The Stationary Phase Method for a Wave Packet in a Semiconductor Layered System. The applicability of the method

Using the formal analysis made by Bohm in his book, {\em "Quantum theory"}, Dover Publications Inc. New York (1979), to calculate approximately the phase time for a transmitted and the reflected wave packets through a potential barrier, we calculate the phase time for a semiconductor system formed by different mesoscopic layers. The transmitted and the reflected wave packets are analyzed and the applicability of this procedure, based on the stationary phase of a wave packet, is considered in different conditions. For the applicability of the stationary phase method an expression is obtained in the case of the transmitted wave depending only on the derivatives of the phase, up to third order. This condition indicates whether the parameters of the system allow to define the wave packet by its leading term. The case of a multiple barrier systems is shown as an illustration of the results. This formalism includes the use of the Transfer Matrix to describe the central stratum, whether it is formed by one layer (the single barrier case), or two barriers and an inner well (the DBRT system), but one can assume that this stratum can be comprise of any number or any kind of semiconductor layers.Comment: 15 pages, 4 figures although figure 4 has 5 graph

Limitations on the superposition principle: superselection rules in non-relativistic quantum mechanics

The superposition principle is a very basic ingredient of quantum theory. What may come as a surprise to many students, and even to many practitioners of the quantum craft, is tha superposition has limitations imposed by certain requirements of the theory. The discussion of such limitations arising from the so-called superselection rules is the main purpose of this paper. Some of their principal consequences are also discussed. The univalence, mass and particle number superselection rules of non-relativistic quantum mechanics are also derived using rather simple methods.Comment: 22 pages, no figure

Fluid dynamics and noise in bacterial cell-cell and cell-surface scattering

Bacterial processes ranging from gene expression to motility and biofilm formation are constantly challenged by internal and external noise. While the importance of stochastic fluctuations has been appreciated for chemotaxis, it is currently believed that deterministic long-range fluid dynamical effects govern cell-cell and cell-surface scattering - the elementary events that lead to swarming and collective swimming in active suspensions and to the formation of biofilms. Here, we report the first direct measurements of the bacterial flow field generated by individual swimming Escherichia coli both far from and near to a solid surface. These experiments allowed us to examine the relative importance of fluid dynamics and rotational diffusion for bacteria. For cell-cell interactions it is shown that thermal and intrinsic stochasticity drown the effects of long-range fluid dynamics, implying that physical interactions between bacteria are determined by steric collisions and near-field lubrication forces. This dominance of short-range forces closely links collective motion in bacterial suspensions to self-organization in driven granular systems, assemblages of biofilaments, and animal flocks. For the scattering of bacteria with surfaces, long-range fluid dynamical interactions are also shown to be negligible before collisions; however, once the bacterium swims along the surface within a few microns after an aligning collision, hydrodynamic effects can contribute to the experimentally observed, long residence times. As these results are based on purely mechanical properties, they apply to a wide range of microorganisms.Comment: 9 pages, 2 figures, http://www.pnas.org/content/108/27/1094

Molecular phylogeny of atractus (Serpentes, dipsadidae), with emphasis on Ecuadorian species and the description of three new taxa

Volume: 661Start Page: 91End Page: 12

SU(16) grandunification: breaking scales, proton decay and neutrino magnetic moment

We give a detailed renormalization group analysis for the SU(16) grandunified group with general breaking chains in which quarks and leptons transform separately at intermediate energies. Our analysis includes the effects of Higgs bosons. We show that the grandunification scale could be as low as $\sim 10^{8.5}$ GeV and give examples where new physics could exist at relatively low energy ($\sim 250$ GeV). We consider proton decay in this model and show that it is consistent with a low grandunification scale. We also discuss the possible generation of a neutrino magnetic moment in the range of $10^{-11}$ to $10^{-10}\mu_B$ with a very small mass by the breaking of the embedded SU(2)$_\nu$ symmetry at a low energy.Comment: (16 pages in REVTEX + 6 figures not included) OITS-49

Identificación y cuantificación de parámetros geológicos para la zonificación de eventos aluvionales en Mendoza

El Gran Mendoza es una de las regiones de la República Argentina que presenta altaocurrencia de peligro aluvional, entendiéndose al mismo como aquel proceso de remociónen masa, llamado técnicamente “inundación de detritos” o “debris flood”. Al producirse estefenómeno en forma extraordinaria por una conjunción de elementos climáticos, hidrológicosy geológicos ocasiona severos daños que pueden llegar a ser catastróficos. Se presenta enesta oportunidad el avance generado por el proyecto homónimo a este trabajo en su primeraño, donde se evidencia la necesidad de tener en cuenta este tipo de fenómenos en lasactividades humanas y en la planificación de la prevención de sus efectos en el GranMendoza.La zona de estudio se encuentra inserta en el piedemonte de la Precordillera Mendocina,planicie oriental de esta unidad geológica de aproximadamente 8 km de longitud y entre 4º a11º de pendiente regular en escala megascópica, pero con enorme cantidad de anisotropíasa escalas mayores, que le imprimen una complejidad apta para el desarrollo de este tipo deeventos.La vinculación entre el ordenamiento territorial y la planificación para la reducción del riesgoaluvional se realiza debido a que, si bien es cierto que los procesos hídricos destructivosacompañan al hombre desde siempre, el advenimiento de la expansión urbana en todo elmundo con el incremento demográfico como también el éxodo del campo a las ciudadesson elementos sociales que han incrementado las consecuencias de estos fenómenos. Paraello, se presentará cartografía SIG tendiente a la identificación, jerarquización y planificaciónmedioambiental, y un plan de mitigación ante estos fenómenos, utilizando diferentestécnicas de clasificación y procesamiento digital de imágenes satelitales actuales. Losavances obtenidos se focalizan principalmente en la generación de cartografía raster(usando métodos con imágenes y DEMs) a partir de digitalización de información obtenidapor clasificación con árbol de decisiones y otros ADI. También se efectuaron salidas decampo con diferentes resultados.Se pretende con esta contribución realizar un avance en la prevención de los efectosnegativos que un proceso de flujo de alta densidad de consideración podría generar en elGran Mendoza, más exactamente en la zona de Godoy Cruz, al suroeste de la ciudadcapital de Mendoza.Se prevé además con este trabajo contribuir a la formación de recursos en investigación,provenientes específicamente de la carrera de Ingeniería en Agrimensura y de otrasrelacionadas a la temática