With the success of the international collaboration Event Horizon Telescope (EHT)observations, culminating in the first image of the black hole shadow in the center of M87galaxy, many studies have been published on the bending of light due to gravitationalfields of compact objects. In particular, as it is expected that the theory of General Relativity(GR) change in quantum regimes, the analysis of the black hole shadow, naturalconfigurations to explore the quantum effects of gravity, can be a tool to help the comparisonbetween black hole solutions in GR and in other theories of gravity. Thus, weanalyze and compare, in this dissertation, the shadow of two charged and rotating blackholes: the Kerr-Sen black hole, which appears as a solution of the equations of low-energyheterotic string theory, one of the main candidates for the quantum theory of gravity; andthe Kerr-Newman black hole, a solution to Einstein-Maxwell equations in GR. We showthat the Kerr-Sen black hole always has a larger shadow for the same physical parametersand observation conditions, and we present an interpretation for these results.

In 1993, Marino proposed the Pseudo-Quantum Electrodynamics (PQED) [E. C. Marino, Nucl. Phys. B, Vol. 408, 551 (1993)], an effective gauge theory in (2+1)D, obtained through the dimensional reduction of the Quantum Electrodynamics (3+1)D (QED) and which, although being a non-local theory, it respects the causality and unitarity, describing the coulombian interaction between static charges in the plane. In the present work, we considered the dimensional reduction that leads us to PQED, such as made by Marino but incorporating the boundary conditions effects imposed by the presence of a partially reflective surface to the electromagnetic field. In this manner, We propose an effective model that fits with the theory recently denominated in the literature as cavity Pseudo-Quantum Electrodynamics. A direct application of this model, also developed in this present work, is the obtaining of the Fermi velocity renormalization for an electron in a graphene sheet near a conducting plate, which is approached in the static interaction regime of the theory.

In the present work, we investigated the electronic properties of structures based on a 2D carbon allotrope formed by rings 5 and 8 called POPGraphene, using the hydrogenation edges of POPGraphene nanoribbons and the doping with N, B, or N and B atoms, in order to propose applications in molecular electronics. For the study, the Density Functional Theory was implemented for the optimization of the structures, and the Density Functional Theory combined with the Non-Equilibrium Green Functions method to obtain the electronic transport properties. The band structure results show that, among the proposed unit cells, the POPGraphene cell has a topological insulator character, and the others have a metallic behavior. The results for the electronic transport properties shows that the POPGraphene nanodevice has characteristics that resemble a Field Effect Transistor in the analyzed range, the hydrogenated one presents a Field Effect Transistor behavior in the range of -0.07 V to 0.07 V and of a Resonant Tunneling Diode in the range from -0.70 V (0.70 V) to -1.00 V (1.00 V), the doped with B presents a Field Effect Transistor behavior in the analyzed range, the doped with N presents a Field Effect Transistor behavior in the range of -0.10 V (0.10 V) to -1.00 V (1.00 V), and the doped with B and N presents a Field Effect Transistor behavior in the range of -0.70 V to 0.70 V and of a Light Emitting Diode in the range of -0.70 V (0.70 V) to -1.00 V (1.00 V). The results show that structures based on POPGraphene appear as a great alternative for the development of nanodevices and applications in molecular electronics.

In the present work, we investigated, using the Pseudo Quantum Electrodynamics, how the renormalization of the Fermi velocity in a graphene sheet is affected by conductive at surfaces in its vicinity. First, we review the effect of a single grounded conductive surface, and later we investigate a cavity formed by two grounded conducting surfaces. We show that the inhibition of the renormalization of the Fermi velocity occurred in the first case is enlarged by the cavity in a non-additive manner. This means that the result of the cavity is not the simple sum of the isolated results coming from two conductive plates. In addition, we also discuss the case in which the graphene is in a dielectric medium with a dielectric constant  separated by a planar interface from another medium with a dielectric constant . For this case, we show that the inhibition of the renormalization of the Fermi velocity is larger for the case  than for the case . The influence of external media on the properties of two-dimensional systems, we call Cavity Pseudo Quantum Electrodynamics.

In the present work was studied the ballistic and inelastic electronic transport properties of single-molecule composed by polyyne coupled to Single-Walled Carbon Nanotubes (SWCNTs) closed on tips under mechanical strain. It was analyzed the I-V, G-V and T-V curves through of the system under mechanical compression, equilibrium and distension. The results showed that while the mechanical strain displace the energy levels and change the band gaps in the closed tips of the SWCNTs (Egap equilibrium ¿ Egap distension ¿ Egap compression), the applied bias voltage break the degeneracy in the states of SWCNTs tips and it defines the conductance (G) along the system. The analysis of Projected Density of States (PDOS) suggests that the higher contribution of the I-V curve comes from the superposition of the closed nanotube tip states, which is in agreement with the T-V curve found. The obtained I-V curve allow us to consider the device obtained as a transistor. Through results obtained by Inelastic Electron Tunneling Spectroscopy (IETS), it was possible to perform a detailed analysis from chemical bonds of the polyyne molecule in three configurations studied. The displacements of raman spectra peaks calculated showed a structural transition from polyyne to cumulene in the configuration where the system is distended.

In this work, an analysis of the Y_1.6Ga_0.4(MoO₄)₃ system submitted to pressure conditions and extreme temperatures is presented. Raman spectroscopy was the main analysis technique used in this work. Auxiliary techniques such as X-ray diffraction, scanning electron microscopy and ab initio calculations were also used. In our analyzes, we suggest that the material Y_1.6Ga_0.4(MoO₄)₃ crystallizes in orthorhombic structure belonging to the space group Pbcn. We identified that the sample consists of grains ranging from 1 μm to 10 μm on average. Measurements of Raman spectroscopy showed that the material presents, under ambient conditions, the presence of water molecules in its structure, which causes a large degree of disorder that is proportional to the number of water molecules. We observed that above 84 °C a structural transformation occurs in the material caused by the exit of the water molecules, which leads to an increase in the degree of crystallinity of the material. It also occurs the appearance of a mode around 878 cm^-1, which is not present in the material not doped with Gallium ions. We conclude, together with the results of the calculations, that this new mode is related to the symmetrical stretch of the molybdate groups close to the Gallium ions. Measurements at low temperatures have shown that the orthorhombic phase of the material does not have a more stable phase with energy close to it that can allow a structural transformation to an even lower energy of this system. Our analyzes of the high-pressure results in the hydrated sample suggest that the material undergoes a hydrostatic pressure-induced amorphization process starting at 4.8 GPa. In the partially hydrated sample, two discontinuous behaviors were observed, where we attributed to an amorphization process and also the presence of domains in the material structure. We have also verified that the presence of water in the structure increases the stability of the crystalline phases, leading to the stability of the amorphization processes to follow the same order of the degree of hydration of the samples. The modifications caused are irreversible. The hydrostatic pressure-induced phase transition reported in the literature for Y₂(MoO₄)₃ material is suppressed because of the high degree of disorder in Y_1.6Ga_0.4(MoO₄)₃ material caused by the presence of water in its structure

In this work the electron transport mechanisms were studied in two isomeric forms of retinol. The electronic transport model used in this work was the ballistic approach. The calculations were performed by using the Nonequilibrium Green’s functions formalism combined with Density Functional Theory (NEGF-DFT). The results obtained were com- pared between the structures and in the one hand it was observed that 11-cis-retinol shows a more symmetrical behavior, on the other hand, all-trans-retinol acts as a molecular rectifier under small voltage ranges. These results suggest that a simple conformational change in the structure modulates the device between a molecular wire (nanowire) and a molecular rectifier, with possible applications in field effect transistors.

We discuss the quantization of gravitational perturbations in spherically symmetric spacetimes, such as Schwarzschild and de Sitter spacetimes and develop some applications. The gravitational perturbations are described using a gauge-invariant formalism, suited to background spacetimes with spherical symmetry. The gravitational perturbations can be divided in three types: scalar-, vector-, and tensor-type perturbations. We compute the emitted power of gravitational radiation from a particle in circular orbit around a Schwarzschild black hole. We obtain numerically the perturbations to compute the total emitted power and the observed power at infinity. For the innermost stable orbits, the emitted power is approximately 25 % smaller than the one computed considering the particle in flat spacetime moving around a stellar object due to a Newtonian force. Additionally, we show, by computing the spectrum of the gravitational radiation, that the low multipoles still give an important contribution to the total emitted power, even for unstable orbits. The scalar radiation emitted by a particle orbiting a regular black hole is also considered, as a first approximation to the gravitational radiation case. We compare it to the radiation in the case where the particle is orbiting a Reissner-Nordström black hole. We also verify the Gibbons-Hawking effect in de Sitter spacetime, by computing the response rate of the graviton field to an external multipole source in the thermal equilibrium state with the Gibbons-Hawking temperature. This response rate is found to be the same as the one obtained in the Bunch-Davies-like
vacuum state in the Poincaré patch.

This dissertation contains a detailed study on the stochastic process of random motion. Demonstrates Chapman's Compatibility Conditions (in which one deduces the Chapman-Kolmogorov equation) which are required to determine if the stochastic process is of the Markovian type. Contain the main properties of probability density for random motion, which in turn has a cases in which the stochastic process is independent, whose deduction is contained in this material.

Here  discusses the  Albert Einstein and Paul Langevin contributions concerning explanation of the Brownian motion, preceded by resolutions of the Langevin equation in normal cases of free particles. In addition, in this material the deduction the Fokker-Planck and Kramers-Moyal equations, followed by a study of the last equation, with the purpose of investigating the influence of their coefficients, in determining the probability density.

There is also in this material, a description about active systems and some motivating models for the study of this type of system, such as the model of Vicsek, Gregóire and Chaté, and Couzin, followed by resolutions, in some cases, of the Langevin equations in the active case of particles free and confined. Here we describe what we call the angular Brownian motion of active particles, which we adopt as a model for the development of computational simulations of an active system of particles considered soft disks, which interact through contact and move through a square network of rigid circular or half-circular obstacles. From this simulation, we obtain the  current of particles (average velocity)  dependent the desord in the system, caused by removals or substitutions of obstacles.

During many decades black holes posed only as mathematical solutions of the equations of
the Theory of General Relativity, until the discovery of its first astrophysical candidate Cygnus
X-1. Currently, black holes are paradigms, both in theoretical Physics and in Astronomy. In Astronomy,
black holes provide a solution in galaxy formation scenarios, in active galactic nuclei
and in the remanent of core-collapse supernovae. The recent detection of gravitational waves,
generated by the fusion of two black holes, opened a new window to observe the Universe and
gave rise to the gravitational-wave Astronomy. These and other experimental advances motivate
a study of the interaction between waves and black holes. We can model this interaction
by considering plane waves impinging upon black holes and studying how they are absorbed
and scattered. The absorption and scattering of waves can be quantified into two quantities: the
absorption and scattering cross sections. In this thesis, we analyze how massless plane waves
are absorbed and scattered by distinct black hole configurations: (i) Schwarzschild surrounded
by thin shell of matter, (ii) Kerr, and (iii) Kerr-Newman. We compute numerically the absorption
and scattering cross sections, and show a selection of results. Our results were obtained for
arbitrary values of frequency and scattering angle of the incident waves, and present excellent
agreement with analytical results obtained in limiting cases.

In this thesis, we present an analysis by resonant Raman spectroscopy (RRS) in single wall carbon nanotubes (SWCNT) and multiple walls (MWCNT) under two different foci. The first investigates the influence of hydrostatic pressure on the mechanical properties of the single wall carbon nanotubes (SWCNTs) dispersed in aqueous sodium dodecyl sulfate (SDS) and fatty acids (Cn), as well as the evidence of their filling by these particles. In this part of the work, the experiments were carried out using two different compressor media, alcohol solution composed of 4 ml of methanol and 1 ml of ethanol (MET-ET) as well as mineral oil (Nujol). The Raman spectra were acquired in the spectral range of the RBM mode and showed that the CNT samples were composed of tubes with diameters varying from 0.85 nm to 1.3 nm. A blueshift to the RBM and G modes is observed due to the increase in pressure. The disappearance of RBM modes assigned to tubes with diameters greater than 1.0 nm indicates that these tubes lose resonance at 2.0 GPa and 3.4 GPa for Nujol and MET-ET, respectively. In the analysis, the G-band component assigned to SWCNTs showed the Breit-Wigner-Fano (BWF) line shape, indicating that the sample is composed mainly of metal tubes. Our results show a dependence on the mechanical properties of the CNTs with the compressor medium, which can be understood from a theoretical prediction previously reported under the filling effect of the tube by fatty acid molecules. The second focus of the work is related to the doping mechanism in MWCNTs filled with aluminum carbide (Al4C3-MWCNTs) where they were studied and interpreted in relation to the changes in their electronic and phonic structures. Unfilled MWCNTs served as standard samples to aid in the interpretation of the fill and doping process. In addition, in order to corroborate the results of Raman spectroscopy, the samples were also characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), through which electron-phonon coupling associated with the Raman intensities of G and G 'modes were studied and interpreted in the light of the doping mechanism in these multiple-walled systems. Our results, corroborated by theoretical calculations, indicate that Al4C3 particles transfer electrons to MWCNTs. These calculations were performed for a system of filled and unfilled tubes of MWCNTs where the calculated electron states density indicated that the two possible systems considered exhibit metallic behavior with the aluminum carbide doping the carbon nanotubes.

In the this work, we have studied the electron transport characteristics in molecular junctions based on opioid derivatives. Four molecules that differ by the presence of acetyl groups were connected as molecular bridges between two metallic Au (111) electrodes. The molecules studied are 3-monoacetylmorphine, 6-onoacetylmorphine, diacetylmorphine and morphine. The theoretical study was performed using the DFT-NEGF methodology.
The results obtained show sequential deacetylation changes drastically the response of the molecular systems to an applied voltage under the system. Rectification effects, Negative Differential Resistance and resonant tunneling are described in terms of transmissions and Density of States. Particularly, diacetylmorphine has electrical current in the microampere scale and rectification pattern to direct polarization, obtaining rectification peaks (R=9,1) at low voltages (0,6 V) and maximum of rectification (R=14,3) in 1,8 V. The deacetylation inverts the rectification for morphine.

The theoretical studies of several magnetic materials are presented in this thesis. To each of them, it was investigated the electronic structure, by means of density functional theory calculations, and/or magnetization dynamics, in the context of atomistic spin dynamics (ASD). For bulk properties, we evaluate the magnon spectra of the heavy rare earths (Gd, Tb, Dy, Ho, Er, and Tm), using the exchange parameters and magnetic moments from first-principles calculations in ASD simulations. Additionally, we performed Monte Carlo simulations that nicely reproduced the qualitative trend of lowering of the critical temperatures across the series. Next, we discuss about the microscopic mechanism of the vanishingly low magnetic anisotropy of Permalloy using the concept of the orbital moment anisotropy for Fe and Ni atoms in the alloy. Turning to surface magnetism, we discuss the use of exchange parameters computed by a non-collinear formalism for 6 monolayers of Fe on the Ir(001) substrate, in order to have a more accurate description of magnons at finite temperature and to obtain good comparison with experimental data. Besides that, we also studied surface magnons on 3 and 9 Ni monolayers on Cu(001) and Cu(111) in order to track the significant surface and/or interface effects and contrast it to properties that are fcc Ni bulk-like. Likewise, we used the Monte Carlo method to estimate the critical temperatures of Ni surfaces and compared with experimental data. Finally, in the field of low dimensional magnetism, we present the ab-initio calculations for the electronic structure of Cr nanostructures of diverse geometries adsorbed on the Pd(111) surface, with focus on the formation of non-collinear spin configurations, either due to geometric frustration or the spin-orbit coupling provided by the substrate.

In the present work, a brief discussion about the application of boundary conditions over electromagnetic vortex systems with gauge-symmetry breaking will be developed both directly over Maxwell-Proca-Chern-Simons model and indirectly through alternative representations of this model.
At first, some breakthrough scientific discoveries related to this research theme will be indicated to present the Maxwell-Proca-Chern-Simons model, per se, with its equations of motion and some of its alternative representation previously known. Posteriorly, the calculations of forces due to the vacuum fluctuations will be developed implementing some apparently different boundary conditions over the other representations mapped before. Since a coincidence between the forces for two supposedly distinct boundary conditions will be shown, an analysis of the system constraints will be deepened in a way to show that such a coincidence, actually, came from the equivalence of the boundary condition. A quantitative approach of the vortex contributions to the force was also made, displaying how strongly the presence of vortices suppresses the vacuum force.

In this work we studied composites formed by the polymers PVA, PVK and PS modified by the presence of ferrites Fe3O4, FeLiO2 and (La0,6Sr0,4) (Co0,2Fe0,8) O3. For the characterization of these composites, Raman, FT-Raman, FT-IR, Optical Spectroscopy (in the case of PVK, for this to absorb and emit light) and X-Ray Diffraction were used. Raman spectra showed the influences of the polymer bands (reduction of intensity generally) when they were doped with the ferrites, facts also observed in FT Raman spectra and in infrared spectra. Raman spectra also showed changes in the line width of PS + Fe3O4, PS + FeLiO2, PVK + Fe3O4 and PVK + FeLiO2 composites. Another important change observed was in the spectrum of PVA modified with FeLiO2, where we observed the displacements of some vibrational modes. In relation to the infrared spectra, we observed important changes in the spectra of the composites, such as the displacement of the modes of the ferrites inserted in the polymer matrices, besides the reduction of the intensity of the modes, with the exception of the composite formed by the PVA and lithium ferrite. Optical spectroscopy showed changes in the absorption and emission bands of the polymers when the ferrites were inserted in their matrices. The X rays diffraction showed changes in the sizes of the ferrite crystallites, when these were inserted in the polymer matrices, the results of this technique also showed that the magnetite inserted in the polymers did not change phase, a fact that occurred when submitted to the laser radiation 785 nm.

In the context of a real massless scalar field in 1+1 D, Muñoz-Castañeda and Guilarte [Phys. Rev. D 91, 025028 (2015)] calculated the Casimir force between partially reflecting mirrors modeled by a term μ δ (x)+ λ δ′ (x) in the Lagrangian, which, in the limit of perfect mirrors, reproduces the Robin boundary conditions. This model, however, presents a problematic aspect: for λ different of 0 the mirrors are not full transparent at high frequencies. In this work, in order to provide a more realistic feature for this model, we propose a modified δ - δ′ point interaction that enables full transparency in the limit of high frequencies. Taking this modified δ - δ′ model into account, we calculate the Casimir force comparing our results with those found in the literature. In addition, we propose a second modification in the δ - δ′ model, so that in the limit of perfect mirrors we obtain the generalized Robin boundary condition.

Temperature- and pressure-dependent studies of Raman and infrared spectra have been
performed on azetidinium zinc formate, [(CH2)3NH2][Zn(HCOO)3] (AZZn), dimethylammonuim
[(CH3)2NH2][Mg(HCOO)3] (DMMg) and [(CH3)2NH2][Cd(HCOO)3] (DMCd). Vibrational spectra
showed distinct anomalies in frequencies modes and bandwidths near 250 and 300 K, which were
attributed to structural phase transitions associated with gradual freezing of ring-puckering motions of
azetidinium cation. Pressure-dependent studies have revealed a transition below 0.94 GPa. Raman spectra
indicate that the structure of the room-temperature high-pressure phase is the same as the monoclinic
structure observed at ambient pressure below 250 K. A second phase transition has been found between
2.26 and 2.74 GPa. This transition has strongly first order character and is associated with strong
distortion of the zinc formate framework and azetidinium cations. For structure of DMMg, studies reveled
three pressure-induced transitions near 2.2, 4.0 and 5.6 GPa. Theses transitions are associated with
significative distortion of the anionic framework and the phase transition at 5.6 GPa has also great impact
on the DMA+ cátion. The DMCd undergoes two pressure-induced phase transitions, the first transition
occured betwee 1.2 and 2.0 GPa and second one near 3.6 GPa. The first transition leads to subtle
structural changes associated with distortion of anionic framework and the later leads to significant
distortion of the framework. In contrast to the DMMg; the third transition associated with distortion of
DMA+ cation is not observed for the DMCd up to 7.8 GPa. This difference can be most likely associated
with larger volume of the cavity occupied by DMA+ cation in the DMCd and thus weaker interactions
between anionic framework and DMA+ cations.

In this work we present the study of the structural stability of a mono crystal all-trans-β-carotene subjected to extreme low temperatures and high hydrostatic pressure through Raman spectroscopy using excitation line 632.8 nm. Computer calculations were performed using the density functional theory (DFT) to determine the vibration modes of molecule all-trans-β-carotene since the vast majority of vibration predicted by group theory for the crystal are related to internal modes. Temperature in the range between 20 and 300 K the Raman spectra exhibit clear changes in the spectral region comprising the lattice modes and the region containing the internal vibrational modes. The temperature dependence of the most intense vibrational modes ν1(1511 cm-1) and ν2(1156 cm-1), which are related with the stretching vibrations (stretch) of the C = C and C − C bonds, respectively, of the chain polyene suffer shift to higher values of wavenumbers. This behavior is similar to that exposed in the theoretical work of Wei-Long Liu et al. We conclude that the all-trans-β-carotene crystal undergoes a phase transition induced by temperature at around 219 K. This transition is interpreted as rotation (around the dihedral angle) experienced by groups known as β-rings, located at each end of molecule. At low temperatures, the new molecular configuration affects the sliding plane of the space group C52h (P21/n), the phase transition leads to an unchanged monoclinic structure. However, the original space group is possibly reduced to the space group C2. At temperatures of 200 and 220 K, the spectral ratio (S) of the intensities integrated of asymmetric and symmetric stretching modes of the methyl group (CH3) undergoes an abrupt change, such change is more an indication of a phase transition. Moreover, the results obtained by differential scanning calorimetry (DSC) showed the occurrence of an exothermic process temperature close to the transition observed by Raman spectroscopy. In regime of high pressures, the Raman spectra showed various modifications as, for example, the disappearance modes and discontinuities. Such changes have taken place both within the region regarding the modes of the crystal lattice and in the region concerning the purely modes of the molecule. Two phase transitions were observed, the first between 1.94 and 2.5 GPa, and the second between 3.76 and 4.78 GPa. The analysis of the characteristic modes ν1 and ν2 shows a behavior similar to that observed for at Wei-Long Liu et al model. Finally, the Raman spectra at different pressures for β-carotene crystal were not compatible to the Raman spectra of the forms cis presented in this work.

O estudo das propriedades de espalhamento de buracos negros de Schwarzschild em dimensões extras para o campo escalar não massivo restrito à brana quadridimensional é apresentado. O estudo de espaços-tempos extra-dimensionais tem aumentado bastante recentemente graças, em parte, aos trabalhos pioneiros de Randall e Sundrum (em cinco dimensões) e Arkani-Hamed, Dimopoulos e Dvali (em d dimensões). Em especial, os últimos conjecturaram a existência de dimensões extras com escala submilimétrica para resolver o problema de hierarquia. Nesta visão, apenas as pertubações gravitacionais são modicadas no espaço-tempo completo, chamado de bulk, sendo que os demais campos do modelo padrão cam restritos à brana. A partir destas ideias, aqui se discute o espalhamento do campo escalar não massivo, no contexto de dimensões extras, restrito à brana do espaço-tempo de Schwarzschild. Utiliza-se o método de ondas parciais para determinação numérica das seções de choque. Aproximações analíticas, tais como a aproximação de Born, a análise perturbativa de geodésicas e a aproximação para o efeito glória, são utilizadas. É mostrado que tais aproximações concordam muito bem com os resultados
numéricos dentro de seus limites de validade.

Neste trabalho foram crescidos os cristais de sulfato de níquel hexahidratado (NSH), o qual foi dopado com os íons Mn+2 e Cu+2, e posteriormente foram caracterizados por difração de raios X de policristais e difração múltipla de raios X (DMX) usando radiação Síncontron. Os cristais foram crescidos pelo m etodo de evaporação lenta e tendo como resultados cristais de boa qualidade, os quais foram utilizados para os experimentos de
difração de raios X de policristais, no laboraório de física experimental e computacional da UFPA, e para os experimentos de difração múltipla realizados no Laboratório Nacional de Luz Síncontron (LNLS). De acordo com os resultados obtidos da difração de raios X de policristais juntamente com o refinamento Rietveld, podemos perceber que apesar da presença do dopante na estrutura cristalina, os cristais dopados de NSH:Mn e NSH:Cu, apresentaram a mesma estrutura tetragonal e grupo especial do NSH puro. Mudanças no parâmetro de
rede foram observadas o que resultou em um aumento da célula unitária, essas mudanças foram anisotrópicas, ou seja, os parâmetros de rede sofreram variações de formas diferentes e foram constatadas, em comum acordo, pelas técnicas utilizadas. O objetivo deste trabalho foi crescer cristais de NSH e dopar com outros íons para entender o processo de dopagem de materiais e estudar suas propriedades físicas por intermédio dos experimentos de difração de raios X. No LNLS, foram realizados os experimentos de difração múltipla, utilizando energias diferentes na estação de trabalho XRD2. As varreduras renninger mostraram que
a introdução dos dopantes na estrutura cristalina provou mudanças nas posições dos picos, intensidades e na largura meia altura. Estas alterac~oes, observadas nos cristais dopados, podem gerar modificações nas propriedades físicas do material, de modo que, podem acentuar ou inibir alguma propriedade que o cristal possua. O estudo da estrutura destes materiais podem auxiliar outras pesquisas que por ventura utilizem estes cristais. Um exemplo disso é a existência de grupos de pesquisa dopando o NSH com outros materiais, a fim de que possam aumentar a temperatura de desidratação. Assim, as aplicações podem ser feitas nas mais diversas áreas como óptica, eletrônica, medicina e na indústria.

General relativity passed all experimental tests in the weak-field regime, what made it the standard theory of gravity. However, the characterization of strong field regime of general relativity is still a challenge, due to the lack of definitive observational data from high curvature regions. It is in the strong regime that general relativity shows its subtleties and where it is possible to test whether or not general relativity should be replaced by an improved theory of gravity. High curvature regions – and therefore regions of strong field – are possible in the vicinity of compact gravitating objects, such as neutron stars, exotic stars, and black holes. Therefore, these compact objects are excellent laboratories to put constraints and to test theories of gravity in the strong regime. The outcome of the investigation of phenomena around these objects may confront crucial characteristics of general relativity as well as rule out possible alternative theories of gravity. With the advent of potential gravitational wave detectors and new or improved telescopes, it is of utmost importance to study the phenomenology around compact astrophysical objects – to understand the nature of the gravitational objects, to improve the description of the current theoretical models, and to understand the theory of gravity itself. In this thesis, we present a collection of studies on compact objects in general relativity and alternative theories of gravity. The thesis is divided in three main parts. In the first part, we discuss some solutions associated with compact objects. In Chapter 1 we obtain, in closed analytic form, slowly rotating black hole solutions of a general class of theories of gravity, for which the Einstein-Hilbert action is supplemented by all possible quadratic, algebraic curvature invariants coupled to a scalar field. We also discuss possible implications of this solution to the description of accretion disk (thermal) emissions. In Chapter 2 we investigate slowly rotating anisotropic neutron stars in general relativity and in scalar-tensor theories of gravity. We discuss the effect of the fluid anisotropy in the so-called spontaneous scalarization of stars. We also discuss possible ways to constraint the anisotropy of neutron stars. In the second part, we discuss wave-emission processes around compact objects and quasinormal modes. In Chapter 3 we calculate the emission of scalar waves by a particle orbiting a Kerr black hole, within the context of quantum field theory in curved spacetimes at tree level. In Chapter 5 we discuss astrophysical signatures of a plausible supplant to black holes: boson stars. We obtain quasinormal modes – polar and axial – of boson stars within a fully relativistic approach. We also compute the emission of gravito-scalar waves by a particle in circular orbits around boson stars, showing that the star modes “resonate” for some orbits. In Chapter 4 we discuss two different methods to compute the quasinormal modes of spherically symmetric astrophysical environments, namely: the direct integration method and the continued fraction method. In Chapter 6 we discuss the effect of accretion and dynamical friction in the motion and gravitational wave emission of a particle orbiting around – and through – a dark matter star. We model the dark matter star using uniform density stars and using boson stars. We discuss the cases for which the motion of the particle is subsonic and supersonic. In the last part, we discuss planar massless scalar waves impinging upon compact objects, considering their absorption and scattering cross sections. In Chapter 8 we compute the absorption cross section of planar massless scalar waves on Kerr black holes. We consider different angles of incidence, such that we explore the role of the rotation of the black hole into the absorption cross section. We also use the “sinc” approximation to compute the absorption cross of Kerr black holes and compare it with our numerical results. In Chapter 9 we analyze a wave incident on a Schwarzschild black hole surrounded by a thin spherical shell. We show that in the low-frequency limit the absorption cross section approaches the area of the black hole, regardless of the shell characteristics (position and mass). However, in the mid-to-high-frequency limit we show, numerically and analytically, that the absorption cross section can considerably differ from the case of an isolated Schwarzschild black hole with the same ADM mass. In Chapters 10 and 11 we study the absorption and scattering of waves, respectively, incident upon a Bardeen regular black hole. We show that Bardeen black holes can mimic some properties of Reissner-Nordstrom black holes, considering absorption and scattering of fields.

Pesquisas  em  matéria  condensada  como  supercondutores  e  isolantes  topológicos   vem garantindo  avanços  principalmente  na  caracterização  das  propriedades  de  superfície. Consciente  da  importância  dessas  propriedades  na  modelagem  de  nano  dispositivo  que nessa  dissertação  apresentamos  um  modelo  de  uma  nova  classe  de  dispositivo nanoeletrônico. Utilizaremos as propriedades teóricas de transporte eletrônico  para estudar este  modelo  que será  investigado  através do método da Função de Green de Não-Equilíbrio [1] .  A  partir da função de Green obtivemos a  fórmula da condutância do sistema. Os gráficos  da  condutância  sinalizam  a  assinatura  eletrônica  do  sistema  com  a  detecção  de quasipartículas, ou seja, de  Férmions de Majorana . Partiremos  de  um ponto  quântico em contato com eletrodos metálicos, de um único nível, sem spin  acoplado a  uma cadeia de Kitaev [2] , sobre um supercondutor topológico com pareamento -wave.

In this work, we used a modied route of the aqueous sol-gel process to produce spinel
nickel manganite and NiO/NiMn2O4 composite in the powder form. This synthesis route
is quite simple and it is based on the use of sorbitol as a chelating agent. The samples
were characterized by X-ray diraction technique, Rietveld method, scanning electron
microscopy and X-ray photoelectron spectroscopy. The magnetic properties of the samples
were investigated by DC magnetization and AC susceptibility measurements. With
respect to the NiMn2O4 compound X-ray diraction data showed that the sample has
structure of a cubic spinel. Magnetization versus temperature data in low magnetic elds
displayed a signature of a spin glass-type state due to magnetic disorder and frustration.
One well dened peak in real part AC susceptibility curve was observed, indicative of the
spin glass-type transition. The frequency dependence of AC susceptibility clearly demonstrated
a spin glass-type behavior with the onset of spin glass freezing below to the Curie
temperature. From DC magnetization data, the NiMn2O4 compound can be identied
as a cluster spin glass or cluster glass with both ferrimagnetic and spin glass-type phases
coexisting together at low temperatures. Thus, the exchange bias eect observed in the
NiMn2O4 compund was proposed to be due to the pinning eect at the ferrimagnetic and
spin glass-type interface. In relation to the results of the pollycrystaline NiO/NiMn2O4
composite, the Rietveld renement data showed that the percentage of the NiMn2O4 phase
in the samples ranged from 62 to 89 wt% and the NiO phase ranged from 48 to 11 wt%.
Magnetic hysteresis cycles of the NiO/NiMn2O4 composite performed at T = 5 K after
cooling the samples in constant magnetic eld (H = 50 kOe) showed a displacement to
negative elds, which is typical of the exchange bias eect. This eect must be presumably
due to exchange coupling at the interfaces between the antiferromagnetic (NiO) and
ferrimagnetic (NiMn2O4) materials. It also investigated the eect of NiO concentration
in the exchange bias eld and the coervicity. The results showed that exchange bias eect
changed with increasing concentration of NiO.

Neste trabalho é apresentado uma investigação do transporte eletrônico na junção de molecula única composta por molécula de bifenil ligada à eletrodos de nanotubos de carbono metálico e semicondutor dopado tipo-p. Os resultados apontam que as características corrente-tensão são assimétricas como um resultado da natureza eletrônica diferença dos eletrodos direito e esquerdo, devido serem semicondutores e metálicos, respectivamente. Uma análise de espectrocopia de voltagem de transição foi feita para cada sistema mediante os gráficos Fowler-Nordheim e Lauritsen-Millikan; estas análises permitem a identificação das posições de ressonâncias e as regiões onde ocorre condutância diferencial negativa. As curvas de transmitância são bem descritas pelas curvas Fano, para ambas as voltagens direta e reversa, mostrando que os orbitais moleculares de fronteiras são efetivamente envolvidos no processo de transporte. Estes resultados dão suporte para a interpretação de espectroscopia de voltagem de transição baseado no modelo de transporte coerente.

Nas últimas décadas, a Cosmologia adentrou na era da precisão". Consideráveis progressos foram alcançados nas medidas de distâncias atraves de técnicas que vão desde a utilização de Supernovas tipo Ia (SNe Ia) como velas padrões, passando pela detecção direta de raios-x, até Oscilação Acústica dos Bárions (OAB) e variações no espectro da radiação cósmica de fundo devido ao efeito Sunyaev-Zel'dovich, entre outras. Esta dissertação é um estudo preliminar sobre a validade da Relação de Etherington (RE) que conecta as duas mais importantes distâncias em Cosmologia através de DL(z)(1 +z)􀀀2=DA(z) = 1, na qual DL e DA são a distância de luminosidade e diâmetro angular respectivamente. Para testar a RE nós utilizamos o metodo estatístico 2 considerando DL(z)(1+z)􀀀2=DA(z) = (z), adotando dados de DL de SNe Ia (UNION 2.1) e as melhores medidas de DA da literatura oriundas de OAB (WiggleZ, SDSS DR7 e BOSS DR11), bem como, adicionamos uma medida de DL de Gamma-Ray Burst em alto redshift para incluir todos os dados da amostra de OAB. Usamos duas parametrizações: (z) = 1+0z e (z) = 1+0z=(1+z), nas quais 0 representa o desvio da RE padrão. Nossos resultados fornecem 0 = 􀀀0; 030; 10 e 0 = 􀀀0; 050; 16 com 1 de nível de confiança estatística. Portanto, nossa análise indica uma excelente concordância entre a RE e as mostras. Mas, esses resultados são preliminares devido a amostra de OAB ser pequena. Novos dados e estudos permitirão investigar melhor a discrepância na RE e sua possível natureza.

Neste trabalho, estudamos as propriedades magnéticas de nanoestruturas adsorvidas sobre superfícies metálicas, via método de primeiros princípios. Usando nanoestruturas de Fe-Pt sobre a superfície de Pt(111), nossos resultados mostram que é possível ajustar a interação de troca e a conguração magnética de ad-átomos de Fe através da introdução de diferentes números de átomos de Pt que ligam os átomos de Fe, ou incluindo efeitos de bordas. Uma nanoestrutura de Fe de geometria triangular sobre a Pt(111) mostrou uma complexa estrutura magnética, com ordenamento magnético do tipo spin-spiral, a qual muda de direção de propagação nos vértices do triângulo. Investigamos também a conguração magnética de nanoestrutura de geometria hexagonal de Fe sobre a superfície de Au(111). Para este sistema vericamos que cálculos sem relaxação estrutural indicam uma conguração magnética aproximadamente ferromagnética. Este ordenamento é modicado para estruturas não-colineares compostas por diferentes sub-redes de spins, quando a relaxação estrutural é incluída nos cálculos. Também apresentamos a estrutura magnética de nanoos e nano-estruturas de Mn adsorvidas sobre a superfície de Au(111), e comparamos estes resultados com os obtidos para geometrias de Mn sobre a superfície de Ag(111). Para nanoos nitos de Mn sobre Ag(111) e Au(111), nossos resultados ab-initio mostram que a grande diferença entre o acoplamento spin-órbita destas superfícies leva a congurações magnéticas bem diferentes. O ordenamento magnético para nanoestruturas de Mn adsorvidas sobre Ag (111) é regido pela forte interação de troca entre os ad-átomos de Mn. Para nano-objetos de Mn sobre o Au(111), a competição entre as interações de Heisenberg e Dzyaloshinskii-Moriya leva a uma complexa estrutura magnética para os sistemas aqui considerados: para o nanoo com sete átomos obtivemos um ordenamento tipo spin-spiral helicoidal, e para um nanoo com o dobro do tamanho, vericamos uma complexa conguração magnética incomensurável com a rede do substrato. O efeito de relaxamento estrutural também foi investigado, mostrando interessantes estruturas magnéticas não-colineares, com um plano de simetria de reexão.

Sob determinadas condições, ondas sonoras em um fluido ideal são governadas pela equação de Klein-Gordon em um espaço-tempo efetivo chamado de espaço-tempo acústico. Os espaços-tempos acústicos são determinados pelas propriedades do fluido não perturbado. Nesta tese, consideramos os espaços-tempos acústicos do buraco acústico canônico, do buraco acústico girante e do vórtice hidrodinâmico. Para o buraco acústico canônico, calculamos as frequências quase-normais e os polos de Regge usando três diferentes métodos. Primeiro, mostramos como oscilações amortecidas surgem a partir da evolução de perturbações gaussianas no domínio temporal, usando o método da diferença finita, e estimamos as frequências quase-normais fundamentais. A seguir, aplicamos o método da expansão geodésica para obter uma expansão para as frequências quase-normais em potências inversas de l + 1=2, onde l é o momento angular associado às ondas sonoras. Testamos o método da expansão geodésica comparando os resultados obtidos via método da diferença finita. O método da expansão geodésica é então usado também para obter os polos de Regge. Para verificar a validade do método da expansão geodésica, calculamos numericamente o espectro dos polos de Regge via método da integração direta no domínio da frequência e comparamos os resultados obtidos. Para o buraco acústico girante, apresentamos uma investigação completa do papel dos modos quase-normais e dos polos de Regge neste espaço-tempo efetivo. Primeiramente, simulamos uma perturbação no domínio temporal aplicando o método da diferença finita para demonstrar a ubiquidade do toque quase-normal. Em seguida, resolvemos a equação de onda no domínio da frequência com o método da fração continuada, para calcular numericamente os espectros dos modos quase-normais e dos polos de Regge. Exploramos a relação geométrica existente entre geodésicas nulas (diretas e retrógradas) no espaço-tempo do buraco acústico girante, e as propriedades do espectro quase-normal/polos de Regge. Aplicamos o método de expansão geodésica para obter uma expressão assintótica para o espectro dos modos quasenormais/polos de Regge, para as funções radiais e para os resíduos. Além disso, estudamos o papel dos polos de Regge nos processos de absorção e espalhamento por meio da aplicação do método do momento angular complexo. Elucidamos a ligação entre polos de Regge e as oscilações no comprimento de choque de absorção. Finalmente, mostramos que os polos de Regge fornecem uma explanação clara acerca de oscilações vistas no comprimento de choque de espalhamento. Por fim, para o vórtice hidrodinâmico, fornecemos uma evidência direta de que este espaçotempo é um sistema instável quando submetido a perturbações lineares. Para mostrarmos isso, calculamos os modos quase-normais deste sistema usando três diferentes métodos (um método no domínio temporal e os outros dois no domínio da frequência). Mostramos que o aparecimento de modos instáveis no vórtice hidrodinâmico está diretamente relacionado à presença de uma ergorregião e à ausência de um horizonte de eventos neste espaço-tempo efetivo.

No presente trabalho, investigamos as propriedades vibracionais de dois materiais da família dos molibdatos de lantanídeos com fórmula geral Ln2Mo4O15 (Ln = Sm, La) através de experimentos de espectroscopia Raman, e com o auxílio dos cálculos de dinâmica de rede, tentamos compreender os efeitos da aplicação de pressão hidrostática nas propriedades estruturais e vibracionais desses materiais. O material Sm2Mo4O15 para o experimento Raman de baixas temperaturas, se mostrou estável até alcançar a temperatura mínima de 10 K, não alterando a sua estrutura cristalina. Esta mesma situação foi observada para o material La2Mo4O15 sob baixas temperaturas.

No experimento de espectroscopia Raman de altas temperaturas, o molibdato de lantânio atingiu uma temperatura de 1173 K e o mesmo foi interrompido nesta etapa, devido ao aumento na relação sinal ruído. Observou-se também que o mesmo atingiu um estado amorfo, mostrando uma degradação da amostra. Já para o molibdato de samário, esta degradação aconteceu para temperatura em torno de 1033 K, sendo que a amorfização é notada por uma diminuição abrupta da intensidade Raman e aumento da relação sinal ruído.

A realização dos experimentos sob altas pressões, para esses dois materiais, revelou que o material Sm2Mo4O15 apresenta uma transição de fase de segunda ordem, passando para uma fase amorfa em torno de 4,5 GPa. Devido ao tipo de transição, conseguimos determinar, através de Teoria de Grupos, a nova fase mais provável que o molibdato de samário possui no regime de altas pressões. No caso do material La2Mo4O15,
observamos que ocorre uma transição de fase de primeira ordem em torno de 2,0 GPa, e acima de 6,8 GPa a amostra tende a amorfizar-se. Ao contrário do molibdato de samário, a identificação da nova fase para o molibdato de lantânio encontra-se em estudo, já que a natureza das transições de fase de primeira ordem é mais complexas que as de segunda ordem, não possibilitando uma análise conclusiva sob o ponto de vista hierárquico de grupo-subgrupo.

Neste trabalho utilizamos a técnica de varredura-Z para a determinação dos
parâmetros ópticos não-lineares (índice de refração não-linear e coeficiente de absorção
não-linear) do ácido oleico e das soluções de beta-caroteno diluído em ácido oleico. As
concentrações das soluções estudadas foram as seguintes: 0, 5:6, 11:3, 22:5, 30, 45 e
60 g=ml, os índices de refração foram calculados levando-se em consideração os dados
experimentais e o modelo teórico previsto por Sheik-Bahae. As soluções de ácido oleico
puro e de ácido oleico com beta-caroteno (60 g=ml) apresentaram absorção não-linear e
seus respectivos coeficientes foram calculados. Obtivemos tambem os índices de refração
não-linear para os óleos de andiroba, buriti e copaíba, todos nativos da região Amazônica.

Neste trabalho investigamos as propriedades magneticas de currais de Fe, Cr e Mn adsorvidos sobre a superfcie de Pt(111) utilizando o metodo RS-LMTO-ASA (Real Space Linear Mun Tin Orbital - Atomic Sphere Approximation), o qual e um metodo de primeiros princpios baseado na Teoria do Funcional da Densidade (DFT-Density Functional Theory), que permite o calculo de estruturas magneticas não-colineares. Obtivemos que os atomos de Fe apresentam momentos magneticos elevados, da ordem de 3.5 µB/atomo, e têm uma interação de troca entre primeiros vizinhos forte e ferromagn etica. Isto leva a um arranjo magnetico colinear no curral. Para os currais de Mn e Cr encontramos que estes possuem elevado momento magnetico, da ordem de 4.51 µB/átomo e 4.15 µB/átomo, respectivamente, e interações de troca entre primeiros vizinhos antiferromagn eticas. Isto conduz a arranjos magneticos colineares em currais simples, assim como interessantes ordenamentos não-colineares, tais como estruturas tipo vortice (skyrmions), para os currais com uma geometria particular onde o antiferromagnetismo se apresenta frustrado.