Exchange coupling between magnetic layers across non-magnetic superlattices

The oscillation periods of the interlayer exchange coupling are investigated when two magnetic layers are separated by a metallic superlattice of two distinct non-magnetic materials. In spite of the conventional behaviour of the coupling as a function of the spacer thickness, new periods arise when the coupling is looked upon as a function of the number of cells of the superlattice. The new periodicity results from the deformation of the corresponding Fermi surface, which is explicitly related to a few controllable parameters, allowing the oscillation periods to be tuned.Comment: 13 pages; 5 figures; To appear in J. Phys.: Cond. Matte

Jet Collimation by Small-Scale Magnetic Fields

A popular model for jet collimation is associated with the presence of a large-scale and predominantly toroidal magnetic field originating from the central engine (a star, a black hole, or an accretion disk). Besides the problem of how such a large-scale magnetic field is generated, in this model the jet suffers from the fatal long-wave mode kink magnetohydrodynamic instability. In this paper we explore an alternative model: jet collimation by small-scale magnetic fields. These magnetic fields are assumed to be local, chaotic, tangled, but are dominated by toroidal components. Just as in the case of a large-scale toroidal magnetic field, we show that the ``hoop stress'' of the tangled toroidal magnetic fields exerts an inward force which confines and collimates the jet. The magnetic ``hoop stress'' is balanced either by the gas pressure of the jet, or by the centrifugal force if the jet is spinning. Since the length-scale of the magnetic field is small (< the cross-sectional radius of the jet << the length of the jet), in this model the jet does not suffer from the long-wave mode kink instability. Many other problems associated with the large-scale magnetic field are also eliminated or alleviated for small-scale magnetic fields. Though it remains an open question how to generate and maintain the required small-scale magnetic fields in a jet, the scenario of jet collimation by small-scale magnetic fields is favored by the current study on disk dynamo which indicates that small-scale magnetic fields are much easier to generate than large-scale magnetic fields.Comment: 14 pages, no figur

Euler-Lagrange equations for composition functionals in calculus of variations on time scales

In this paper we consider the problem of the calculus of variations for a functional which is the composition of a certain scalar function $H$ with the delta integral of a vector valued field $f$, i.e., of the form $H(\int_{a}^{b}f(t,x^{\sigma}(t),x^{\Delta}(t))\Delta t)$. Euler-Lagrange equations, natural boundary conditions for such problems as well as a necessary optimality condition for isoperimetric problems, on a general time scale, are given. A number of corollaries are obtained, and several examples illustrating the new results are discussed in detail.Comment: Submitted 10-May-2009 to Discrete and Continuous Dynamical Systems (DCDS-B); revised 10-March-2010; accepted 04-July-201

Lorentz-breaking effects in scalar-tensor theories of gravity

In this work, we study the effects of breaking Lorentz symmetry in scalar-tensor theories of gravity taking torsion into account. We show that a space-time with torsion interacting with a Maxwell field by means of a Chern-Simons-like term is able to explain the optical activity in syncrotron radiation emitted by cosmological distant radio sources. Without specifying the source of the dilaton-gravity, we study the dilaton-solution. We analyse the physical implications of this result in the Jordan-Fierz frame. We also analyse the effects of the Lorentz breaking in the cosmic string formation process. We obtain the solution corresponding to a cosmic string in the presence of torsion by keeping track of the effects of the Chern-Simons coupling and calculate the charge induced on this cosmic string in this framework. We also show that the resulting charged cosmic string gives us important effects concerning the background radiation.The optical activity in this case is also worked out and discussed.Comment: 10 pages, no figures, ReVTex forma

Algoritmo genético para construção de ensembles de redes neurais: aplicação à língua eletrônica.

bitstream/CNPDIA-2009-09/11846/1/CiT34_2006.pd

Simulation of VUV electroluminescence in micropattern gaseous detectors: the case of GEM and MHSP

Electroluminescence produced during avalanche development in gaseous avalanche detectors is an useful information for triggering, calorimetry and tracking in gaseous detectors. Noble gases present high electroluminescence yields, emitting mainly in the VUV region. The photons can provide signal readout if appropriate photosensors are used. Micropattern gaseous detectors are good candidates for signal amplification in high background and/or low rate experiments due to their high electroluminescence yields and radiopurity. In this work, the VUV light responses of the Gas Electron Multiplier and of the Micro-Hole Strip Plate, working with pure xenon, are simulated and studied in detail using a new and versatile C++ toolkit. It is shown that the solid angle subtended by a photosensor placed below the microstructures depends on the operating conditions. The obtained absolute EL yields, determined for different gas pressures and as functions of the applied voltage, are compared with those determined experimentally.Comment: Accepted for publication in Journal of Instrumentatio

Re-design de vestuário mais um passo no slow design

This article is prepared in the course of a dissertation for the Master in Design and Marketing. The focus of research concerns the exploitation of human and natural resources according to textile industry business objectives and their respective wastes that pollute the environment. Moreover, the consumers pressure so that textile industry responds quickly to their requirements and vice versa, is from the creation and development of a garment, to its disposal, a process unsustainable - designated as "Fast-fashion." This is where the center of the work is derived, namely the reuse of garments, with careful collection and classification, which can be recovered in a current approach to fashion increasing its life cycle. "Design" as an essential tool of the "Life Cycle" of products, is responsible for maximizing the value of a piece of clothing in the search for minimizing its environmental and socio - economic impacts. Thus, the methodology adopted for the study in question will be "Up-cycling" with the reuse of garments belonging to a random sample.Este artigo é elaborado no decurso de uma dissertação para o Mestrado em Design e Marketing. O foco de investigação diz respeito à exploração de recursos humanos e naturais em função dos objetivos comerciais da indústria têxtil e dos respetivos desperdícios que poluem o meio ambiente. Além disto, a pressão colocada pelos consumidores à indústria têxtil, para que esta responda rapidamente às suas exigências e vice-versa, faz desde a criação e desenvolvimento de uma peça de roupa, até ao seu descarte, um processo insustentável – designado como “Fast-fashion”. É neste ponto que o cerne do trabalho deriva, ou seja, o reaproveitamento de peças de vestuário, com recolha e classificação criteriosa, passíveis de serem recuperadas numa abordagem atual de moda concretizando mais um acréscimo no seu ciclo de vida. O “Design”, enquanto ferramenta essencial do “Life Cycle” dos produtos, é o responsável para a maximização do valor de uma peça de roupa na procura da minimização dos seus impactos ambientais e sócio - económicos. Deste modo, a metodologia adotada para o estudo em questão será a de “Up-cycling” com vista à reutilização de peças de vestuário pertencentes a uma amostra aleatória

Qualitative analysis of a scalar-tensor theory with exponential potential

A qualitative analysis of a scalar-tensor cosmological model, with an exponential potential for the scalar field, is performed. The phase diagram for the flat case is constructed. It is shown that solutions with an initial and final inflationary behaviour appear. The conditions for which the scenario favored by supernova type Ia observations becomes an attractor in the space of the solutions are established.Comment: Latex file, 9 pages, 1 figur