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Estudo da impedância de plasmas anisotrópicos por meio do Teorema de Poynting

2015-01-01T00:00:00Z

Título: Estudo da impedância de plasmas anisotrópicos por meio do Teorema de Poynting Autor(es): Silva, Jailson Gomes da Primeiro Orientador: Farias, Eliel Eleutério Abstract: We know that matter in the plasma state becomes good conductors and respond strongly to electromagnetic fields. This response may be used to probe quantities related to plasma, and is defined in terms of a conductivity tensor. In this work we present a brief study about the conductivity tensor calculus for anisotropic plasmas using the notation of Einstein and the Levi-Civita symbol. Making use of this conductivity tensor particularized for cold plasma model, we seek to determine expressions for resistance, reactance and plasma impedance defined by the Poynting’s theorem. In a simple analysis of the expression impedance we show an alternative way for determining the plasma density and plasma frequency in terms of easily measurable experimental parameters such as radius, distance between electrodes, the potential difference and external resistance to plasma. The method presented here is invasive as well as in cases where the diagnosis frequency and plasma density are performed by Langmuir probe, however the mapping through the impedance is shown mathematically and experimentally simpler to implement, and whith the possibility of being applied to pulsed systems as Tokamak and Theta-Pinch. Editor: Universidade Federal de Roraima Tipo: Dissertação

Modelo de ising frustrado e aleatoriamente decorado com interações de primeiros e segundos vizinhos com campo magnético

2012-01-01T00:00:00Z

Título: Modelo de ising frustrado e aleatoriamente decorado com interações de primeiros e segundos vizinhos com campo magnético Autor(es): Azevedo, Jonas Menezes de Primeiro Orientador: Araújo, Ijanílio Gabirel de Abstract: The model studied in this dissertation consists of planes, in which the nodal spins (vertices of a square lattice) have antiferromagnetic (J1 and J2) interactions with the first (J1 < 0) and second (J2 < 0) neighboring spins (representing the interactions in the bond between the Cu −Cu copper ions in the CuO2 plane) and with ferromagnetic interaction (JF > 0) with the decorator spins (simulating the Cu − O interactions) randomly distributed quenched (quenched) over a two-dimensional lattice, and finally , the nodal spins between planes have weak antiferromagnetic interaction (λ J1 < 0, λ = 10−5). In this model, the action of a magnetic field and the effective interaction Je f between the nodal spins are considered, determined with the contribution of the decorator spin, through the decoration-iteration transformation technique. The phase transition of the model was studied through the effective field theory for a cluster with a spin, combined with the differential operator technique. Then, we discussed the critical concentration pc at which the Ne'el temperature tends to zero for various values of the spin frustration parameters α1 and α2 (α1 = JF /J1, α2 = J2/J1) under the influence of the magnetic field H. For some of these values, we observe a reentrant behavior at low temperatures. Phase diagrams were also obtained in the plane ( KB TN, H ), and for some values of H , in the planes ( p, KB TN ), (KB TNKB TNJ1 J1KB TNJ1, α1), (J1 , α2) and ( λ ,J1 ). Finally, the dependence of the sublattice magnetization as a function of temperature was calculated, helping to determine the uniform (m) and remaining (residual, ms) magnetizations, in order to analyze the existence of the spin frustration phenomenon, and the reentrant behavior in the phase diagrams of the planes ( KB TN , m ) and ( KB TN , ms) under the effect of the magnetic field. Editor: Universidade Federal de Roraima Tipo: Dissertação

Nova abordagem da teoria de campo efetivo aplicada ao modelo de Ising com interações competitivas

2013-01-01T00:00:00Z

Título: Nova abordagem da teoria de campo efetivo aplicada ao modelo de Ising com interações competitivas Autor(es): Castro, Eronildo Cornélio de Primeiro Orientador: Pacobahyba, Josefa Teixeira de Mendonça Abstract: The research of high temperature superconducting compounds formed by plans CuO2, has motivated the theoretical study in proposing a model able to describe their characteristics. In particular, these compounds have a common antiferromagnetic ordering, that for small doping, the Néel temperature is drastically reduced to zero when a critical concentration is reached pc. In this work, was used the randomly antiferromagnetic Ising model decorated with spin dfrac12 and competitive interactions to describe properties CuO2. This model consists of plans, in which the nodal spins interact antiferromagnetically J1 with its nearest neighbors (Cu - Cu interactions) and ferromagneticamente JF with the spins decorators (Cu - Ointeractions) that are randomly distributed ( textit quenched) over a three-dimensional network. It used the effective field theory based on operator technique for studying the differential phase transition in a cluster formed by two sites in a cubic lattice. The system was analyzed for the almost-twodimensional topologies and three-dimensional, initially, from the perspective of methodology Zernike, usually used in statistical mechanics, where you can analyze the phase diagram and the behavior of the magnetization. posteriorly, was applied a new methodological approach decoupling the model, which was analyzed only the phase diagram. Discussed the variation of the critical concentration Pc where the Néel temperature tends to zero for various values of the frustration parameter . For certain values of this parameter was observed reentrant behavior at low temperatures. Was calculated dependence of magnetization subnet as a function of temperature for analyzing the behavior observed in reentrant of phase diagrams. Editor: Universidade Federal de Roraima Tipo: Dissertação

Estudos de um fluido cosmológico com produção de partículas em N dimensões

2013-01-01T00:00:00Z

Título: Estudos de um fluido cosmológico com produção de partículas em N dimensões Autor(es): Souza, Rochelle Gomes Primeiro Orientador: Batista, Miguel Gustavo de Campos Abstract: We study a cosmological model where the number of particles do not onserve in a spacetime with extra dimensions. The open system cosmological model by I. Prigogine considered the particle formation at expenses of the gravitational field with intent to explain the entropic content of the universe, and presently, this model can be an alternative to explain the accelerated expansion of the universe. We generalize the open system cosmological model including an extra dimension and the creation process of particles is due to the compactification of the extra dimension, evolving for the usual quadri-dimensional spacetime. Editor: Universidade Federal de Roraima Tipo: Dissertação