Dissertações em Física (Mestrado) - PPGF/ICEN
URI Permanente para esta coleçãohttps://repositorio.ufpa.br/handle/2011/2340
O Mestrado Acadêmico iniciou-se em 2003 e pertence ao Programa de Pós-Graduação em Física (PPGF) do Instituto de Ciências Exatas e Naturais (ICEN) da Universidade Federal do Pará (UFPA).
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Item Acesso aberto (Open Access) Criação de fótons no efeito Casimir dinâmico em cavidade oscilante com condições de Neumann e Dirichlet(Universidade Federal do Pará, 2005-03) GRANHEN, Edney Ramos; ALVES, Danilo Teixeira; http://lattes.cnpq.br/6642002445572793We consider a massless scalar field in a two-dimensional space-time inside an oscillating cavity with mixed boundary conditions. In order to discuss particle creation phenomenum, we consider the situation of parametric resonance in which the oscillating frequency is twice the frequency of the first mode of the static cavity. For convenience, we suppose that one of the boundaries is at rest and imposes Neumann condition to the field, whereas the other is in non-relativistic motion and imposes a Dirichlet condition on the field. Following the procedure developed by Dodonov and Klimov (Phys. Rev. A, 56, 2664 (1996)), we compute the number of created particles, the generation rate and the energy in the cavity. We compare our results with those found in the literature for the Dirichlet-Dirichlet case.Item Acesso aberto (Open Access) Densidade de energia e força de radiação sobre fronteiras em movimento num espaço-tempo bidimensional(Universidade Federal do Pará, 2009-03) REGO, Andreson Luis Carvalho; ALVES, Danilo Teixeira; http://lattes.cnpq.br/6642002445572793In the present work, we investigate the energy density and the quantum radiation reaction force on one moving boundary which imposes on a massless scalar field a Dirichlet or a Neumann boundary condition. Though we assume a particular motion for the boundary, introduced by Walker and Davies many years ago (J. Phys. A, 15 L477, 1982), we consider new possibilities for the initial field state, namely, thermal state and coherent state. We also investigate the problem of a cavity with one of the boundaries in the particular motion proposed by Walker and Davies, taking into account the vacuum, thermal and coherent initial field states. Finnaly we investigate the case of a non-static boundary which imposes the Robin boundary condition to the field.Item Acesso aberto (Open Access) Fonte escalar acoplada ao campo de Klein-Gordon orbitando um objeto estelar(Universidade Federal do Pará, 2006-02) MEIRA FILHO, Damião Pedro; CRISPINO, Luís Carlos Bassalo; http://lattes.cnpq.br/4033994493756291In this work we determine, using Quantum Field Theory in tree level, the scalar radiation emitted by a source in uniform circular motion in Minkowski spacetime, assuming Newtonian gravitation, and in the curved spacetime of a chargeless black hole with null angular momentum, assuming General Relativity. We perform this calculation analitically for the case of Minkowski spacetime and numerically for Schwarzschild spacetime. In the black hole case we obtain the analytic form and the normalization of the modes in the asymptotic regions. We verify, for stable circular orbits acording to general relativity, that the emitted power in Schwarzschild spacetime is lower than the one obtained in Minkowski spacetime assuming Newtonian gravitation. We obtain that only a little amount of the emitted radiation is absorbed by black hole. We also verify that the difference between the emitted powers in Schwarzschild and Minkowski cases decreases if the mass of ¯eld is increased. In Schwarzschild spacetime, the amount of radiation absorbed by the black hole increases for higher values of the mass of the scalar field.Item Acesso aberto (Open Access) As funções do grupo de renormalização no modelo com auto-interação quártica fermiônica(Universidade Federal do Pará, 2009) NASCIMENTO, Leonardo Oliveira do; ALVES, Van Sérgio da Silva; http://lattes.cnpq.br/5346258049047843Four femion interactions models have been studied to clarify their conceptual aspects and possible applications in quantum field theory. In this work we present the structure of the renormalization group in the Nambu-Jona-Lasinio model up to 1-loop order. The model is not perturbatively renormalizable in the usual power counting sense, but it is treated as an effective theory, valid in a scale of energy where p <<^, being p the external moment of the loop and ^ a massive parameter that characterizes the coupling of the non-renormalizable vertex. We clarify the tensorial structure of the interaction vertices and calculate the functions of the renormalization group. The analysis of the fixed points of the theory is also presented using Zimmermann procedure for reducing the coupling constants. We find that the origin is an infrared-stable fixed point at low energies and has also a nontrivial ultraviolet stable fixed point, indicating that the theory could be perturbatively investigated if the momentum is low enough.Item Acesso aberto (Open Access) Investigação da transição isolante-metal do CDM via algoritmos computacionais e estudo de nano-dispositivos orgânicos(Universidade Federal do Pará, 2007) AMAZONAS, Járlesson Gama; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899We present in this work a theoretical study on conjugated organic polymers. These systems, in general semiconductors or insulators, when doped, show an increase in electrical conductivity. Under action of electric field, small oligomers can present a similar behavior to of usual devices, but with innumerable advantages as, for example, size extremely reduced (some nanometers). In the first chapter we will make a brief introduction on organic polymers conjugated showing some experimental results for 4-dicianometileno-4,4-ciclopenta polymer [2,1-b: 3,4b'] ditiofeno - CDM, that is the central object of study. Chapter 2 deals with the used quantum methods. We will cite the Hartre-Fock (HF) approximation and its semiempirical derivations. The Configuration Interaction (CI) method and the Density Functional Theory (DFT) also will be treated in this chapter. Chapter 3 is dedicated to describe the characteristics of some usual devices as diodes and transistors. We will still cite some extremely small electronic devices. Chapter 4 describes our results and quarrels about the possibility the of the transistion insulator-metal in CDM when doped. We got the characterization optics of oligomers of CDM with null charge and in the presence of defects of the type bipolaron. From the Hartree-Fock level we calculate the Structure of Bands and the Density of States (DOS) for the PCDM in the neutral and doped state. The calculation of the DOS and the Dispersion had been carried through computational programs developed here in the Group GFMA. We still present in this chapter the theoretical absorption spectra for oligomers of CDM with geometry fully optimized by DFT. In chapter 5 we have the relative results to the analysis of nanodevice based on tetramers of CDM with and without charge. The curves of the charge displacement versus voltage present characteristics of curves of usual devices. We also analyze the theoretical absorption spectra of the nanodevice for values of null tension and in points of current saturation.Item Acesso aberto (Open Access) Potenciais pseudo-newtonianos e a radiação escalar emitida por uma fonte girando ao redor de um objeto estelar(Universidade Federal do Pará, 2010) CRUZ FILHO, Jaime Luis Cardoso da; CRISPINO, Luís Carlos Bassalo; http://lattes.cnpq.br/4033994493756291We use the pseudo-newtonian potentials proposed by Paczynski and Wiita, Nowak and Wagoner and Artemova et al. to calculate the scalar radiation emitted by a source in uniform circular motion around a stellar object. We compare the results obtained in this approach with the results obtained via quantum eld theory in Schwarzschild spacetime. We find that, up to the marginally stable circular orbit (R = 6M) the potential that better reproduces the Schwarzschild result is the Nowak and Wagoner one. Between this orbit and the last unstable circular orbit (R = 3M) neither one of the pseudo-newtonian potentials produce satisfactory results, and the newtonian potential turns out to be the best approximation. The Paczy nski and Wiita potential, the most used in the literature to analyze accretion disks, generates the less satisfactory results for this situation.Item Acesso aberto (Open Access) Quantização canônica do campo de Proca no espaço-tempo de Rindler e interação de uma fonte uniformemente acelerada com o banho térmico de Unruh(Universidade Federal do Pará, 2010) CORRÊA, Emerson Benedito Sousa; CASTIÑEIRAS RODRÍGUEZ, Jorge; CRISPINO, Luís Carlos Bassalo; http://lattes.cnpq.br/4033994493756291; http://lattes.cnpq.br/5047254977931140We perform the canonical quantization of the massive vector field, first with respect to inertial observers and then with respect to accelerated observers. We investigate how the uniformly accelerated source in Minkowski inertial vacuum interacts with the massive vector field through the computation of its total response rate. This response rate is computed with respect to two different frames, one inertial and the other co-accelerated with that source. According with the Unruh effect, in the accelerated frame, the inertial vacuum corresponds to a thermal bath of particles. Taking into account this effect, we show explicitly that theses response rates are identicals. This result can be used to describe the interaction of static electrons with the Z 0 particles present in the Hawking radiation, provided the electrons are very close to the black hole event horizon.Item Acesso aberto (Open Access) Quebra dinâmica da simetria quiral na pseudo eletrodinâmica quântica em (2 + 1) dimensões(Universidade Federal do Pará, 2011) ELIAS, Walace de Sousa; ALVES, Van Sérgio da Silva; http://lattes.cnpq.br/5346258049047843In the present work, we study the chiral symmetry breaking in pseudo quantum electrodynamics in (2+1) dimensions using the Schwinger-Dyson equation formalism and investigate the similarities of this model with the criticality in both QED3 and QED4. Using the rainbow-quenched approximation we show that there is a critical coupling αcc = π/16 above which there is a mass generation for fermions and hence the chiral symmetry breaking occurs. We also study the N massless fermions flavors version using the 1/N expansion in rainbow-unquenched approximation and we obtain a critical value for the number of active fermions below which there is chiral symmetry breaking. We show that at strong gauge coupling constant (g -- ∞) this critical number is the same as in QED3 in large N expansion.Item Acesso aberto (Open Access) Radiação emitida por uma carga elétrica orbitando um buraco negro de Schwarzschild segundo Teoria Quântica de Campos(Universidade Federal do Pará, 2004-01-21) FIGUEIRA, Rodrigo Murta de Andrade; CRISPINO, Luís Carlos Bassalo; http://lattes.cnpq.br/4033994493756291We performthe quantization of the massless vector field in Minkowski and Schwarz-schild spacetimes, and calculate the radiated power by an electric charge in a circular orbit around an object with mass M in both spacetimes. In the Minkowski case wend the analytical expression for the radiated power using quantum field theory and assuming Newtonian gravity. It coincides with classical Larmors result, since the calculations are performed at the tree level. Since in the Schwarzschild case it is not possible to express the solution of the radial equation in terms of well known special functions, we adopt the following two approaches: analytical approximation in the low frequency limit and numerical computing. The first approach was used as a consistency check for the numerical one. We also use quantum eld theory at tree level in the Schwarzschid case, and the radiated power is obtained both in the low frequency limit as well as numerically. After comparing the results, we conclude that for the same angular velocity of the charge (as measured by asymptotical static observers), the radiated power in Minkowski spacetime is bigger than in Schwarzschild case.