http://repositorio.ufc.br/handle/riufc/218 Wed, 10 Jun 2026 13:26:10 GMT 2026-06-10T13:26:10Z 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 Thu, 01 Jan 2026 00:00:00 GMT http://repositorio.ufc.br/handle/riufc/86441 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/86087 Título: Electron-phonon coupling in low-dimensional metal halide perovskites Autor(es): Soares, Cássio César Silva Abstract: Metal halide perovskites (MHPs) are versatile, low-cost materials for optoelectronic applications like solar cells and LEDs, offering tunable architectures that enhance optical absorption and carrier mobility. Low-dimensional metal halide perovskites (LDMHPs) have specifically gained attention for their superior stability and molecular-level quantum confinement, where structural constraints create 2D layers, 1D chains, or 0D clusters of the inorganic framework. The "soft" lattice nature of LDMHPs facilitates rapid structural distortions upon excitation as the charge carriers of the quasiparticles called excitons, formed by bound electron-hole pairs through Coulomb forces, interact with lattice ions through phonons. This relationship is governed by electron-phonon coupling (EPC), which forms quasiparticles called polarons that significantly dictate optoelectronic properties such as photoluminescence (PL). Understanding the mechanisms and strength of EPC is therefore essential for developing high-efficiency materials, particularly for white light illumination sources capable of achieving substantial global energy savings. In this context, this thesis focuses on the study of EPC in LDMHPs, primarily concerning their optoelectronic properties, such as low-temperature PL. The first family of LDMHPs studied was the 0D halide perovskites A2SnBr6 (A = Cs, Rb). For Cs2SnBr6, the analysis revealed a strong EPC, as evidenced by multiphonon scattering, leading to intense overtones up to the fourth order. Such strength of the EPC was confirmed by the Huang–Rhys factor S = 24.4 from the PL phenomenon arising from self-trapped excitons (STE) with the contribution of phonon modes related to the Sn–Br asymmetric stretching predominating in the electronic transition. Furthermore, for Rb2SnBr6, temperature-dependent PL measurements, complemented by other characterization techniques and theoretical calculations, revealed broadband emission with a significant Stokes shift attributed STE state. The Fröhlich mechanism, mediated by interactions between excitonic charge carriers and longitudinal optical (LO) phonons, primarily accounted for the emission broadening through phonon-assisted radiative recombination. The EPC strength was evaluated through the Huang–Rhys factor S = 34, confirming strong correlations between electronic and vibrational properties. The possible mechanism of STE formation was evaluated by the Fröhlich parameter α of 2.78 for electrons and 4.41 for holes, which pointed out a major contribution of the hole-polaron quasiparticle on exciton trapping. Finally, the last family of LDMHPs investigated consisted of low-dimensional lead bromide perovskites with chiral spacers based on α-methylbenzylamine (MBA). It was found that the use of a racemic mixture of this organic molecule yielded a reduction of dimensionality from the 2D (R-/S-MBA)2PbBr4, obtained when using the enantiomers R- and S-MBA of the spacer, to the 1D (Rac-MBA)3PbBr5·H2O phase, which exhibited a different PL behavior compared to the 2D phases and two regimes of EPC with Huang–Rhys factors of S = 67 above 110 K and S = 189 below this threshold that were further correlated with the degree of octahedra distortions of the inorganic framework upon lowering temperature. The results obtained through this thesis pave the way for developing new and improved devices and technologies towards optoelectronic applications. Tipo: Tese Thu, 01 Jan 2026 00:00:00 GMT http://repositorio.ufc.br/handle/riufc/86087 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/86072 Título: Emergência e leis de escala em sistemas complexos: uma abordagem orientada por dados Autor(es): Ferreira, Antônio Edinaldo de Oliveira Abstract: In this work, we investigate two complex systems of distinct natures under a single data-driven approach: (i) viscoelastic networks immersed in a fluid and (ii) the urban–economic system of Ceará, structured by multilayer trade flows among 184 municipalities. In both cases, we deal with many interacting elements, structural heterogeneity, and multiscale coupling, seeking to infer emergent macroscopic properties from high-dimensional local data through collective descriptors. In the viscoelastic study, we consider a model of an elastic network with sublinear dissipation, characterized by the parameter α ∈ [0,1] in the drag force fa = γvα. In homogeneous and rigid networks, the drag exponent α governs the transition between different relaxation regimes: within a specific sublinear range of α, the force response takes the form of a power law, F(t) ∼ t−β, whereas outside this range, exponential-type relaxation responses become predominant. In heterogeneous Gaussian networks, disorder broadens the range of α values that give rise to power-law relaxations, while preserving a robust interval of β values, which are mainly sensitive to the mean stiffness level and only weakly dependent on the width of its distribution. In the urban–economic system, we analyze more than 2.5 billion formal transactions (electronic invoices, NF-e, 2016–2019) and show that purchases, sales, and internal trade scale superlinearly with population, with distinct exponents. The economic indicator RCA (Revealed Comparative Advantage), used to characterize patterns of regional productive specialization, follows Zipf–Mandelbrot-type distributions for the three flow layers, revealing hierarchies and diversification limits. Municipal Fitness, an economic metric that measures the degree of diversification and productive complexity of cities based on their export products, also scales superlinearly, indicating a disproportionate accumulation of structural capabilities, especially on the supply side and in internal recirculation. The integration of these results shows that power laws, collective emergence, hierarchies, critical transitions, and multiscale organization are recurrent patterns in both viscoelastic materials and urban systems. We demonstrate that the combination of numerical simulations, machine learning, network analysis, Bayesian inference, and multivariate techniques consolidates Data Science as a unified framework for unveiling deep structures in seemingly disparate complex systems. Tipo: Tese Thu, 01 Jan 2026 00:00:00 GMT http://repositorio.ufc.br/handle/riufc/86072 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/85730 Título: Ácido adipico em condições extremas de temperatura e pressão Autor(es): Silva, Rômulo Sampaio da Abstract: Dicarboxylic acids are organic compounds characterized by the presence of two carboxyl groups (COOH) in their molecular structure, which are responsible for their acidity. These compounds are found in more complex organic systems and play important roles in various chemical and biological processes. In addition, they attract significant interest due to their wide applicability in different industrial fields, making it essential to understand their physical and chemical properties under varying temperature and pressure conditions. In this context, this work investigates the structural behavior of adipic acid C6H10O4 under extreme temperature and pressure conditions through a combination of Raman spectroscopy, powder X-ray diffraction, and density functional theory (DFT) calculations. Initially, vibrational characterization was carried out under ambient conditions, allowing the classification and assignment of the observed modes with the support of computational calculations performed on the unit cell. At low temperatures, the sample was cooled down to 10 K, where a phase transition previously reported in the literature was observed in the range between 135 K and 130 K, accompanied by crystal fragmentation and significant changes in the Raman spectrum. In order to better understand these results, powder X-ray diffraction measurements were performed, revealing phase coexistence below the transition temperature, persisting down to approximately 10 K. Under high pressures, the sample was investigated up to values above 7 GPa in two independent Raman spectroscopy experiments. Significant changes in intermolecular vibrational modes, together with an increase in the number of modes above 1.6 GPa, indicated a phase transition associated with a reduction in the symmetry of the crystal structure. At higher pressures, variations in the intensities of internal modes suggest possible conformational changes of the molecules within the unit cell. The results obtained highlight the strong dependence of adipic acid on thermodynamic conditions, contributing to the understanding of phase transition mechanisms in this molecular system. Tipo: Tese Thu, 01 Jan 2026 00:00:00 GMT http://repositorio.ufc.br/handle/riufc/85730 2026-01-01T00:00:00Z