For thin epitaxial metal films, when the thickness is on the order of the Fermi wavelength, [lambda subscript F], quantum confinement can dramatically alter the physical properties of the film. These so-called Quantum Size Effects (QSE) can dramatically alter the morphology of thin films by an intricate interplay between kinetics and surface energy driven thermodynamics. These effects lead to rich growth-related phenomena in Pb(111) films grown on semiconductor substrates such as Si(111). For example, QSE can drive flat film formation when growth is dominated by surface energy oscillations. This is rather surprising for Pb/Si systems because of a rather high lattice mismatch. However, these films are not defect free, but rather show common occurrences of three defect types. Low Temperature Scanning Tunneling Microscopy (LT-STM) was utilized to characterize these defects on the atomic scale. Furthermore, these defects create modulations in the electron density resulting in fluctuations in QWS near defect sites. Another topic of recent interest is how QSE affect adsorption of as well as how adsorbates modify QSE for these Pb films. In this thesis, LT-STM and first principles calculations were utilized to study Cs adsorbates on Pb film surfaces, defects, and step edges. Cs adsorption is intricately related to the electronic structure of the surface, especially the defect sites which can act as surface traps. These Cs adsorbates, which are assumed to be ionized, enhance elastic surface scattering of empty-state electrons. This results in observable wave interference patterns near Cs impurities. Furthermore, Cs adsorbates, by an overall step energy reduction, can promote QSE-related nanostructures, which are otherwise too weak when kinetic effects cannot be ignored. This enhancement of "quantum stability" is driven by favorable Cs step binding and can be explained within the contexts of Density Functional Theory (DFT). / text

An (e,2e) electron scattering spectrometer has been constructed and used for the first time to investigate the spectral momentum density of the valence bands of a solid target. This technique provides fundamental information about the electronic structure of both crystalline and amorphous solids. The three fundamental quantities, the band structure, electron density of states, and electron momentum distribution can be simultaneously derived from the measured (e,2e) cross section. A review of single electron and (e,2e) scattering theory is given with an emphasis on scattering from solids. The effects of multiple scattering are discussed and a method of deconvoluting those effects from the measured (e,2e) cross section is developed. There is a detailed description of the spectrometer design and operation with particular attention given to the electron optics and voltage distribution. The algorithms and software for computer aided data acquisition and analysis are also outlined, as is error analysis. The techniques employed in the preparation and characterization of extremely thin film samples of a-C and single crystal graphite are described. An analysis of the data taken for a-C samples is given. The data are compared with the results of complementary experiments and theory for graphite, diamond, and a-C which are given in a review of the literature. The existence of a definite dispersion relation ε(q) in amorphous carbon is demonstrated. The a-C band structure appears to be more similar to that of graphite than to that of diamond, however it differs significantly from both in some respects. The measured spectral momentum density seems compatible with a model of a-C based on small, randomly-oriented islands of quasi-2D graphite-like continuous random network structures. However, no definitive interpretations can be made until higher resolution experiments are performed on both a-C and single crystal graphite. / Ph. D.

Thesis (MSc (Physics))--University of Stellenbosch, 2007. / A relativistic plane wave model is developed for electromagnetic production of unbound hyperons with free kaons from nuclei. The differential cross section is expressed as a contraction of leptonic and hadronic tensors. The leptonic tensor is constructed by using the helicity representation of a free Dirac spinor. A model for the corresponding elementary process is used to calculate the hadronic tensor, in which the hadronic current operator Jˆμ is written as a linear combination of six invariant amplitudes and six Lorentz and gauge invariant quantities. The kinematics for this process is assumed to be a quasi-free process i.e., the electron interacts with only one bound nucleon inside the nucleus. The bound state wavefunction of the bound nucleon is calculated within the framework of the relativistic mean-field approximation. The unpolarized differential cross section for the K+ electroproduction process, e + A −→ e + K+ + A + Aresidual is calculated as a function of the hyperon scattering angle.

Recent experimentation with excitation of surface plasmons on a gold film in the Kretschmann configuration have shown what appears to be a superconductive effect. Researchers claimed to see the existence of electron pairing during scattering as well as magnetic field repulsion while twisting the polarization of the laser. In an attempt to explain this, they pointed to a combination of electron-electron scattering in external fields as well as dynamic screening via intense laser radiation. This paper expands upon the latter, taking a look at the properties of a dynamic polarization function, its effects on bulk and surface plasmon dispersion relations, and its various consequences.

Orientadores: Antonio J. Ramirez, Maria Clara F. Ierardi / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-09T21:56:05Z (GMT). No. of bitstreams: 1 Seraphim_RafaelMolena_M.pdf: 6485115 bytes, checksum: 380cfffee5fa982ec74a9cdc78c88e3a (MD5) Previous issue date: 2007 / Resumo: O avanço cientifico no estudo de átomos, moléculas e proteínas tem recebido uma contribuição significativa dos aceleradores de partículas, que em sua grande maioria são utilizados para a geração de radiação síncrotron. Por isso, o aprimoramento destas máquinas mostra-se de extrema importância para manter a ciência em seu contínuo avanço. O sistema de vácuo destas máquinas apresenta-se como um importante parâmetro para o seu bom funcionamento e melhorias neste promovem diretamente melhorias na qualidade da radiação síncrotron gerada. Logo, o desenvolvimento deste trabalho teve como principal objetivo o estudo das juntas soldadas das câmaras de vácuo que compõem o acelerador do Laboratório Nacional de Luz Síncrotron (LNLS). As juntas foram preparadas com o processo de soldagem GTAW (Gás Tungsten Arc Welding) mediante a utilização de diferentes gases de proteção, como Argônio, Hélio e Nitrogênio, e misturas entre estes. Os estudos concentraram-se na análise da influência dos gases de proteção na dessorção de gases das juntas soldadas. Para a caracterização destas utilizaram-se as técnicas de análise de superfícies Dessorção Estimulada por Elétrons (DEE) e Dessorção Estimulada por Fótons (DEF). Adicionalmente, realizou-se um estudo comparativo entre as duas técnicas buscando-se estabelecer relações entre os resultados fornecidos pelas mesmas. Os resultados mostraram que a dessorção de gases independe do gás de proteção de soldagem, mas depende sim do nível de oxidação das juntas soldadas, as quais podem apresentar altas taxas de dessorção caso estejam muito oxidadas e os óxidos não sejam camadas contínuas. Em conclusão pode-se dizer que as juntas soldadas para câmaras de vácuo de aceleradores de partículas devem apresentar baixos níveis de oxidação, e apesar das semelhanças encontradas entre a DEE e a DEF é aconselhável utilizar a DEF para a caracterização de superfícies para trabalharem com radiação síncrotron / Abstract: The scientific advance of atoms, molecules and proteins studies has received an important contribution from particle accelerators, which are mainly used to generate synchrotron radiation. Hence, the improvement of these machines is necessary to maintain the continuous advance of science. One of the key components for the adequate operation of these accelerators is the vacuum system. Thus, its improvement directly impacts on the quality of the generated synchrotron radiation. Therefore, the principal purpose of this work is to study the gas desorption from the welding joints on the particle accelerators vacuum chambers. The welds were prepared using the GTAW (Gas Tungsten Arc Welding) process with different shielding gases, as argon, helium and nitrogen, and some mixtures of them. The study concentrated on the analysis of the shielding gas influence on the gas desorption from the welded joints. The gas desorption of the joints was evaluated using Electron Stimulated Desorption (ESD) and Photon Stimulated Desorption (PSD). In addition, it was carried out a comparative study between both desorption techniques to establish relationships between the results provided by them. The results showed that the gas desorption from the welding joints does not depend on the welding shielding gas, but on the oxidation level of the joint, which can present high desorption yield if it is highly oxidized and these oxides are not a continuous film. Therefore, it is advisable to minimize the oxidation during welding of vacuum chambers for particle accelerators. Regarding the comparison between ESD and PSD they were found some similarities. However, PSD will have better performance characterizing surfaces that will be exposed to synchrotron radiation / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica