https://repositorio.ufu.br/handle/123456789/5494 2026-04-07T13:37:18Z https://repositorio.ufu.br/handle/123456789/48521 Title: Estudo optoestrutural de perovskitas de iodeto de césio e chumbo em vidro borossilicato: efeitos da dopagem com terras raras e nióbio Abstract: In this work, the structural and optical properties of CsPbI3 perovskites embedded in a borosilicate glass matrix, doped with rare-earth ions (Nd3+, Er3+, Sm3+, and Ho3+) and co-doped with Nb⁵⁺, were investigated. The results show that increasing the Nb5+ concentration favors its incorporation into the glass matrix and promotes the formation of the cubic perovskite phase. The addition of rare-earth ions also contributes to the stabilization of this phase, although no dominant element could be identified. Raman spectroscopy confirmed the presence of vibrational modes associated with the SNAB glass matrix and the role of Nb5+ as a network modifier. Temperature-dependent photoluminescence spectra exhibited emission bands attributed to CsPbI3, with a maximum at ~720 nm (1.72 eV) at 77 K, shifting to ~700 nm (1.78 eV) with increasing temperature, accompanied by spectral broadening and thermal quenching. Among the investigated samples, the Er3+-doped compositions are the most promising, due to the effective stabilization of the cubic phase and their potential application as optical thermometers, exhibiting a relative sensitivity of 4.3% K-1. These results demonstrate that co-doping with Nb5+ and rare-earth ions is an efficient strategy to control the structural stability and optical properties of CsPbI3 perovskites embedded in glass matrices. 2026-02-02T00:00:00Z https://repositorio.ufu.br/handle/123456789/48141 Title: Kondo effect under extreme conditions: testing the limits of the Numerical Renormalization Group Abstract: The study of physical phenomena under extreme conditions (very low temperatures, for example) has historically shown the potential to uncover unexpected states in Condensed matter Physics, at the same time, it poses a stiff challenge to both experimental and theoretical research techniques. One celebrated example was the discovery of superconductivity when it became possible to liquefy helium and thus perform resistivity measurements at temperatures of a few Kelvin. Following this approach, in this Thesis we study the Kondo effect under two extreme conditions: (i) systems where the metallic host density of states presents a strong divergency near the Fermi energy; (ii) T-shape double quantum dot (DQD) systems, where the energy scale of interest spans 20 orders of magnitude. Both systems were studied using the Numerical Renormalization Group (NRG) method. In (i), we obtained that relatively soft divergencies, of the $\omega^{{-1}/{2}}$-type, generate a series of many-body bound-states with peculiar properties. We thus believe that stronger divergencies (associated to flat bands, for example) have the potential to uncover additional surprises. As to the DQD system, we obtained results that allow us to guide the experimentalists on how to tune the DQD characteristics in order to clearly distinguish two regimes with very different properties, viz., the two-stage Kondo regime and the molecular regime. To analyze the Kondo effect in these so-called extreme conditions (strong host DOS divergencies and extreme energy scale variation) it was necessary to force the NRG into high-demand scenarios, pushing its parameters to extreme values. 2025-11-10T00:00:00Z https://repositorio.ufu.br/handle/123456789/46894 Title: On electronic and magnetic properties of distorted kagome lattices Abstract: Many-body physics, frustrated magnetism and the knobs for tuning quantum materials intersect at a rich frontier of modern condensed matter physics. The kagome lattice, with its unique geometry and yet not completely understood phase diagram, has emerged as a key platform for exploring such phenomena. In this thesis, we investigate the effects of applied distortions on the magnetic properties of kagome lattices, focusing on the interplay between Geometric Frustration (GF) and correlations. We study a simple Hubbard model, and employ the Density Matrix Renormalization Group (DMRG) method to study the ground state properties of strained kagome systems. Two quantifiers for GF are proposed, and their predictive capabilities evidenced. Based on the literature review we conducted, one of said quantifiers features as the first local correlation-only way to "probe" GF. Our findings are thus a proof of concept in favor of not only that strain can significantly change "effective dimensionalities" and magnetic order, but also modify the "frustration content" and electronic structure of interacting kagome lattices, potentially leading to novel phases and emergent phenomena. This work paves the way for future studies on the interplay between strain, electronic correlations, frustration and elastic properties in interacting kagome lattices, with potential implications for the design of novel quantum materials and devices. 2025-08-15T00:00:00Z https://repositorio.ufu.br/handle/123456789/46755 Title: Síntese e espectroscopia aplicada na caracterização de copmplexos de curcumina com íons metálicos Abstract: This study investigated the structural and spectroscopic properties of metal-based complexes derived from curcumin (CurcZn, CurcPb, and CurcCa) using Raman spectroscopy, FTIR, optical absorption spectroscopy, photoluminescence, and emission ellipsometry. The results revealed significant interactions between the central oxygenated groups of curcumin and the metals, with variations observed among the complexes. Raman and FTIR analyses identified shifts in vibrational modes, particularly in the enol and C=O groups, confirming metal coordination. However, it was also observed that the complexation of the CurcCa sample did not occur satisfactorily. The optical absorption spectra showed changes in peak positions and the appearance of shoulders, indicating alterations in electron density and molecular geometry. Photoluminescence measurements highlighted subtle changes in emission maxima due to metal-induced electronic transitions. Notably, the new technique of emission ellipsometry combined with applied magnetic fields provided a final perspective on the variation of Stokes parameters as a function of the applied magnetic field. This revealed distinct magneto-optical effects for the complexes: CurcZn exhibited a change in the preferred direction of emitted polarization, while CurcPb and CurcCa showed an increase in the percentage of polarized light in their preferred direction. Thus, we emphasize the impact of metal-curcumin interactions on optical properties and the introduction of a novel technique. 2024-12-20T00:00:00Z