http://repositorio.ufc.br/handle/riufc/212 2026-05-31T16:46:23Z 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 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 2023-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/85551 Título: Condições de contorno para nanofitas de fosforeno com bordas enviesadas Autor(es): Costa, Tiago da Silva Abstract: Phosphorene, a monolayer of black phosphorus, exhibits a honeycomb-like structure in top view and a puckered structure in other directions. This is due to sp3 hybridization, in which each phosphorus atom establishes three covalent bonds with neighboring atoms in the monolayer. As a result, phosphorene-based nanostructures can present different edge terminations, called normal and skewed, of the zigzag and armchair type. In the effective theoretical description of the phosphorene monolayer, the literature provides tight-binding models that account for two, five, or ten hopping interactions. Initially, this research employed these models to describe the band structure of the infinite system, exploring the role of each energetic connection between sites in the three theoretical models. It was found that adjustments in the interaction strengths, in-plane and out-of-plane, can drastically change the electronic dispersions of the system’s energy bands. Subsequently, using the three tight-binding models, phosphorene nanoribbons with different widths and distinct edge termination types were studied: normal zigzag, normal armchair, skewed zigzag, and skewed armchair. Results on the dispersion relations and wave functions of edge states were discussed. It was verified, for the case of nanoribbons with normal zigzag and skewed armchair edges, that adjustments in the interaction contributions allow quasi-flat edge states to become perfectly flat. As a perspective, it is intended to employ the continuum approximation, derived from the Taylor series expansion around the Γ point of the tight-binding model, to describe the electronic properties of different phosphorene nanoribbons. To this end, suitable boundary conditions will be proposed to describe the electronic properties of phosphorene nanoribbons with skewed and normal edges. The analytical results can then be compared with those provided by the tight-binding models. Finally, we seek to demonstrate the dependence of the bandgap on the width of the nanoribbons. Tipo: Dissertação 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/85550 Título: Escoamento bifásico em meio poroso: análise dos regimes do estado estacionário Autor(es): Magalhães, Alessandro Peixoto Abstract: Immiscible displacement in disordered porous media exhibits a wide range of steady-state interfacial morphologies whose collective organization cannot be inferred from transport laws alone. These patterns arise from the competition between viscous, capillary, and inertial forces; however, descriptions that span different regimes and simultaneously connect dimensionless control parameters, morphology, and collective structure remain limited. In this work, we simulate the Navier-Stokes equations with periodic boundary conditions to resolve statistically steady two-phase flow in a disordered porous matrix consisting of a random array of non-overlapping disks, over wide ranges of pressure gradient and interfacial tension, covering multiple orders of magnitude in the Capillary and Forchheimer numbers. Using unsupervised machine learning, we identify three robust steady-state morphologies — bubbles, stripes, and mixtures — organized in a regime diagram (Fo, Ca) with sharp and consistent transitions governed by the balance between capillary, viscous, and inertial effects. Despite these pronounced morphological reorganizations, macroscopic transport varies smoothly: stripe and mixture states follow a Forchheimer-type V versus |∇p| relation, while bubble states deviate systematically due to the additional energy barrier arising from interface deformations. To characterize the collective organization underlying these morphologies, and following the precedent of pseudo-thermodynamic analogies for two-phase flows, we coarse-grain steady-state configurations into binary fields and infer pairwise maximum-entropy models. The inferred Hamiltonians reproduce the fitted first- and second-order statistics and, near some morphological transitions, accurately predict out-of-sample higher-order correlations. Thermodynamic analysis of the inferred models—in which temperature is a parameter of the statistical model, unrelated to the physical temperature of the fluid—reveals regime-dependent specific heat signatures: a considerable fraction of stripe morphologies operate near an order-disorder transition point, while bubble and mixture states correspond to supercritical organization. Taken together, the hydrodynamic regime map and the maximum-entropy representation establish a unified, dimensionless framework connecting interfacial morphology, macroscopic transport, and collective statistics in two-phase porous flows. Tipo: Dissertação 2026-01-01T00:00:00Z