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Made available in DSpace on 2014-06-11T19:32:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2001-11Bitstream added on 2014-06-13T20:03:16Z : No. of bitstreams: 1 lunardi_jtt_dr_ift.pdf: 559817 bytes, checksum: e535803e37e925b888797ba55f2016c3 (MD5) / Neste trabalho consideramos alguns aspectos da teoria de campo escalar em interação no formalismo de Duffin-Kemmer-Petiau (DKP). No nível clássico estudamos dois casos: a interação mínima eletromagnética e a interação gravitacional no contexto das teorias de Einstein-Cartan e Teleparalelismo. No primeiro caso, mostramos que o formalismo DKP é completamente equivalente ao formalismo de Klein-Gordon (KG). No segundo caso, mostramos que a presença de torção no contexto da teoria de Einstein-Cartan destrói essa equivalência. Enquanto no formalismo KG não há interação entre campo escalar e a torção, no formalismo DKP essa interação surge de maneira natural. Discutimos a diferença entre a interação com a torção nos contextos das teorias de Einstein-Cartan e do Teleparalelismo. No nível quântico formulamos a eletrodinâmica escalar via formalismo DKP no contexto da teoria de perturbação causal de Bogoliubov-Epstein-Glaser, de maneira a definir a matriz S de espalhamento de uma forma não efetiva e matematicamente rigorosa, livre de divergências. Consideramos, em ordem mais baixa de teoria de perturbação, o espalhamento Compton, a polarização do vácuo, a auto-energia da partícula escalar e a correção de vértice no limite de momento transferido nulo. Todos os resultados obtidos nesses cálculos concordam com aqueles obtidos na teoria DKP efetiva usual e nos tratamentos via formalismo de Klein-Gordon, corroborando assim a equivalência entre ambos os formalismos nesse contexto / Abstracts: In this work we consider some aspects of the interacting scalar field in the Diffin-Kemmer-Patiau (DKP) formalism. At the classical level we study two cases: the minimal electromagnetic interaction and the gravitational interaction in the context of Einstein-Cartan and Teleparalelismo theories. In the former case we show that DKP formalism is completely equivalent to the Klein-Gordon (KG) one. In the later case we show that a nonvanishing torsion in einstein-Cartan theory breaks this equivalence. While in the KG formalism the scalar field does not interact with torsion, in DKP formalism this interaction naturally arises. We discuss the difference between this interaction with torsion in the context of Einstein-Cartan and Teleparalelism. At the quantum level we formulate scalar electrodynamics via DKP formalism in the framework of Bogoliubov-Epstein-Glaser causal perturbation theory. We define the S-matrix in a non effective and mathematically rigorous way. We consider, in the lowest order of perturbation theory, the Compton scattering, vacuum polarization, scalar self-energy and the vertex function at the limit of zero transferred momentum. In these computations all our results agree with those obtained first in the usual effective theory and in the Klein-Gordon based approaches, which corroborate the statement about the equivalence of both formalism in this context

xi, 63 f. : il. / Submitted by Bárbara Gomes (barbara.lrg@gmail.com) on 2010-02-04T20:24:28Z No. of bitstreams: 1 2007_PedroHenriquedeOliveiraNeto.pdf: 2980777 bytes, checksum: ecf175e7cdf2af2d4c9ac79738f04834 (MD5) / Approved for entry into archive by Marília Freitas(marilia@bce.unb.br) on 2010-02-04T22:24:08Z (GMT) No. of bitstreams: 1 2007_PedroHenriquedeOliveiraNeto.pdf: 2980777 bytes, checksum: ecf175e7cdf2af2d4c9ac79738f04834 (MD5) / Made available in DSpace on 2010-02-04T22:24:08Z (GMT). No. of bitstreams: 1 2007_PedroHenriquedeOliveiraNeto.pdf: 2980777 bytes, checksum: ecf175e7cdf2af2d4c9ac79738f04834 (MD5) Previous issue date: 2007-01 / A dinâmica de foto-excitação como mecanismo de formação de pares de pólarons em cadeias acopladas de poliacetileno é o principal objeto de estudo desse trabalho. A interação entre duas cadeias paralelas é investigada numericamente a partir da dinâmica de sistemas elétrons-rede na presença de um campo elétrico, utilizando uma versão estendida do modelo de Su-Schrieffer-Heeger. O campo elétrico é introduzido em termos do potencial vetor dependente do tempo aparecendo no Hamiltoniano através da substituição de Peierls no fator de fase da inte- gral de transferência. A dinâmica de relaxação da cadeia de um estado foto-excitado para pólarons carregados é determinada pela resolução das equações de movimento eletrônica e da rede. Mostramos que a interação entre cadeias é determinante no processo de geração de pólarons sem polímeros conjugados. _______________________________________________________________________________ ABSTRACT / The dynamics of photo-excitations to describe the generation mechanism of charged polaron pairs belonging to parallel conjugated polymer chains is inves- tigated. The interchain interaction of two parallel chains is studied numerically through the dynamics of the electron-lattice system in the presence of an electric field by using extended version of Su-Schrieffer-Heeger model. The electric field is introduced in terms of a time-dependent vector potential which is present in the Ha- miltonian through a Peierls substitution of the phase factor to the transfer integral. The lattice dynamic relaxation from the photo-excited states in a few hundreds of femtosecond to charged polarons belonging to neighboring chains is determined by the numerical resolution of the equations of motion for the ¼-eletrons and for the chain. It is shown in the present study that interchain interactions are fundamental in the description of polaron generation in conjugated polymers.

Dissertação (mestrado)—Universidade de Brasília, Faculdade UnB Planaltina, Programa de Pós-Graduação em Ciência de Materiais, 2015. / Submitted by Tania Milca Carvalho Malheiros (tania@bce.unb.br) on 2015-11-23T17:03:53Z No. of bitstreams: 1 2015_LucasRangelThomas.pdf: 1678275 bytes, checksum: a2abc9937d70e20ec0273a467f7d3f90 (MD5) / Approved for entry into archive by Marília Freitas(marilia@bce.unb.br) on 2016-01-26T12:09:00Z (GMT) No. of bitstreams: 1 2015_LucasRangelThomas.pdf: 1678275 bytes, checksum: a2abc9937d70e20ec0273a467f7d3f90 (MD5) / Made available in DSpace on 2016-01-26T12:09:00Z (GMT). No. of bitstreams: 1 2015_LucasRangelThomas.pdf: 1678275 bytes, checksum: a2abc9937d70e20ec0273a467f7d3f90 (MD5) / O objetivo deste trabalho é estudar as transições de fase metal-isolante geradas pela interação spin-órbita de Rashba e potencial químico modulados por um campo elétrico externo aplicado a um fio quântico. Utilizamos de métodos numéricos de diagonalização de matrizes para obter as autoenergias e autoestados que descrevem o sistema. Analisando o espectro de energias, observamos o aparecimento de gaps estruturais nas fronteiras das zonas de Brillouin reduzidas do sistema. Através do controle do preenchimento da rede, é possível promover transições de fase metal-isolante percorrendo as bandas de energia e, com isso, produzir uma chave de corrente baseada no grau de liberdade de spin dos elétrons. A análise dos autoestados mostrou a presença de ondas de densidade de carga e a inversão de spin, consistentes com a fenomenologia inferida a partir do estudo das autoenergias. A investigação do efeito do número de onda da modulação externa sobre o espectro de energias revelou a formação de padrões fractais conhecidos como borboletas de Hofstadter. Realizamos um estudo de caso com dados experimentais para um fio formado em um poço quântico de InAs a fim de verificar a viabilidade prática do dispositivo proposto. ______________________________________________________________________________ ABSTRACT / The goal of the present work is to study the metal-insulator phase transistion in a quantum wire driven by a Rashba spin-orbit interaction and a chemical potential, when both are modulated by an external eletric array. We use numerical methods to diagonalize the Hamiltonian matrix in order to obtain the eigenenergies and eigenvalues of our system. From analyzing the energy spectrum we observe that multiple structural gaps arise at the boarders of the system’s reduced Brillouin zones. Through an external control of the band filling, the system can be carried along the energy bands, undergoing a metal-insulator phase transitions across the gaps. Since this transition relies on the presence of the Rashba spin-orbit interaction, it can be viewed as a current switch based on the electron spin degree of freedom. The analysis of the eigenstates reveals charge density waves and spin flip, in agreement with the phenomenology inferred from the spectrum. An investigation of the effect of the external modulation wave number on the eigenenergies spectrum uncovers fractal patterns known as Hofstadter butterflies. We present a case study with experimental data for a wire structured in a InAs quantum well in order to verify the practical viability of the proposed device.