-solidified Ni52.2Mn34.3In13.5 melt-spun ribbons; a characterization by dc magnetization measurements was carried out. These alloy ribbons transform martensitically from a single austenitic (AST) parent phase with B2-type crystal structure. For an applied magnetic field μ 0 H of 1 T and up to 9 T, a moderate but progressive KA of the MT is observed. The metastability of the non-equilibrium field-cooled glassy state was characterized by introducing thermal and magnetic field fluctuations. It was found that the total magnetization difference ( Δ σ) between the zero field-cooling (ZFC) and field-cooling (FC) pathways of the temperature dependence of magnetization Δ σ( T) shows irreversible and reversible components; the former decreases as the temperature decreases. After a short thermal annealing of 10 min at 1073 K AST shows a highly ordered L21-type crystal structure and the kinetic arrest of martensitic transformation is no longer observed suggesting a connection between the crystal structure order of austenite and the appearance of the kinetic arrest phenomenon. <![CDATA[Study of the critical-fields and the thermal broadening in polycrystalline Ag<sub>2</sub>FeGeSe<sub>4</sub> semiconducting compound]]> Abstract We show that a phenomenological model based on sublattice magnetization describes the temperature and field dependent magnetism in the Ag2FeGeSe4 semiconductor compound with wurtz-stannite-type structure. The model successfully finds the antiferromagnetic (AF), spin flop (SF) and paramagnetic (P) phases for all magnetization curves below the Néel temperature. The Langevin classical function instead of the Brillouin one is used in the analysis of the phase transitions to take into account the randomness of the magnetic moments given the polycrystalline nature of the samples. The critical-fields and the thermal broadening of the phase transitions were also found. The model was tested in the Ba3Cu3In4O12 and Ba3Cu3Sc4O12 compounds, and was successfully identified AF, SF and P phases in these materials.<hr/>Resumen Hemos demostrado que un modelo fenomenológico basado en la magnetización de la subred describe el magnetismo dependiente del campo y la temperatura en el compuesto semiconductor Ag2FeGeSe4 con estructura tipo wurtz-estanita. El modelo exitosamente encuentra las fases anti ferromagnéticas (AF), espín-flop (SF) y paramagnética (P) para todas las curvas de magnetización por debajo de la temperatura de Néel. Se utiliza la función clásica de Langevin en lugar de la función de Brillouin en el análisis de las transiciones de fases a fin de tomar en cuenta la aleatoriedad de los momentos magnéticos dada la naturaleza policristalina de las muestras. También se encontraron los campos críticos y los anchos térmicos de las transiciones de fases. El modelo fue probado en los compuestos Ba3Cu3In4O12 y Ba3Cu3Sc4O12 y se identificaron exitosamente las fases AF, SF y P en estos materiales. <![CDATA[The initial value problem method for time-dependent harmonic oscillator]]> Abstract The initial value problem method is formulated to calculate the propagator for time- dependent harmonic oscillators. The method is based on finding the initial position operator from Heisenberg equations. The investigated models in this paper are the damped harmonic oscillator, the harmonic oscillator with strongly pulsating mass, and the harmonic oscillator with mass growing with time. The comparison of the initial value problem method with Feynman path integral and Schwinger method is also described. <![CDATA[Gravitational radiation from a rotating magnetic dipole]]> Abstract The gravitational radiation emitted by a rotating magnetic dipole is calculated. Formulas for the polarization amplitudes and the radiated power are obtained in closed forms. A comparison is made with other sources of gravitational and electromagnetic radiation, particularly neutron stars with extremely powerful magnetic fields. <![CDATA[Study of CdTe recrystallization by hydrated-CdCl<sub>2</sub> thermal treatment]]> Abstract CdTe thin films solar cells are currently produced using a layer sequence of glass/FTO/CdS/CdTe/metal contact (Cu/Ag), these films are deposited by two different techniques, chemical bath deposition (CBD) and close space vapour transport (CSVT). In order to reach reasonable conversion efficiencies, the device has to be thermally treated in a hydrated-CdCl2 atmosphere. This study was carried out using X-ray diffraction (XRD), photoluminescence, SEM-EDS, four probe method and SIMS profiling of CdTe. These analyses confirm the presence of hydrated CdCl2 and CdTe phases on CdTe surface and shown a good recrystallization morphology helping to the carriers mobility along the structure. Using the thermal treatment was possible to reduce the resistivity of CdTe thin film; it is a result to the Cl migration along the CdTe solar cell structure, reducing the defects between CdS and CdTe thin films. A strong CdTe thin film recrystallization was observed by the implementation of a hydrated-CdCl2 treatment doing to this a good candidate to CdTe solar cells process. <![CDATA[<em>SU</em>(2) <strong>symmetry and conservation of helicity for a Dirac particle in a static magnetic field at first order</strong>]]> Abstract We investigate the spin dynamics and the conservation of helicity in the first order S-matrix of a Dirac particle in any static magnetic field. We express the dynamical quantities using a coordinate system defined by the three mutually orthogonal vectors; the total momentum k = p f + p i, the momentum transfer q = p f - p i, and l = k × q. We show that this leads to an alternative symmetric description of the conservation of helicity in a static magnetic field at first order. In particular, we show that helicity conservation in the transition can be viewed as the invariance of the component of the spin along k and the flipping of its component along q, just as what happens to the momentum vector of a ball bouncing off a wall. We also derive a “plug and play" formula for the transition matrix element where the only reference to the specific field configuration, and the incident and outgoing momenta is through the kinematical factors multiplying a general matrix element that is independent of the specific vector potential present. <![CDATA[Spectral line broadening by electron collisions in plasmas]]> Abstract In this work we compute the broadening of the spectral line shape in a plasma. Precisely we focus our study on the broadening of the spectral line shape by the electrons collisions with the ions of the plasma. During the collision, the electron moves in the effective potential created by all the plasma components (ions and free electrons). Whereas the interaction between the colliding electron and the ion (assumed at the rest) is those of Deutsch. The latter takes into account the quantum effect at short distance. The corresponding broadening is computed for the case of the spectral line for Lyman-alpha of Li+2 (Hydrogen-like of Lithium) and compared with the case where the interaction is that of Coulomb. <![CDATA[Análisis de cromitas de lantano de estructura perovskita con aplicación en ánodos para celdas SOFC]]> Resumen Uno de los desafíos más importantes para avanzar en la tecnología de las celdas de combustible de óxidos sólidos (SOFC), es el estudio y mejoramiento de las propiedades de materiales anódicos que las conforman. En este sentido la conductividad electrónica, porosidad, densidad de potencia, coeficiente de expansión térmico (TEC), densidad de corriente y estabilidad de los materiales; deben ser adecuadas para un uso eficiente a largo plazo. Con el objetivo de evitar problemas como la formación de carbón (hidrocarburo), intolerancia a impurezas de azufre, y cambios de volumen asociados a los ciclos redox, se han estudiado materiales alternativos al cermet YSZ/Ni como ánodo para celdas SOFC. Recientemente, se han publicado importantes avances en los materiales anódicos, como las cromitas de lantano, de estructura perovskita. Estos materiales han demostrado tener aceptable actividad catalítica para la oxidación de combustibles como el metano e hidrógeno, que pueden obtenerse en procesos térmicos de biomasa como la pirólisis y la gasificación, para aprovecharse en la tecnología de celdas de combustible SOFC. En este estudio se analizan los avances en la investigación de las cromitas de lantano, con énfasis en su aplicación para celdas SOFC.<hr/>Abstract One of the most important challenges in advancing solid oxide fuel cell (SOFC) technology is the study and improvement of the properties of anodic materials that conform. In this sense the electronic conductivity, porosity, power density, coefficient of thermal expansion (TEC), current density and stability of materials; must be suitable for long-term efficient use. In order to avoid problems such as the formation of coke (hydrocarbon), intolerance to sulfur impurities, and changes in volume associated with redox cycles, alternative materials to the YSZ/Ni cermet have been studied as anode for SOFC cells. Recently, important advances have been made in the anodic materials, such as the chromites of lanthanum, of perovskite structure. These materials have been shown to have acceptable catalytic activity for the oxidation of fuels such as methane and hydrogen, which can be obtained in biomass thermal processes such as pyrolysis and gasification, to be exploited in SOFC fuel cell technology. This study analyzes the advances in the research of lanthanum chromites, with emphasis on their application to SOFC cells.