https://repositorio.ufpb.br/jspui/handle/tede/5435 PPGF Mon, 05 Oct 2026 22:06:20 GMT 2026-10-05T22:06:20Z https://repositorio.ufpb.br/jspui/handle/123456789/39423 Título: Dopagem substitucional em nanofitas de grafeno com borda zigue-zague: estabilidade, estrutura eletrônica e resposta óptica preliminar via DFT Autor(es): Cavalcante, Carlos Rodrigo Moura Orientador: Azevedo, Sérgio André Fontes Abstract: The absence of an intrinsic band gap in graphene restricts its application in logic nanoelectronic devices, motivating the investigation of nanostructures such as zigzag graphene nanoribbons (ZGNRs). Although lateral quantum confinement promotes the opening of a band gap in ZGNRs associated with inter-edge antiferromagnetic ordering, the high reactivity of their pristine edges requires passivation and modulation, such as substitutional doping. This thesis systematically investigates the thermodynamic stability, electronic structure, and optical response of pristine ZGNRs and those edge-doped with nitrogen or boron, as a function of the ribbon width (n = 2 to 16). Density functional theory was employed, using the PBE functional for geometric optimization and the hybrid HSE06 functional to mitigate the self-interaction error and obtain better quantitative descriptions of the electronic states and the band gap. The optical properties were studied within the framework of the independent particle approximation. The results reveal that nitrogen doping induces a slight lattice contraction with a low-energy cost, preserving the global stability of the system in a manner analogous to pristine ribbons. In contrast, boron doping presents a higher energy cost, especially in the strong confinement regime. The electronic structure analysis demonstrates that all nanoribbons operate as narrow-gap semiconductors, whose magnitude decays asymptotically with widening. It was verified that nitrogen doping preserves the geometric tunability of the band gap and induces a spin splitting in the edge states on the order of 0.2 eV. In boron-doped ribbons, a strong spatial localization of the edge state was observed, associated almost exclusively with the px orbital of the dopant, conferring stability to the magnitude of the band gap against variations in width. In the optical domain, a redshift of the absorption threshold was identified with the widening of the nanoribbon; additionally, boron doping promotes a significant redistribution of the oscillator strength, intensifying absorption in the ultraviolet range (4.0 to 6.0 eV). It was also demonstrated that optical edge effects lose relevance for widths n ≥ 14, where transitions from the graphene-like central portion begin to dominate. It is concluded that N-doped ZGNRs emerge as candidates for spin logic architectures and tunable optoelectronics, while B-doped ones are promising for application in spin filters, consolidating a relevant predictive map for the design of carbon-based nanodevices. Editor: Universidade Federal da Paraíba Tipo: Tese Thu, 23 Jul 2026 00:00:00 GMT https://repositorio.ufpb.br/jspui/handle/123456789/39423 2026-07-23T00:00:00Z https://repositorio.ufpb.br/jspui/handle/123456789/39417 Título: Ação de um laser randômico no regime de localização induzida em uma suspensão coloidal Autor(es): Gomes, Anderson Alexandre Vieira Orientador: Villar, Ernesto Jimenez Abstract: Anderson localization of light and random lasing at the localization transition are an open research frontier. The first approach to demonstrate experimentally Anderson localization of light in three dimensional systems was based on the increase of the samples dispersion by increasing the scatterer density with the goal of obtaining the Ioffe-Regel criterion of kl = 1, where k is the wavevector and l is the transport length. Howe- ver, this strategy decreases the separation between scatterers and consequently increases near field coupling which hampers Anderson localization. Another, second and less well- known approach, is to introduce correlation in the scatterers position, that is, inducing a certain order in the disorder or, in other words, to introduce disorder in an ordered photonics structure. This strategy was recently demonstrated theoretically by Kravtsov et al and also Dal Negro et al, showing the universality of this phenomenon. They cal- led this phenomenon of correlated long-range hopping correlation-induced localization. We demonstrated experimentally localization of light in 3D induced by correlation in the scatterers position, which was attributed to the long-range Coulomb interaction between the scatterers. Additionally, we proposed that an increase of the silica shell thickness should lead to an increase of the scatter?s hydrodynamic diameter, which would induce a stronger and longer-range Coulomb interaction between the scatterers. In this manuscript we show random lasing in correlated colloidal systems origi-nated by long-range Coulomb interactions between core-shell TiO2@Silica nanoparticles suspended in ethanol solution of Rhodamine 6G. TiO2 nanoparticles with two different silica shell thicknesses (40 nm and 70 nm) were prepared. We find that the Random laser performance improves when the silica shell is thicker (70 nm). Also, light diffraction patterns in both TiO2@Silica suspensions showed strong correlation in the scatterers po- sition, being stronger when the silica shell is thicker. Together with other experimental evidence, we demonstrate that stronger and longer-range Coulomb interaction between the scatterers, caused by a thicker shell in TiO2@Silica nanoparticles, causes a stronger localization of light (higher density of localized states) induced by stronger correlation in the scatterers position. Editor: Universidade Federal da Paraíba Tipo: Tese Fri, 03 Sep 2021 00:00:00 GMT https://repositorio.ufpb.br/jspui/handle/123456789/39417 2021-09-03T00:00:00Z https://repositorio.ufpb.br/jspui/handle/123456789/39368 Título: Quantificações de interferência em sistemas quânticos multipartidos Autor(es): Brito, Rejane Alves de Orientador: Bernardo, Bertúlio de Lima Abstract: By using the density matrix formalism, we analyze the phenomenon of genuine quantum interference for multipartite systems. The characterization and quantification of quantum correlations is of paramount importance to better understand quantum theory, as well as to correctly differentiate quantum correlations from classical ones. Among the quantum correlations, the most prominent one is entanglement, however, it is by no means the only one. Furthermore, the more complex the system, with regard to the number of constituents and the dimension of the Hilbert space that describes them, more types of quantum correlations can arise. Regarding the genuine quantum interference of two and three particles, studied in the course of this work, its manifestation occurs when analyzing the rates of joint detection of the constituent particles. In this case, there has been a consensus that, for this type of phenomenon to be observed, entanglement is necessary. However, as we will demonstrate at the end of this work, entanglement is not a requirement to verify these special types of interference, according to the approach presented here. In addition, we found that the interference quantifiers proposed here also serve to quantify entanglement, as long as they are applied to pure states. Editor: Universidade Federal da Paraíba Tipo: Tese Fri, 28 May 2021 00:00:00 GMT https://repositorio.ufpb.br/jspui/handle/123456789/39368 2021-05-28T00:00:00Z https://repositorio.ufpb.br/jspui/handle/123456789/39351 Título: Da Teoria de Gauge métrica-afim da gravitação ao teleparalelismo generalizado: equações de campo e a energia gravitacional Autor(es): Duarte, João Artur Cordeiro Orientador: Formiga, Jansen Brasileiro Abstract: This dissertation proposes a generalization of the teleparallel equivalent of general relativity based on the metric-affine gauge theory of gravitation (MAG). Furthermore, the relation between the teleparallel frame and the affine connection is analyzed, and a study of the gravitational energy problem is carried out. By applying the variational formalism of MAG, a geometric model is constructed that has degrees of freedom associated with torsion and nonmetricity, and it is shown that it reproduces Einstein’s equations. The analysis of the teleparallel frame shows that teleparallel theories without nonmetricity have bases with a constant scalar product as their teleparallel frame, while theories without torsion use coordinate bases. With regard to the gravitational energy problem, definitions of the 4-momentum and of the energy-momentum tensors of the gravitational field and spacetime are developed. The application of the formalism to the case of plane-fronted gravitational waves (pp-waves) provides results consistent with those found in the literature. These results indicate that generalized teleparallelism is suitable for the analysis of gravitational field energy and allows greater flexibility in choosing the teleparallel frame. Editor: Universidade Federal da Paraíba Tipo: Dissertação Mon, 23 Feb 2026 00:00:00 GMT https://repositorio.ufpb.br/jspui/handle/123456789/39351 2026-02-23T00:00:00Z