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https://repositorio.ufu.br/handle/123456789/5166
2017-09-16T01:52:37ZDesenvolvimento e caracterização fototérmica de novos materiais vítreos dopados com íons emissores terras-raras
https://repositorio.ufu.br/handle/123456789/19674
Title: Desenvolvimento e caracterização fototérmica de novos materiais vítreos dopados com íons emissores terras-raras
Abstract: In this work two new glass matrices has been developed, a phosphate and borate one, PAN (40P2O5.20 Al2O3. 40Na2CO3 (mol%)) and BAN (40B2O3. 20 M2O3. 40Na2CO3 (mol%)), namely. The matrices were doped with increasing concentration of neodimium and praseodymium rare-earth ions in order to study its thermal and optical properties. To obtain these properties, initially, measurements of optical absorption, photoluminescence, lifetime, refractive index, mass density and specific heat were performed. In addition, applying the time resolved mode mismatched dual beam thermal lens technique the thermal-optical properties were found. It was found that the PAN and BAN thermal diffusivities were quite similar. By using a reference sample the quantum efficiency of the neodymium sample were determined. It was noticed that the insertion of dopant fens causes a luminescence quenching. This effect became more evident in the decreasing of the quantum efficiency (and lifetime) when concentration is increased. Finally, from the above results it was possible to calculate the nonradiative relaxation rate. All the found results were analised as a function of concentration. In particular, the PAN matrix presents a good combination of thermal and optical properties, which are essential to possible aplication in optical devices.2009-07-27T00:00:00ZEstudo de efeito spin-orbita na adsorção de Bi em superfícies de Si(111)
https://repositorio.ufu.br/handle/123456789/19610
Title: Estudo de efeito spin-orbita na adsorção de Bi em superfícies de Si(111)
Abstract: This work consists on a deep analysis through ab initio of Bi/Si(111) surfaces in order to characterize magnetism, band and topological properties. Initially, we did a detailed review of the theoretical bases for characterization of topological systems and the Densty Functional Theory (DFT). It’s worth mentioning that in the review of the topological insulation characterization methods, we start from the fundamental concepts presented by M.V Berry going to the methods the Wannier functions and parity.
With such tools in hand, we began our studies with ab initio calculations of the bilayer of Bi, characterizing it topologically through parity, thus we verify that the topological properties of the sheet are given by it’s parity in point M and not point r, as proposed in bibliography. We also perform the evolution of the Wannier Charge Centers for the bilayer with the intention of comparing the results between methods. The second step consists on the study of the stability of Bi’s adsorption in Si(111)y/3 x v^3 including the spin orbit coupling, thus confirming that the Milkstool Structure is the most stable structure as predicted by the calculation without SO coupling and more than that, the Spin-Orbit increases the energy difference between Milkstool and concurrent structures. Later on, we perfom band structure calculations through DFT, showing that SO coupling is little relevant in Milktool band structures calculations. Finally, we work on different Si-(111) structures and conclude that Bi layers do have strong interaction with the Si layers, displaying metallic states of Fermi level.2017-02-24T00:00:00ZEstados ligados de Andreev e Majorana em pontos quânticos
https://repositorio.ufu.br/handle/123456789/18741
Title: Estados ligados de Andreev e Majorana em pontos quânticos
Abstract: We present a numerical study of the emergence of Majorana and Andreev bound states in a system composed of two quantum dots, one of which is coupled to a conventional superconductor, SC1, and the other connects to a topological superconductor, SC2. By controlling the interdot coupling we can drive the system from two single (uncoupled) quantum dots to double (coupled) dot system configurations. We employ a recursive Green’s function technique that provides us with numerically exact results for the local density of states of the system. We first show that in the uncoupled dot configuration (single dot behavior) the Majorana and the Andreev bound states appear in an individual dot in two completely distinct regimes. Therefore, they cannot coexist in the single quantum dot system. We then study the coexistence of these states in the coupled double dot configuration. In this situation we show that in the trivial phase of SC2, the Andreev states are bound to an individual quantum dot in the atomic regime (weak interdot coupling) or extended over the entire molecule in the molecular regime (strong interdot coupling). More interesting features are actually seen in the topological phase of SC2. In this case, in the atomic limit, the Andreev states appear bound to one of the quantum dots while a Majorana zero mode appears in the other one. In the molecular regime, on the other hand, the Andreev bound states take over the entire molecule while the Majorana state remains always bound to one of the quantum dots.2017-02-15T00:00:00ZEstudo dos parâmetros termo-óticos do vidro fosfato PANK:Nd3+ em função da temperatura
https://repositorio.ufu.br/handle/123456789/18416
Title: Estudo dos parâmetros termo-óticos do vidro fosfato PANK:Nd3+ em função da temperatura
Abstract: This work describes a spectroscopic analysis of phosphate glass called PANK ( 4 0 P22 0 5 ■ 2 0A120 3 ■ 3 5 N a20 ■ 5 K20 ( m o 1 %)) + xNd2O3 with x = 1, 2, 3, 4 e 5 (wt%), as a function of temperature in the range from 80 K to 480 K. Phosphate glasses have received a lot of attention of the researchers by their good thermal-optical properties, as well as, a high quantum efficiency when doped with Nd ion. These properties allow them to be applied as active laser medium, optical fibers and optical amplifiers. In general the studies of properties are performed at room temperature (300 K). However it is necessary execute these studies as a function of temperature, because these properties may be temperature dependent. Applying the thermal lens technique and using the measure of the lifetime of luminescence, was possible to find temperature dependence of quantum efficiency of luminescence. To obtain it was used the normalized lifetime method. As a support to the study were performed measurements of photoluminescence spectra. A discontinuity in thermal diffusivity was observed about 180 K. In the same region an inversion of the thermal lens sign occurred, probably associated in a signal change of the variation of the optical path with the temperature. Both effects were observed for all samples. In this study, it was clear that both the increase of the concentration of Nd ion and increase of the material temperature produced a quenching of luminescence. This effect is noticed in lifetime and quantum efficiency results. Analyzing the result of lifetime in the limit of zero concentration (x ^ 0), it was observed its value has a thermal quenching from 80 - 200 K and it is related with the thermal excitation of the population in the lower level to the upper Stark level of the metastable level 4F3/2. After 200 K its value keeps constant and it is in good agreement of the radiative lifetime calculated by Judd-Ofelt theory. Based on the behavior of the quantum efficiency in the limit of zero concentration, it is possible to see two regions with thermal quenching. The first one agrees with the same thermal quenching that diminished the value of lifetime while the second one is related to an energy loss by thermal activation of one vibrational mode, confirmed by Raman spectrum. Finally, the results obtained in this work are fundamental to increase the discussion of the temperature dependence of thermal-optical properties, which is important to develop of new solid state laser.2017-02-14T00:00:00Z