http://repositorio.ufc.br/handle/riufc/212 Mon, 15 Jun 2026 20:13:09 GMT 2026-06-15T20:13:09Z 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 Thu, 01 Jan 2026 00:00:00 GMT http://repositorio.ufc.br/handle/riufc/86564 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 Thu, 01 Jan 2026 00:00:00 GMT http://repositorio.ufc.br/handle/riufc/86563 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 Thu, 01 Jan 2026 00:00:00 GMT http://repositorio.ufc.br/handle/riufc/86391 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/86122 Título: Pontos quânticos em bicamada de grafeno rotacionada na presença de junções p-n Autor(es): Bernardo, Brehmer Braga Abstract: Since the first experimental isolation of graphene in 2004, a vast number of research studies on this material have been conducted owing to its exotic physical properties, which are mostly due to its linear dispersion relation near the Fermi level. An additional degree of freedom to tune the electronic properties of graphene-based systems is provided by the stacking order, achieved by considering the overlaying and coupling of its monolayer compounds with offsets or rotations. In a bilayer system with the rotation of one of the graphene layers, figures known as Moiré patterns appear: quasi-periodic structures that, for certain magic angles, become periodic, forming Moiré superlattices. When the twist angle is close to the magic angle, the band structure of bilayer graphene near the Fermi energy becomes flat, leading to an unconventional superconductor-like behavior. Within the context of doped bilayer graphene-based systems and in addition to optoelectronic devices, it is natural to study junctions formed by excess or lack of electrons, that is, p-n junctions that can be generated by doping or application of external potentials, being such junctions a key piece for semiconductor devices, such as the diode. Thus, in this work, we theoretically investigate, using the tight-binding approach, the behavior of electrons and holes in circular twisted bilayer graphene quantum dots for potential applications in p-n junction formation. The quantum dots are defined by an infinite-mass potential, so the specific edge effects are absent. The effects of (i) the twisting angle, (ii) the p-n junction alignment, and (iii) the applied magnetic field perpendicular to the QD surface on the energy levels and on the spatial distribution of the confined states are studied. Tipo: Dissertação Sun, 01 Jan 2023 00:00:00 GMT http://repositorio.ufc.br/handle/riufc/86122 2023-01-01T00:00:00Z