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Programa de Pós-Graduação em Física - PPGF/ICEN

URI Permanente desta comunidadehttps://repositorio.ufpa.br/handle/2011/2338

O Programa de Pós-Graduação em Física (PPGF) do Instituto de Ciências Exatas e Naturais (ICEN) da Universidade Federal do Pará (UFPA) foi iniciado em março de 2003 em nível de Mestrado. Em março de 2010, foi iniciado o Curso de Doutorado em Física do PPGF/UFPA, consistindo no primeiro curso de Doutorado em Física da Região Amazônica.

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Agora exibindo 1 - 20 de 39

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/4033994493756291
We 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.
Acesso aberto (Open Access)
Métodos teóricos na investigação da estrutura eletrônica do resveratrol e derivados
(Universidade Federal do Pará, 2004-09) COSTA, Sheila Cristina dos Santos; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899
In this work we report na ab initio, density functional and a semiempirical investigation of the ground and vertical excited states and of the spectroscopic properties of resveratrol, a phytoalexin with known antioxidative properties, and of structurally related derivatives as piceatannol, paravinylphenylphenol and resveratrol-dihydroxyl_N (N=1,2 and 3). The results of the simulation of bond length resulting from the photoexcitation and of the corresponding theoretical spectra gives us new insight of how chemical modifications of these molecules could affect the possible physiological properties of resveratrol. Also we report new molecules (piceatannol and resveratrol-dihydroxyl_3) as antioxidative properties by the analysis of molecular parameters and electronic properties.
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/6642002445572793
We 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.
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/4033994493756291
In 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.
Acesso aberto (Open Access)
Investigação das características I x V e C x V de NCPS puro, com nitrogênio substitucional carregado (-1 e +1) e com grupos doador (NO2)-aceitador (NH2) através de métodos derivados de hartree-fock
(Universidade Federal do Pará, 2006-06-15) SILVA JÚNIOR, Carlos Alberto Brito da; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899
In this work, we relate an investigation on the theoretical study of the I x V and C x V characteristics os SWCN pure, with substitutional Nitrogen charged with charge -1 (n-type material) and +1 (p-type material) and in the presence of donor (NO2)-aceptor (NH2) grups, through of the computational simulation of the SWCN ground states as well as your electronic structure and optics properties utilizing quantum chemical approaches: AM1 (Austin Model 1) and ZINDO/S-CIS (Zerner’s Intermediate Neglect of Differential Orbital/Spectroscopic – Configuration Interaction Simple) semi-empirical parametrization, Hartree-Fock based theory. By mean of this theoretical model, we analyse the electrons and optics properties, of more interest for that materials, the end understand the best shape of interation of this materials in the fabrication of electronic devices such as FETs (Field-Effect Transistors) or in optoelectronic applications such as LEDs (Light-Emitting Devices). We observate that SWCN with substitutional Nitrogen presents conformational defects of the polaron type. We made the curves of the UV-visivel Absorpion Spectra for armchair and zigzag SWCN pure, with substitutional Nitrogen charged with charge -1 (n-type material) and +1 (p-type material) and in the presence of donor (NO2)-aceptor (NH2) grups, when they are perturbed by different intensity of electric field. We examine in zigzag SWCN when it has na increase in the intensity of the electric field, yours curves suffer large perturbations. We get the curves p x E, I x V and C x V for this SWCN, we conclude that armchair armchair possess resistor behavior, therefore yours curves are linears and zigzag possess similar behavior at the importants electronic devices for the tecnologics advance. Our results are good agreement with the experimentals and theoretical results of the SWCN pure and with Nitrogen found in the literature.
Acesso aberto (Open Access)
Análise da potencialidade a condução do BDT através da densidade eletrônica de estados via tight binding e desenvolvimento de software (B3J)
(Universidade Federal do Pará, 2007) GUIMARÃES, Jeconias Rocha; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899
In this work we studied the potentiality of the polymer BDT (1,3-benzodithiole 4Hciclopenta[2,1-b:3,4b’]) to become a conducting polymer. Ground state conformations and the electronic structure have been calculated through quantum chemical methods in order to obtain the density of electronic states. The presence of several types of conformational defects and the variation of doping level has been considered. One of the most used theories to study the doping effects on conjugated polymers is the Huckel method. It is based on the separability of the sigma and pi bonds of planar molecules. BDT oligomers are not planar molecules and an extended Huckel method that includes all the atomic valence functions has been used. An original computation code, the B3J program, has been developed for this purpose. Simulations of the absorption spectrum in the UV-visible-near IR region have seen performed as well. Program B3J calculates the density of states of polymeric systems. The band structure of BDT was obtained with this software. We calculated the density of states of the neutral system and doped systems including several doping levels and both random and uniform distribution of defects. The square of the coefficients expansion of the wave function was obtained for polymers including up to 20 monomers. For these calculations we used geometries calculated by AM1 and PM3 quantum chemical methods. Finally, we studied the asymmetry of the charge separation induced by the presence of an external static electrical field in BDT oligomers suitably substituted by electron acceptor/donor species. The results are consistent with a transistor-like behavior of neutral and charged molecules.
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/5168545718455899
We 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.
Acesso aberto (Open Access)
Espectroscopia de Lente Térmica aplicada ao sistema Ácido oléico/Beta Caroteno
(Universidade Federal do Pará, 2007) PASSOS, João Paulo Rocha dos; MOREIRA, Sanclayton Geraldo Carneiro; http://lattes.cnpq.br/7312223977002681
In this work we present a study in the mixture formed by the beta-carotene diluted in oleic acid in different concentrations being used for that the technique called of Thermal Lens. We tried to vary the different control parameters as: the thickness of the sample, his/her concentration, the potency of the excitation laser, etc. We applied the aberrant model developed by Shen to find the thermal diffusivity, the variation of the refraction index with the temperature (dn/dT), and the magnitude of the sign of thermal lens, in the samples. We verified, with base in the theoretical model presented in the chapter 2, that if the increase of the potency and of the concentration it is very big and it reaches a certain value, parameters as thermal diffusivity start to present distortions in the measured values. We also observed that the mixture BC:AO presents nonlinear absorption, even for small potencies of the excitation laser used. The contributions presented in this work are unpublished and they should contribute to a better understanding of the thermal properties of the system BC:AO or of similar systems as some vegetable oils that are rich in acid oleic and betacarotene.
Acesso aberto (Open Access)
Modelo teórico de sensores ópticos baseados em fibras com grade de Bragg
(Universidade Federal do Pará, 2007) CUNHA, José Renato Ferreira Alves da; ALCANTARA JUNIOR, Petrus Agrippino de; http://lattes.cnpq.br/8771105422068014
By applying an external stress to an optical fiber its refractive index can change due to the photoelastic effect. When the optical fiber contains a Bragg grating inscribed in its core besides of the refractive index their spacial period changes too, allowing its application as high sensitive optical sensors for measurements of temperature and stress. From the theoretical point of view, up to date the operation of sensors using optical fiber with Bragg gratings (FBG) is explained by the photoelastic effect through appropriate components of the photoelastic tensor but for germanosilicated optical fibers only, which may to represent limitations to understand the responses of other classes of FGB used as optical sensors. In this work we show an alternative theoretical formulation for the studies of the operation properties of an optical sensor of longitudinal stress based on optical fiber with FBG, which do not depends from previous knowledge of the components of the photoelastic tensor, but just depends on the refractive index of the core, Poisson’s ratio, and relative deformation. The results shown good agreement with the literature and experimental data, allowing to estimate other features related with the reflectivity of the FGB.
Acesso aberto (Open Access)
Estudo de solvatocromismo em líquidos moleculares orgânicos via método seqüencial Monte Carlo/Mecânica Quântica
(Universidade Federal do Pará, 2007-03) MARTINS, Hardiney dos Santos; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899
The Sequential Monte Carlo / Quantum Mechanical method was applied to study the solvatochromic shifts and the dipole moments for organic molecules: Uracil in water, - carotene in oleic acid, ricinoleic acid in methanol and in ethanol and oleic acid in methanol and in ethanol. The optimizations and charges distributions had found by Density Functional Theory through the B3LYP functional and sets basis functions 6-31G(d) to all molecules except water and Uracil, whose was applied the sets basis functions 6-311++G(d,p). In the classical approach, Monte Carlo, was applied the algorithm Metropólis through the DICE program. The sampling of statistically relevant configurations to averaged calculations of the properties was used with auto-correlation function performed to each system. The Radial Distribution Function of the molecular liquids was applied to separate the first shell solvation for each system, which gives the main interaction between solute-solvent. The statistically relevant configurations of the first shell of solvation of the each system were subject a quantum mechanical calculations at the semi-empiric level with the method ZINDO/S-CI. The absorptions spectrum had found to solutes in gas phase and to the systems of molecular liquids spoken. The electric dipole moments of these were also found. All the bands of spectrum absorptions had a blue shift, except the second band of the -carotene in oleic acid that was a red shift. The results found have a very good agreement with the values found in the literature. All the system had increase in the electric dipole moments because the solvents molecules are polar molecules. The system of fatty acids in alcohols had results very similar, in other words, the fatty acids mentioned had characteristics spectroscopic similar submitted to same solvents. The simulations whose with Sequential Monte Carlo / Quantum Mechanical method were studied show the methodology is effective to find the spectroscopic proprieties of molecular liquids analyzed.
Acesso aberto (Open Access)
Investigação da dinâmica sequêncial Monte Carlo/Mecânica Quântica para sitemas moleculares orgânicos
(Universidade Federal do Pará, 2007-03) GESTER, Rodrigo do Monte; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899
The main objective of this work is to investigate theoretically the electronic and optical properties of organic compounds on liquid enviroment. The understanding of interactions on liquid systems is very important for the description of phenomena in many parts of the Science, like Physics, Chemistry, Biology and Materials. Also as in the development of optical materials. The thermal fluctuations make that lots of configurations can be generated. This is the reason of the statistical behavior realized in liquid systems. Considering these difficulties, to describe liquid systems, the sequential Monte Carlo / quantum mechanics (S-MC/QM) treatment is used in this work. On this procedure, first the liquid structure is generated by MC simulation and later, only the most important part of the system is treated as quantum mechanics.Using the above procedure, the quantum properties of the Methyl Orange (MO) and O 2,3-dimethyl thiene[3,4-b]pirazine (DTP) were investigated. MO is a well known pH indicator and can be found under basic and acidic conditions. Its minimal energy geometries were obtained by Density Functional Theory by the B3LYP functional, being the system described by Pople basis with one polarization function (6-31G*). To obtain the average properties of the quantum observables of these systems quantum chemistry calculations were performed within semi-empirical INDO/S-CI approach. About the absorption spectra of the MO, the experimental data presented in scientific literature reports a broad band placed on the lowest energies, more precisely between 400 and 600 nm. Our theoretical results for the alcaline form show an intense π → π* transition at about 432.4±0.03 nm and under acidic conditions, considering the MO acidic form, this transition appears at about 507.4±0.12, 496.4±0.28 or 545.3±0.10 nm, showing good agreement with experimental results. The DTP is a particular system used in the production of low band gap polimers. Its electrical and optical properties were obtained by the novel Average Solvent Electrostatic Configuration (ASEC) methodology. The ASEC procedure include a solvent molecules point charges and permits obtaining quantum mechanical quantities performing only few quantum calculations. For DTP, using the Mφller- Plesset second order perturbation thoery (MP2) and the aug-cc-pVDZ, the electric dipole moment 1.16 D was reached performing only four quantum calculations on representative configurations, presenting an increase of 42% when compared to the isolated dipole. The polarizability also was apparised considering the same level of dipole calculation, the average value 132.7 a30 was observed. The lowest energy region of the absorption spectra was also investigated by ASEC procedure and using both, semi-empirical and DFT quantum approaches. This absorption region is the reason of contraditory conclusions about of the n → π* and π → π* transitions. Our results show these excitations are observed and they can overlap themselves. As example, our DFT results using the B3LYP functional show that these transitions are placed at about 360.63 and 351.15 nm, respectively, being in good agreement with experimental predictions.
Acesso aberto (Open Access)
Propriedades ópticas e térmicas da oleína de palma
(Universidade Federal do Pará, 2008) NUNES, Edson Carlos de Barros; ALCANTARA JUNIOR, Petrus Agrippino de; http://lattes.cnpq.br/8771105422068014
We used an experimental methodology to investigate optical and thermal properties of palm olein (PO), the liquid fraction of the palm oil (Elaies guineensis) obtained by natural fractionation with physical properties very different of the oil. In a first approach, we study both the absorption and emission spectra monitoring the modifications in the spectral curves caused by the increase of the concentration of β-carotene diluted into PO. This procedure allowed to find the molar absorption coefficient of the β-carotene (ε = 920,802 L.mol-1.cm-1). This value is very small compared with β-carotene diluted in pure solvent like benzene or n-hexane. Measurements of the absorption spectra of β-carotene in pure hexane allow us to measure ε = 117.900 L.mol-1.cm-1, only 18% below of that reported in the literature. In a second approach, the absorption spectra were used to measure the absorption coefficient of the PO, A = 0,028 cm-1 at 532 nm, the wavelength of a laser used as excitation source to investigate thermal properties of the PO by thermal lens spectroscopy. In this sense, was measured too the thermal coefficient of the refractive index of the PO, dn/dt = -3,821 x 10-4°C-1, using an Abbé’s refractometer coupled to a thermal bath with controlled temperature. These results associated with the aberrant model for thermal lens spectroscopy studies, allowed to measure the thermal diffusion coefficient, D = 2,19 ± 0,11 x 10-4 cm2/s, and the thermal conductivity, k = 5,96 ± 0,08 W/m°C, from the palm olein. Although the thermal properties measured for the PO not been knowledge in the literature yet, one observes that they has the same order of magnitude of the similar thermal properties measured in other vegetable oils reported in the literature.
Acesso aberto (Open Access)
Propriedades magnéticas de nanoestruturas de metais de transição 3d adsorvidas na superfície de Pt(111)
(Universidade Federal do Pará, 2008) CORRÊA JÚNIOR, Gregório Barbosa; KLAUTAU, Ângela Burlamaqui; http://lattes.cnpq.br/4323567432559309
In this work we use the first principles RS-LMTO-ASA (“Linear Muffin-Tin Orbital - Atomic Sphere Approximation”) method, based on the Density Functional Theory and implemented to calculate non-collinear magnetic structures, to investigate the magnetic properties of 3d (Cr, Mn, Fe, Co and Ni) nanostructures adsorbed on Pt(111). Different geometries such as adatoms, dimmers, trimmers, linear and zig-zag wires have been considered, with cluster sizes varying between 2 and 7 atoms. The Fe, Co and Ni clusters are found to order ferromagnetically regardless of the cluster geometry. We find enhanced spin and orbital moments at Fe, Co and Ni sites compared to what is found in bulk, which is attributed to the reduced coordination number present at the surface. For Mn and Cr clusters on Pt(111), antiferromagnetic exchange interactions between nearest neighbors are found to cause collinear antiferromagnetic ordering when the geometry allows it. If the antiferromagnetism is frustrated by the cluster geometry, noncollinear ordering is found. The values are in general in good agreement with experiment and other calculations, when available in the literature.
Acesso aberto (Open Access)
Seção de choque de absorção de buracos negros de Schwarzschild e de buracos acústicos canônicos
(Universidade Federal do Pará, 2008-02-22) OLIVEIRA, Ednilton Santos de; CRISPINO, Luís Carlos Bassalo; http://lattes.cnpq.br/4033994493756291
In this dissertation we compute the absorption cross section of Schwarzschild black holes for the massless scalar and electromagnetic fields. We also compute the absorption cross section of canonical acoustic holes for sound waves. We use a numerical method to obtain the results in arbitrary frequencies. We also obtain analytic expressions for the low- and high-frequency absorption cross sections. The numerical results are in excellent agreement with the low- and high-frequency absorption cross section values obtained analytically. In the zero-frequency limit the absorption cross section tends to the event horizon area value for both the massless scalar field in Schwarzschild spacetime and the canonical acoustic hole cases. However, as the frequency increases, these two results become very different. This shows that, although the spacetime geometry does not influence the absorption cross section in the zero-frequency limit, it is important for arbitrary frequencies. We also see that massless scalar and electromagnetic absorption cross section values for the Schwarzschild black hole coincide for high enough frequencies and angular momenta. The spin of the scattered particle, in this case, although being very important for low frequencies, becomes less relevant to the absorption cross section value as the frequency and the angular momentum of the incident particle increase.
Acesso aberto (Open Access)
Medidas de espectroscopia Raman em cristais KDP e Nanotubos de Carbono: implementação da técnica
(Universidade Federal do Pará, 2008-08-29) FERREIRA, Marcel Luiz Rodrigues; REMÉDIOS, Cláudio Márcio Rocha; http://lattes.cnpq.br/7146076994037490; MOREIRA, Sanclayton Geraldo Carneiro; http://lattes.cnpq.br/7312223977002681
Currently, physicist have developed new techniques for characterization materials, mainly in the field of new materials that are being produced from existing materials such as glass, ceramics, polymers, semiconductor and superconductor materials, magnetic materials, etc. In that line, the Group of Physic of materials of Amazon has acquired new techniques for material characterization and this work is the beginning of the implantation of Raman spectroscopy technique. In this work, we used sample of KH2PO4 crystals (KDP) and carbon single wall nanotube (SWNT). The KDP was chosen because their spectra are well-known and their preparation quite simple. The KDP pure or doped is still researched due have a ferroelectric transition and their several applications in laser, sensors, second and third harmonic generation. SWNT was used in this work because are materials of great scientific and technological interest with applications in nanotechnology such as transistor, diode. KDP samples were submitted uniaxial pressure and high temperatures to investigated possible phase transition. Both decomposition and phase transition was observed through Raman spectra of KDP. In the SWNT Raman spectra was identified well known D and G bands. The results support implantation of Raman spectroscopy in the Laboratory of spectroscopy of UFPA.
Acesso aberto (Open Access)
Propriedades dielétricas e térmicas das blendas de poliestireno com ácido oléico e poliestireno com betacaroteno
(Universidade Federal do Pará, 2008-08-29) BELO, Ezequiel de Andrade; REMÉDIOS, Cláudio Márcio Rocha; http://lattes.cnpq.br/7146076994037490; MOREIRA, Sanclayton Geraldo Carneiro; http://lattes.cnpq.br/7312223977002681
The thermal, dielectrics and optics properties of vegetable oils have been studied by Amazonian Materials Physic Group since 1996 in the Department of Physic of UFPA. Recently collaboration with Physical-chemistry of polymer Laboratory in the Chemistry Institute of UnB allowed studying these oils and its components as polymeric blends. In this work, we search to contribute for this study and we explore thermal and dielectric properties as temperature dependence of blends: Polystyrene (PS) modified with Oleic Acid (OA) and polystyrene modified with Beta-Carotene (BC). We performed dielectric constant and thermal diffusivity measurements resorting Capacitance Bridge and photopyroelectric technical, respectively. We estimate moment of dipole for PS/OA blend in the Debye, Onsager and Kirkwood theoretical models and fitting linear regression of experimental data. The results show the phase transition of OA persists in the blend and it is displacement for high temperature.
Acesso aberto (Open Access)
Densidade de energia e força de radiação sobre fronteiras em movimento para estados arbitrários do campo: aplicação ao estado coerente
(Universidade Federal do Pará, 2008-08-29) LIMA, Mateus Gomes; ALVES, Danilo Teixeira; http://lattes.cnpq.br/6642002445572793
In this work we consider a real massless scalar field in a two-dimensional spacetime, satisfying Dirichlet or Neumann boundary condition at the instantaneous position of a moving boundary. For a relativistic law of motion, we show that Dirichlet and Neumann boundary conditions yield the same radiation force on a moving mirror when the initial field state is invariant under time translations. We obtain the exact formulas for the energy density of the field and the radiation force on the boundary for vacuum, coherent and squeezed state. In the nonrelativistic limit, our results coincide with those found in the literature. We also investigate the field inside an oscillating cavity. Considering Neumann and Dirichlet boundary conditions, we write the exact formula for the energy density inside a non-static cavity for an arbitrary initial field state. Taking as basis the Moore equation, we calculate recursively the energy density and investigate its time evolution for the coherent state.
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/5346258049047843
Four 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.
Acesso aberto (Open Access)
Fenômeno de interferência na densidade de energia em cavidades com duas fronteiras oscilantes
(Universidade Federal do Pará, 2009) PIRES, Wagner Pinheiro; ALVES, Danilo Teixeira; http://lattes.cnpq.br/6642002445572793
We consider a real massless scalar field inside a cavity with two moving mirrors in a two-dimensional spacetime, satisfying Dirichlet boundary condition at the instantaneous position of the boundaries, for arbitrary and relativistic laws of motion. Considering vacuum as the initial field state, we show that the exact value of the the energy density in the cavity can be obtained by tracing back a sequence of null lines, connecting the value of the energy density at the given spacetime point to a certain known value of the energy density at a point in the region where the initial field modes are not affected by the boundaries motion. We obtain formulas for the energy density of the field and the quantum force acting on the boundaries. We also show that for a large class of movements, including resonant cases found in the literature, the exact method proposed here enable us to find analytical solutions written as series in the parameter that controls the amplitudes of the movements. Using these analytical solutions, we investigate the interference phenomena in the energy density and total energy stored in the cavity.
Acesso aberto (Open Access)
Efeitos de temperatura e potencial químico em Teoria Quântica de Campos
(Universidade Federal do Pará, 2009-03) MACIEL, Soraya Galdino; PEREZ, Silvana; http://lattes.cnpq.br/3585326481456738
In this work we study the e ects of a nonzero chemical potential in (1 + 1) dimensions quantum eld models at nite temperature. We particularly consider massless fermions in an abelian gauge eld background and calculate the e ective action by evaluating the n-point functions. We nd that the structure of the amplitudes corresponds to a generalization of the structure noted earlier in a calculation found in the literature without a chemical potential (the associated integrals carry the dependence on the chemical potential). Our calculation shows that the chiral anomaly is una ected by the presence of a chemical potential at nite temperature. However, unlike in the absence of a chemical potential, odd point functions do not vanish. In fact, we nd that all the even point functions are even functions of while the odd point functions are odd functions of . We show that the origin of the structure of the amplitudes is best seen from a formulation of the theory in terms of left and right handed spinors. The calculations are also much simpler in this formulation and it clari es many other aspects of the theory.