http://repositorio.ufc.br/handle/riufc/169 2026-06-16T13:35:55Z http://repositorio.ufc.br/handle/riufc/86564 Título: Cenários de mundo-brana em gravitação teleparalela: uma abordagem com invariante de Gauss-Bonnet Autor(es): Silva, Pedro Uchoa Araújo Abstract: The present dissertation investigates an alternative gravity model that integrates the teleparallel formulation with Gauss-Bonnet invariant corrections within brane-world scenarios. The work is motivated by the fact that phenomena such as dark energy, cosmological inflation, and the hierarchy problem challenge the Standard Model and General Relativity. It begins with a brief review of General Relativity. Subsequently, the three theoretical pillars of the model are discussed. The first pillar is Teleparallel Gravity. Next, the Gauss-Bonnet term and its teleparallel counterpart are examined. Finally, the addition of an extra dimension via the Randall-Sundrum model is addressed. Methodologically, the reduced action formalism is adopted with a tetrad ansatz compatible with the warped metric. To this end, it is necessary to introduce a lapse function for the extra dimension in order to ensure the appropriate number of equations of motion. From this configuration, the field equations are derived for the general action F(T,GT ), where T is the torsion scalar and GT is the teleparallel equivalent of the Gauss- Bonnet term. The analysis of the model's dynamics reveals that the algebraic complexity of the field equations leads to several consequences, such as the emergence of dynamic for these newlapse function. The study delves into the quadratic case (α GT + βG_T^2). In the thin-brane limit, constraints on the model's parameters are established, demonstrating that the bulk cosmological constant naturally emerges from the geometric corrections. For thick branes, two exact and original solutions were constructed: one kink-like and other 2-kink-like solution. These solutions indicate that the first and second-order Gauss-Bonnet corrections may either interact in a mutually compensating manner, approaching the usual case, or acting together to generate more complex structures, analogous to those produced by multiple scalar fields. Tipo: Dissertação 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/86563 Título: Excitons de transferência de carga em bicamadas de semicondutores rotacionadas Autor(es): Albuquerque, João Italo Carneiro de Abstract: Two-dimensional semiconductors constitute a promising platform for engineering next-generation optoelectronic devices. In addition, van der Waals structures formed by the vertical stacking of these materials have further expanded the range of systems to be studied, since this stacking can give rise to optical, electronic, and mechanical properties that do not exist in the individual constituent materials. Furthermore, the relative rotation between layers in these structures introduces an additional degree of freedom. This rotation gives rise to a periodic pattern, called a moiré pattern, which induces spatial modulations in the valence and conduction bands along the material plane. These modulations can be viewed in the continuum model as an effective periodic potential felt by the charge carriers present in the semiconductor. The moiré exciton is a quasiparticle formed when the electron-hole interaction is strong, where the electron and hole are confined in the same region of the material. In the case where the moiré potential is comparable to the interaction potential, the exciton center-of-mass coordinates cannot be separated from the relative coordinates, and the concept of moiré exciton fails. In this work, we propose a model that allows the transition between the regime of moiré electrons and holes, in-plane spatially indirect excitons, and the regime of moiré excitons. Our results show that even in situations where the moiré potential confines electrons and holes in the same regions of the plane, the conventional models used in the literature to calculate the exciton binding energy underestimate their actual values. Tipo: Dissertação 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/86441 Título: Aspects of field theories with Lorentz symmetry violation Autor(es): Belchior, Fernando de Menezes Abstract: In recent years, studies on Lorentz symmetry violation (LSV) in field theory have gained significant prominence in the literature. Such investigations have led to the construction of the Standard Model Extension (SME), which possesses both minimal and non-minimal versions. The minimal SME contains only renormalizable terms, whereas the non-minimal formulation includes all non-renormalizable operators. In this thesis, we investigate a variety of theoretical and phenomenological aspects of LSV within the frameworks of the SME, very special relativity (VSR), and related effective field theories, which employ background vector and tensor fields. In the first part of this work, we focus on aspects of LSV in flat spacetime, including one-loop radiative corrections and dualities. We begin by revisiting the gauge and fermionic sectors of Lorentz-violating (LV) quantum electrodynamics (QED) with CPT-odd and CPT-even terms. We show that the axial coupling 𝑏𝜇¯ 𝜓 𝛾𝜇𝛾5𝜓 induces coefficients that depend on the choice of regularization scheme and on the treatment of 𝛾5 in different dimensions. We also study radiative corrections in a vector model with spontaneous LSV, known in the literature as the bumblebee model. In addition to Lorentz symmetry breaking, this model exhibits gauge symmetry violation. To restore gauge symmetry, we introduce a Stueckelberg field and calculate the two-point function by employing the principal-value (PV) prescription. The result is non-transversal, leading to a massive excited mode. Furthermore, we investigate the classical duality between the 𝑆𝐼𝑀(1) - Maxwell-Chern-Simons (MCS) model and its self-dual counterpart. Initially, our analysis focuses on free-field cases in order to establish equivalence through two distinct approaches: comparison of the equations of motion and the use of the master Lagrangian method. Another topic addressed involves the classical duality between the 𝑆𝐼𝑀(2) - Maxwell-Kalb-Ramond (MKR) model and its non-gauge-invariant self-dual counterpart. We establish the equivalence for free-field theories by employing two methods: direct comparison of the equations of motion and the master Lagrangian approach. In both methodologies, the classical correspondence between the MKR model and self-dual fields exhibits modifications due to VSR. In the second part of the thesis, we examine aspects of LSV in curved spacetime, deriving a black hole solution with a global monopole in a modified gravity framework that includes a self-interacting Kalb-Ramond tensor field non-minimally coupled to gravity. Finally, the thesis discusses the phenomenological implications and experimental prospects of the studied models by establishing bounds on LV coefficients and comparing theoretical consistency conditions with current and future experimental sensitivities. The results presented here contribute to a deeper understanding of how LSV may manifest in field theories and gravitational contexts. Tipo: Tese 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/86391 Título: Injeção de esferas magnéticas em uma cavidade circular parcialmente preenchida com um substrato Autor(es): Silva Junior, Márcio Vaz da Abstract: Magnetic particle packing exploits dipolar interactions for spontaneous self-organization into chains or ordered structures, with applications in self-responsive systems and the understanding of complex biological structures. This study investigates packing patterns in a two-dimensional circular cavity, where linear chains of neodymium spheres are injected diametrically into a Hele-Shaw cell containing an inert granular substrate. The analysis reveals that the initial dipole configurations and the substrate area fraction (η) govern the emerging geometry. For initially parallel dipoles, the system forms loops whose areas follow a log-normal distribution, indicating hierarchical fragmentation of space, and whose quantity scales via a power law, while the number of contacts (Nc), between magnetic particles, remains invariant.Topological analysis via graphs revealed that increasing the substrate alters the network connectivity: the shortest-path distribution transitions from unimodal (η = 0.0 , cohesive network) to bimodal (η > 0.2 ), evidencing the formation of clusters isolated by low-coordination bottlenecks. Through the BFGS energy minimization method, it was identified that the system tends to organize into square symmetry networks with antiferromagnetic domains, presenting energies per particle (uN) close to the theoretical limit of linear chains (u∞ ≈ −2.404). Electrical transport properties were modeled by treating contacts as resistors and utilizing the Moore-Penrose inverse of the Laplacian matrix. It was observed that the equivalent resistance reaches peaks at diferente packing stages as η increases, reflecting the transition from parallel to serial conduction paths. In the critical regime of η = 0.83, substrate saturation prevents loop nucleation, resulting in monotonic resistive growth. The study demonstrates that current distribution concentrates on the network backbone, validating the integration between contact morphology and transport efficiency in heterogeneous media.The study expands the understanding of magnetic matter behavior in confined systems. Tipo: Dissertação 2026-01-01T00:00:00Z