Scielo RSS <![CDATA[Revista mexicana de física]]> http://www.scielo.org.mx/rss.php?pid=0035-001X20140005&lang=es vol. 60 num. 5 lang. es <![CDATA[SciELO Logo]]> http://www.scielo.org.mx/img/en/fbpelogp.gif http://www.scielo.org.mx <![CDATA[<b>Origin of conical dispersion relations</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500001&lng=es&nrm=iso&tlng=es A mechanism that produces conical dispersion relations is presented. A Kronig Penney one dimensional array with two different strengths delta function potentials gives rise to both the gap closure and the dispersion relation observed in graphene and other materials. The Schrodinger eigenvalue problem is locally invariant under the infinite dimensional Virasoro algebra near conical dispersion points in reciprocal space, thus suggesting a possible relation to string theory. <![CDATA[<b>Caracterización por SEM, EDS y micro-Raman de la influencia de la relación molar SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> y temperatura de reducción sobre el sistema CuZSM5</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500002&lng=es&nrm=iso&tlng=es Se presenta un estudio por microscopia SEM, EDS y espectroscopia micro-Raman sobre los cambios morfológicos y espectroscópicos causados por la reducción en hidrogeno a 150 y 350°C en la CuZSM5, con relaciones molares (RM) SiO2/Al2O3 de 30 y 70 y 1 % en peso de cobre incorporado por intercambio iónico. La microscopia SEM del conjunto de muestras de CuZSM5 mostro que el tratamiento reductor a 350° C produce la aglomeración de partículas y textura superficial más suave que el tratamiento a 150° C, siendo este efecto más pronunciado para la RM 30. Además, las partículas correspondientes para la RM 30 son de forma irregular y textura rugosa (cristales cúbicos y rectangulares, salientes), mientras que las partículas para la RM 70 son en su mayoría esféricas y de textura suave (cristales en forma de hoja/plaquetas, recostadas). El análisis elemental EDS pone de manifiesto una heterogénea distribución del cobre y otros elementos en cada muestra. La espectroscopia micro-Raman muestra algunos cambios en los modos vibracionales para las muestras estudiadas.<hr/>The morphological and spectroscopic changes caused by reduction of ion-exchanged CuZSM5 samples with SiO2/Al2O3 molar ratios (MR) of 30 and 70 in H2 flow at 150 and 350°C are discussed. The nominal copper loading was of 1 % by weight. SEM microscopy of CuZSM5 sample set showed that the treatment in reducing atmosphere at 350° C produces particle agglomeration and smoother surface texture that treatment at 150° C; this effect was more pronounced for the sample with RM 30. Furthermore, the particles corresponding to the sample with RM 30 are of irregular shape and have rough texture (cubic or rectangular crystals, protrusions) while the particles of the sample with RM 70 are in the majority of spherical smooth texture (leaf-shaped crystals/platelet, reclining). EDS elemental analysis reveals a heterogeneous distribution of copper as well other elements in each sample. The micro-Raman spectroscopy shows some changes in the vibrational modes for the studied samples. <![CDATA[<b>Coulomb effects in isobaric cold fission from reactions <sup>233</sup>U(<i>n</i><sub>th</sub>,f), <sup>235</sup>U(<i>n</i><sub>th</sub>,f), <sup>239</sup>Pu(<i>n</i><sub>th</sub>,f) and <sup>252</sup>Cf(sf)</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500003&lng=es&nrm=iso&tlng=es The Coulomb effect hypothesis, formerly used to interpret fluctuations in the curve of maximal total kinetic energy as a function of light fragment mass in reactions 233U(n th,f), 235U(n th,f) and 239Pu(n th,f), is confirmed in high kinetic energy as well as in low excitation energy windows, respectively. Data from reactions 233U(n th,f), 235U(n th,f), 239Pu(n th,f) and 252Cf(sf) show that, between two isobaric fragmentations with similar Q-values, the more asymmetric charge split reaches the higher value of total kinetic energy. Moreover, in low excitation energy windows, between two isobaric charge splits with different Q-values, the more asymmetrical fragmentations is preferred. <![CDATA[<b>Quasinormal frequencies of the Dirac field in a D-dimensional Lifshitz black hole</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500004&lng=es&nrm=iso&tlng=es In a D-dimensional Lifshitz black hole we calculate exactly the quasinormal frequencies of a test Dirac field in the massless and zero angular eigenvalue limits. These results are an extension of the previous calculations in which the quasinormal frequencies of the Dirac field are determined, but in four dimensions. We discuss the four-dimensional limit of our expressions for the quasinormal frequencies and compare with the previous results. We also determine whether the Dirac field has unstable modes in the D-dimensional Lifshitz spacetime. <![CDATA[<b>Enhancement of vectorial nonlinearity in rubidium vapor by using an additional pump beam</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500005&lng=es&nrm=iso&tlng=es We report the enhancement of nonlinearity to absorption ratio in rubidium vapor for a signal beam in 87Rb transition D2 line with Fg = 1 by means of an additional pump beam. Experimental results for a pump beam in co- and counter-propagation configurations are reported. The ratio increment of approximately 2 times is obtained for the co-propagating case when the pump beam is tuned at Fg = 1.<hr/>Reportamos el incremento de la razón entre la no-linealidad y la absorción en vapor de rubidio para un haz señal en la línea de transición D2 del 87Ru con Fg = 1 utilizando un haz de bombeo adicional. Se reportan resultados experimentales cuando el haz de bombeo se propaga en la misma dirección del haz señal y en dirección opuesta a este. Un incremento de aproximadamente 2 veces se obtiene cuando los haces de bombeo y señal se propagan en la misma dirección y el láser de bombeo se encuentra sintonizado en la línea de transición Fg = 1. <![CDATA[<b>Higher dimensional Elko theory</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500006&lng=es&nrm=iso&tlng=es We show that the so-called Elko equation can be derived from a 5-dimensional Dirac equation. We argue that this result can be relevant for dark matter and cosmological scenarios. We generalize our procedure to higher dimensions. <![CDATA[<b>Probabilidad de transmisión en cadenas desordenadas de ADN</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500007&lng=es&nrm=iso&tlng=es En los últimos años ha crecido el interés de utilizar los ácidos nucleídos y proteínas en nanodispositivos. Sin embargo, algunas de sus propiedades electrónicas aún no se entienden claramente. Abordamos el problema de la transmisión electrónica a través de un segmento finito de ADN, usando un modelo de cadena lineal dentro del formalismo de enlace fuerte con interacción a primeros vecinos y un orbital por sitio para representar el ADN. Los efectos de hidratación se incluyen como variaciones aleatorias de las autoenergías. La probabilidad de transmisión se calcula a partir del concepto de matriz de transferencia.<hr/>In the last years have grown the interest to use nucleic acids and proteins in nanodevices. However, some their electronic properties remain unclear and there is still much more to be done. We address the problem of charge transport across a finite segment of DNA, using a linear chain model into tight binding framework with nearest neighbors interaction and one orbital per site to represent the DNA. Hydrations effects are taken into account as random variations of the self-energy. Transmission probability is calculated from transfer matrix concept. <![CDATA[<b>Preparation, structural and NLO-optical characterization of LB-molecular films from asymmetric Bent-Core liquid crystals</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500008&lng=es&nrm=iso&tlng=es Molecular mono- and multilayered films of a polar asymmetric bent-core ("banana-shaped") liquid crystalline (LC) compound with hydrocarbon end-chains were prepared by the Langmuir-Blodgett (LB) technique. Langmuir films were characterized by surface pressure isotherms and Brewster angle microscopy (BAM). Likewise, LB-films deposited onto glass substrates were characterized by UV-VIS spectroscopy, the optical second harmonic generation (SHG) technique and atomic force microscopy (AFM). Results show that the asymmetric structure of bent-core liquid crystals may promotes an unstable multi-layered (n > 10 LB-layers) LB-architecture which leads to a rapid collapse of Z-type arrangements, giving rise to a drastic decrease of the nonlinear optical (NLO) properties and film quality. Indeed, measurements evidence a tolerable and uniform molecular coverage on the glass substrates with anisotropic orientational distribution for a moderate number of layers only (n ≤ 10 LB-layers); where, according to NLO-experimental data, the net molecular polarization is aligned outward the substrate layer. This observation leads us to implement a simplified model based on the monomeric rod-like approximation, in order to estimate significant NLO-tensorial components and an effective molecular hyperpolarizability βeff -coefficient along the polar axes of the 2D-polar LC-compound within the mechanical stable LB-monolayer arrangements.<hr/>Películas moleculares mono y multicapa a base de un compuesto asimétrico líquido cristalino (LC) con propiedades polares (del tipo "banana") y conteniendo cadenas terminales hidrocarbonadas, fueron preparadas sobre substratos de vidrio mediante la técnica de depósito Langmuir-Blodgett (LB). Previo al depósito, las películas de Langmuir fueron caracterizadas mediante isotermas de presión superficial y microscopía de ángulo de Brewster (BAM). Asimismo, las películas depositadas LB fueron caracterizadas por espectroscopía UV-VIS, por medio de la técnica de generación del segundo armónico óptico (SHG) y por microscopía de fuerza atómica (AFM). Los resultados muestran que estos compuestos LC asimétricos promueven la formación de estructuras LB multicapa altamente inestables (para n > 10 depósitos LB del tipo Z), lo cual conlleva a un rápido colapso de estos sistemas y a una drástica disminución de sus propiedades ópticas nolineales (NLO) y de la calidad estructural de las películas. En efecto, los resultados experimentales evidencian sistemas multicapa razonablemente homogéneos con una distribución molecular anisotrópica para sistemas LB con al menos un numero moderado de depósitos (para n ≤ 10). De acuerdo a mediciones de ONL, el momento dipolar neto de estos sistemas está alineado hacia afuera del plano del substrato; esto nos llevó a la implementación de un modelo simplificado (para el sistema mono-capa con mayor estabilidad mecánica), basado en la aproximación de moléculas tipo "rodillo"; esto para la evaluación de las componentes tensoriales más significativas de las propiedades de ONL de estas moléculas, así como para la estimación del coeficiente de híper-polarizabilidad molecular βeff a lo largo del eje polar de este tipo de compuestos con estructura 2D. <![CDATA[<b>Changes of representation and general boundary conditions for Dirac operators in 1+1 dimensions</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500009&lng=es&nrm=iso&tlng=es We introduce a family of four Dirac operators in 1+1 dimensions: ĥA = -iħcˆΓA ∂/∂x (A = 1, 2, 3,4) for x ∉ Ω = [α, b]. Here, {ˆΓA} is a complete set of 2 x 2 matrices: ˆΓ1 = ˆ1, ˆΓ2 = ˆα, ˆΓ3 = ˆβ, and ˆΓ4 = iˆβˆα, where ˆα and ˆβ are the usual Dirac matrices. We show that the hermiticity of each of the operators ĥA implies that C A (x = b) = C A (x = α), where the real-valued quantities C A = cψ†ˆΓAψ, the bilinear densities, are precisely the components of a Clifford number Ĉ in the basis of the matrices ˆΓA; moreover, Ĉ/2cρ is a density matrix (ρ is the probability density). Because we know the most general family of self-adjoint boundary conditions for ĥ2 in the Weyl representation (and also for ĥ1), we can obtain similar families for ĥ3 and ĥ4 in the Weyl representation using only the aforementioned family for ĥ2 and changes of representation among the Dirac matrices. Using these results, we also determine families of general boundary conditions for all these operators in the standard representation. We also find and discuss connections between boundary conditions for the free (self-adjoint) Dirac Hamiltonian in the standard representation and boundary conditions for the free Dirac Hamiltonian in the Foldy-Wouthuysen representation. <![CDATA[<b>Heat engines and the Curzon-Ahlborn efficiency</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2014000500010&lng=es&nrm=iso&tlng=es The so-called Curzon-Ahlborn efficiency is becoming a paradigmatic result with regards to thermodynamic optimization of power cycles. Its wide applicability and sole dependence on the external heat bath temperatures (as the Carnot efficiency does) allows for an easy and fairly comparison with experimental efficiencies of striking validity. Different analytical derivations are presented in order to assess its validity and limitations for a broad variety of thermal cycles and steady state systems based on Finite-Time, Linear-Irreversible and Equilibrium Thermodynamic frameworks. Some conclusions and future perspectives are also outlined.