Research on spintronics has galvanized the design of new devices that exploit the electronic spin in order to augment the performance of current microelectronic technologies. The sucessful implementation of these devices is largely contingent on a quantitative understanding of nonequilibrium magnetism in conducting ferromagnets. This thesis is largely devoted to expanding the microscopic theory of magnetization relaxation and current-induced spin torques in transition metals ferromagnets as well as in (III,Mn)V dilute magnetic semiconductors. We start with two theoretical studies of the Gilbert damping in electric equilibrium, which treat disorder exactly and include atomic-scale spatial inhomogeneities of the exchange field. These studies enable us to critically review the accuracy of the conventional expressions used to evaluate the Gilbert damping in transition metals. We follow by generalizing the calculation of the Gilbert damping to current-carrying steady states. We find that the magnetization relaxation changes in presence of an electric current. We connect this change with the non-adiabatic spin transfer torque parameter, which is an elusive yet potentially important quantity of nonequilibrium magnetism. This connection culminates in a concise analytical expression that will lead to the first ab initio estimates of the non-adiabatic spin transfer torque in real materials. Subsequently we predict that in gyrotropic ferromagnets the magnetic anisotropy can be altered by a dc current. In these systems spin-orbit coupling, broken inversion symmetry and chirality conspire to yield current-induced spin torques even for uniform magnetic textures. We thus demonstrate that a transport current can switch the magnetization of strained (Ga,Mn)As. This thesis concludes with the transfer of some fundamental ideas from nonequilibrium magnetism into the realm of superconductors, which may be viewed as easy-plane ferromagnets in the particle-hole space. We emphasize on the analogies between nonequilibrium magnetism and superconductivity, which have thus far been studied as completely separate disciplines. Our approach foreshadows potentially new effects in superconductors. / text

Potentially toxic elements (PTEs) can be absorbed and accumulated in vegetables organ and tissues, and this is an important channel through which PTEs can enter into the food chain. Soil management may interfere in the behavior of PTEs in the system. So, the study of the dynamics of elements in the soil is fundamental to a better understanding of factors that influence soil-plant transfer. In the state of Pernambuco, Brazil, the production of vegetables is mainly concentrated in small or medium organic and conventional farms located in the Agreste and Sao Francisco regions. Our aims in this research were: i) to determine the contents of PTEs in soil and in vegetables produced under conventional and organic systems in garden soils of Pernambuco; ii) to investigate the forms of the PTEs and their uptake by the vegetables based on different soil-plant transfer factors; iii) to use the calculated transfer factors to assess the human risk exposure; iv) to assess the effect of aging and organic matter (OM) on copper (Cu) desorption in garden soils; v) to determine if manure application to soils increases water soluble Cu concentrations; and vi) to verify the efficiency of machine learning techniques in the classification of vegetables produced under organic and conventional systems. Acid and complexing solutions were employed to extract total contents of PTEs in plants and to extract pseudototal, reactive and bioavailable fractions of the PTEs in soils. Soil-plant transfer factors and human risk assessment were calculated based on the determined levels of PTEs. Kinetic assays were developed using rates of organic composts added to Cu-spiked soils to model the Cu desorption. Water extractable Cu and dissolved organic carbon (DOC) were extracted from soils amended with dairy manure, and the Cu-DOC interactions were studied using spectroscopic techniques. Lettuce (Lactuca sativa) leaves had the highest capacity to accumulate PTEs; conventional system promoted higher PTEs values in soils and plants than the organic production; regardless of the scenario, children and adults in contact to vegetables cropped in organic system have a lower health risk than individuals exposed to lettuce leaves conventionally produced. The addition of OM promoted greater Cu desorption, while aging lead to a reduction in the Cu release. The Fouriertransform infrared spectroscopy spectra showed increase in the presence of aliphatic and carboxylic groups in the extracts from the manure-amended soils. Carboxylic acid functional groups on the DOC formed strong bonds with Cu, which can promote Cu transport as dissolved Cu-organic complexes in the soil profile. The Support Vector Machine had an accuracy over 90% to classify the vegetables in the two groups (organic and conventional); and this high assertiveness rate highlights the potential of using elemental quantification and algorithms as support techniques in the process of authenticity and inspection of organic products. / Elementos potencialmente toxicos (EPTs) podem estar biodisponiveis para serem absorvidos e acumulados nos tecidos e orgaos vegetais, sendo esta uma importante rota de entrada de EPTs na cadeia alimentar. O manejo do solo pode interferir no comportamento dos EPTs no sistema. Desta forma, o estudo da dinamica desses elementos no solo e fundamental para melhor entendimento dos fatores que influenciam a transferencia solo-planta. No estado de Pernambuco, a producao de hortalicas se concentra em intensos cultivos organicos e convencionais localizados em pequenas ou medias propriedades, principalmente na regiao Agreste e Sao Francisco. Nossos objetivos neste trabalho foram: i) determinar os teores de EPTs no solo e nas hortalicas produzidas em sistemas convencionais e organicos em hortas de Pernambuco; ii) investigar as formas de EPTs e sua absorcao pelas hortalicas com base em diferentes fatores de transferencia solo-planta; iii) usar os fatores de transferencia calculados para investigar o risco de exposicao aos humanos; iv) estudar o efeito do tempo e da materia organica (MO) na dessorcao de cobre (Cu) em solos de horta; v) determinar se a aplicacao de esterco aumenta a concentracao de Cu soluvel em agua; e vi) verificar a eficiencia de tecnicas machine learning na classificacao de hortalicas produzidas em sistemas organicos e convencionais. Solucoes acidas e complexantes foram utilizadas para extrair os teores totais de EPTs nas plantas e os teores pseudototal, reativo e biodisponivel dos EPTs no solo. Fatores de transferencia solo-planta e avaliacao de risco humano foram calculados com base nos teores de EPTs. Estudos de cinetica foram desenvolvidos apos adicao de composto organico em solos contaminados com Cu para modelar a dessorcao de Cu. Cu soluvel em agua e carbono organico dissolvido (COD) foram extraidos de solos adubados com esterco, a interacao Cu-COD foi estudada usando tecnicas espectroscopicas. Folhas de alface (Lactuca sativa) apresentaram a maior capacidade de acumular EPTS; o sistema convencional promoveu maiores tores de EPTs nos solos e plantas do que o sistema organico; independentemente do cenario, criancas e adultos em contato com vegetais produzidos em sistema organico tem um menor risco do que os individuos que consomem folhas de alface convencionalmente produzidas. A adicao de MO promoveu maior dessorcao de Cu, enquanto o aumento no tempo de contato reduziu a liberacao de Cu. Os graficos da espectroscopia de infravermelho com transformada de Fourier mostraram um aumento na presenca de grupos funcionais alifaticos e carboxilicos nos extratos de solos adubados com esterco. Esses grupos funcionais no COD formam ligacoes fortes com Cu, o que promove o transporte do elemento na forma de complexos organicos dissolvidos no perfil do solo. O modelo Support Vector Machine apresentou acuracia superior a 90% na classificacao das hortalicas em dois grupos (organicas e convencionais); e a alta taxa de assertividade mostra o potencial do uso de quantificacao elementar e algoritmos como tecnicas auxiliares no processo de autenticidade de inspecao de produtos organicos.

Nesta tese de doutorado apresentamos um estudo sobre o comportamento de diversos sistemas irrevers?veis, caracterizados pela existencia de estados absorventes, atraves de abordagens distintas. Utilizamos aproximacoes de campo medio dinamico, simulacoes numericas usuais, mudanca de ensemble e expanso em serie. Alem disso, mostramos numa parte deste trabalho que a abordagem proposta para o estudo de sistemas irrevers?veis no ensemble em que o numero de part?culas e constante tambem pode ser estendida para sistemas em equil´?brio termodinamico, descrito pela distribuicao de probabilidades de Gibbs. Finalmente mostramos problemas em aberto para trabalhos futuros. / In this PHD thesis, we have presented a study about several nonequilibrium systems with absorbing states by means of different approaches, such as mean-field analysis, usual numerical simulations, analysis in another ensemble and perturbative series expansions. In a specific part of this thesis, we have shown that the approach proposed here for describing nonequilibrium systems in the constant particle number ensemble can also be used to caracterize equilibrium systems, described by Gibbs probability distribution. Finally, we have shown open problems for future researchs.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Num regime balísstico e a baixas temperaturas, a fórmula de Landauer dá uma boa descrição do transporte de calor para nano-junções conectadas a dois fios acoplados a banhos térmicos a temperaturas diferentes. Partindo de um modelo microscópico e utilizando o método de funções de Green fora do equilíbrio, é possível obter uma expressão para a condutância térmica na nano-junção equivalente a fórmula de Landauer. Esta depende dos valores das constantes de acoplamento entre os modos de fônons da região central e dos fios, além do gradiente térmico. A expressão para a condutância térmica é muito semelhante aquela obtida para a condutância elétrica. Neste trabalho nós apresentamos o método para o cálculo de grandezas relacionadas ao transporte térmico em um regime onde não há um gradiente de temperatura entre os reservatórios mas o sistema sofre uma perturbação dependente do tempo. Ou seja, com uma escolha conveniente da parametrização temporal dos termos de acoplamento entre a nano-junção e os fios é possível produzir uma corrente de calor na ausência de diferença de temperaturas entre os banhos térmicos aos quais os fios estão conectados. Esse fenômeno caracteriza o bombeamento de calor. Desenvolvemos uma teoria de transporte dependente do tempo para descrever o bombeamento. A teoria é geral, dependendo da densidade de fônons, da intensidade e dependência temporal do acoplamento. Aplicamos o formalismo em um modelo simples demonstrando que, em princípio, é possível bombear calor através de uma cadeia linear de átomos sem gradiente térmico. / In the ballistic regime at low temperatures, the Landauer formula gives a good description of heat transport for nano-junctions, connected to two leads attached to thermal baths at different temperatures. Starting from a microscopic model and using the nonequilibrium Green functions, it is possible to obtain an expression for the thermal conductance in nano-junction equivalent to the Landauer formula. The latest depends on the values of the coupling constants between phonon modes of the central region and leads, as well as on the thermal gradient. The expression for the thermal conductance is quite similar to that obtained for electrical conductance. In this work we present the method to calculate quantities related to heat transport in a regime where there is no temperature gradient between the reservoirs, but the system suffers a time depending perturbation. That is, with a convenient choice of time parameterization of the coupling terms between the nano-junction and the leads it is possible to produce a heat flow in the absence of a temperature difference between the thermal baths connected to the leads. This phenomenon characterizes the heat pumping. We develop a time-dependent transport theory to describe the pumping. The theory is general, depending on the phonons density, intensity and time dependence of the coupling. We apply the formalism in a simple model showing that in principle it is possible to pump heat through a linear chain of atoms without thermal gradient.

Potentially toxic elements (PTEs) can be absorbed and accumulated in vegetables organ and tissues, and this is an important channel through which PTEs can enter into the food chain. Soil management may interfere in the behavior of PTEs in the system. So, the study of the dynamics of elements in the soil is fundamental to a better understanding of factors that influence soil-plant transfer. In the state of Pernambuco, Brazil, the production of vegetables is mainly concentrated in small or medium organic and conventional farms located in the Agreste and Sao Francisco regions. Our aims in this research were: i) to determine the contents of PTEs in soil and in vegetables produced under conventional and organic systems in garden soils of Pernambuco; ii) to investigate the forms of the PTEs and their uptake by the vegetables based on different soil-plant transfer factors; iii) to use the calculated transfer factors to assess the human risk exposure; iv) to assess the effect of aging and organic matter (OM) on copper (Cu) desorption in garden soils; v) to determine if manure application to soils increases water soluble Cu concentrations; and vi) to verify the efficiency of machine learning techniques in the classification of vegetables produced under organic and conventional systems. Acid and complexing solutions were employed to extract total contents of PTEs in plants and to extract pseudototal, reactive and bioavailable fractions of the PTEs in soils. Soil-plant transfer factors and human risk assessment were calculated based on the determined levels of PTEs. Kinetic assays were developed using rates of organic composts added to Cu-spiked soils to model the Cu desorption. Water extractable Cu and dissolved organic carbon (DOC) were extracted from soils amended with dairy manure, and the Cu-DOC interactions were studied using spectroscopic techniques. Lettuce (Lactuca sativa) leaves had the highest capacity to accumulate PTEs; conventional system promoted higher PTEs values in soils and plants than the organic production; regardless of the scenario, children and adults in contact to vegetables cropped in organic system have a lower health risk than individuals exposed to lettuce leaves conventionally produced. The addition of OM promoted greater Cu desorption, while aging lead to a reduction in the Cu release. The Fouriertransform infrared spectroscopy spectra showed increase in the presence of aliphatic and carboxylic groups in the extracts from the manure-amended soils. Carboxylic acid functional groups on the DOC formed strong bonds with Cu, which can promote Cu transport as dissolved Cu-organic complexes in the soil profile. The Support Vector Machine had an accuracy over 90% to classify the vegetables in the two groups (organic and conventional); and this high assertiveness rate highlights the potential of using elemental quantification and algorithms as support techniques in the process of authenticity and inspection of organic products. / Elementos potencialmente toxicos (EPTs) podem estar biodisponiveis para serem absorvidos e acumulados nos tecidos e orgaos vegetais, sendo esta uma importante rota de entrada de EPTs na cadeia alimentar. O manejo do solo pode interferir no comportamento dos EPTs no sistema. Desta forma, o estudo da dinamica desses elementos no solo e fundamental para melhor entendimento dos fatores que influenciam a transferencia solo-planta. No estado de Pernambuco, a producao de hortalicas se concentra em intensos cultivos organicos e convencionais localizados em pequenas ou medias propriedades, principalmente na regiao Agreste e Sao Francisco. Nossos objetivos neste trabalho foram: i) determinar os teores de EPTs no solo e nas hortalicas produzidas em sistemas convencionais e organicos em hortas de Pernambuco; ii) investigar as formas de EPTs e sua absorcao pelas hortalicas com base em diferentes fatores de transferencia solo-planta; iii) usar os fatores de transferencia calculados para investigar o risco de exposicao aos humanos; iv) estudar o efeito do tempo e da materia organica (MO) na dessorcao de cobre (Cu) em solos de horta; v) determinar se a aplicacao de esterco aumenta a concentracao de Cu soluvel em agua; e vi) verificar a eficiencia de tecnicas machine learning na classificacao de hortalicas produzidas em sistemas organicos e convencionais. Solucoes acidas e complexantes foram utilizadas para extrair os teores totais de EPTs nas plantas e os teores pseudototal, reativo e biodisponivel dos EPTs no solo. Fatores de transferencia solo-planta e avaliacao de risco humano foram calculados com base nos teores de EPTs. Estudos de cinetica foram desenvolvidos apos adicao de composto organico em solos contaminados com Cu para modelar a dessorcao de Cu. Cu soluvel em agua e carbono organico dissolvido (COD) foram extraidos de solos adubados com esterco, a interacao Cu-COD foi estudada usando tecnicas espectroscopicas. Folhas de alface (Lactuca sativa) apresentaram a maior capacidade de acumular EPTS; o sistema convencional promoveu maiores tores de EPTs nos solos e plantas do que o sistema organico; independentemente do cenario, criancas e adultos em contato com vegetais produzidos em sistema organico tem um menor risco do que os individuos que consomem folhas de alface convencionalmente produzidas. A adicao de MO promoveu maior dessorcao de Cu, enquanto o aumento no tempo de contato reduziu a liberacao de Cu. Os graficos da espectroscopia de infravermelho com transformada de Fourier mostraram um aumento na presenca de grupos funcionais alifaticos e carboxilicos nos extratos de solos adubados com esterco. Esses grupos funcionais no COD formam ligacoes fortes com Cu, o que promove o transporte do elemento na forma de complexos organicos dissolvidos no perfil do solo. O modelo Support Vector Machine apresentou acuracia superior a 90% na classificacao das hortalicas em dois grupos (organicas e convencionais); e a alta taxa de assertividade mostra o potencial do uso de quantificacao elementar e algoritmos como tecnicas auxiliares no processo de autenticidade de inspecao de produtos organicos.

No description available.

No description available.

No description available.

Previous research in the Ouachita Mountains, Arkansas suggests that, on relatively thin soils overlying bedrock, individual trees locally thicken the regolith by root penetration into bedrock. However, that work was conducted mainly in areas of strongly dipping and contorted rock, where joints and bedding planes susceptible to root penetration are more common and accessible. This project extended this concept to the Cumberland Plateau, Kentucky, with flat, level-bedded sedimentary rocks. Spatial variability of soil thickness was quantified at three nested spatial scales, and statistical relationships with other potential influences of thickness were examined. In addition, soil depth beneath trees was compared to that of non-tree sites by measuring depth to bedrock of stumps and immediately adjacent sites. While soil thickness beneath stumps was greater in the Ouachita Mountains compared to the Kentucky sites, there were no statistically significant differences in the difference between stump and adjacent sites between the two regions. In both regions, however, soils beneath stumps are significantly deeper than adjacent soils. This suggests the local deepening effects of trees occur in flat-bedded as well as steeply dipping lithologies. Regression results at the Cumberland Plateau sites showed no statistically significant relationship between soil depth and geomorphic or stand-level ecological variables, consistent with a major role for individual tree effects. Nested analysis of variance between 10 ha stands, 1.0 ha plots, and 0.1 ha subplots indicates that about 67 percent of total depth variance occurs at, or below, the subplot level of organization. This highly localized variability is consistent with, and most plausibly explained by, individual tree effects. The effects of biomechanical weathering by trees are not limited to areas with strongly dipping and contorted bedrock. Variability of soil depth in the Cumberland Plateau is likely influenced by positive feedbacks from tree root growth, that these interactions occur over multiple generations of growth, and that the effects of trees are the dominant control of local soil thickness. Since lateral lithological variation was minimal, this study also provides evidence that the positive feedback from biomechanical weathering by trees leads to divergent development of soil thickness.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Num regime balísstico e a baixas temperaturas, a fórmula de Landauer dá uma boa descrição do transporte de calor para nano-junções conectadas a dois fios acoplados a banhos térmicos a temperaturas diferentes. Partindo de um modelo microscópico e utilizando o método de funções de Green fora do equilíbrio, é possível obter uma expressão para a condutância térmica na nano-junção equivalente a fórmula de Landauer. Esta depende dos valores das constantes de acoplamento entre os modos de fônons da região central e dos fios, além do gradiente térmico. A expressão para a condutância térmica é muito semelhante aquela obtida para a condutância elétrica. Neste trabalho nós apresentamos o método para o cálculo de grandezas relacionadas ao transporte térmico em um regime onde não há um gradiente de temperatura entre os reservatórios mas o sistema sofre uma perturbação dependente do tempo. Ou seja, com uma escolha conveniente da parametrização temporal dos termos de acoplamento entre a nano-junção e os fios é possível produzir uma corrente de calor na ausência de diferença de temperaturas entre os banhos térmicos aos quais os fios estão conectados. Esse fenômeno caracteriza o bombeamento de calor. Desenvolvemos uma teoria de transporte dependente do tempo para descrever o bombeamento. A teoria é geral, dependendo da densidade de fônons, da intensidade e dependência temporal do acoplamento. Aplicamos o formalismo em um modelo simples demonstrando que, em princípio, é possível bombear calor através de uma cadeia linear de átomos sem gradiente térmico. / In the ballistic regime at low temperatures, the Landauer formula gives a good description of heat transport for nano-junctions, connected to two leads attached to thermal baths at different temperatures. Starting from a microscopic model and using the nonequilibrium Green functions, it is possible to obtain an expression for the thermal conductance in nano-junction equivalent to the Landauer formula. The latest depends on the values of the coupling constants between phonon modes of the central region and leads, as well as on the thermal gradient. The expression for the thermal conductance is quite similar to that obtained for electrical conductance. In this work we present the method to calculate quantities related to heat transport in a regime where there is no temperature gradient between the reservoirs, but the system suffers a time depending perturbation. That is, with a convenient choice of time parameterization of the coupling terms between the nano-junction and the leads it is possible to produce a heat flow in the absence of a temperature difference between the thermal baths connected to the leads. This phenomenon characterizes the heat pumping. We develop a time-dependent transport theory to describe the pumping. The theory is general, depending on the phonons density, intensity and time dependence of the coupling. We apply the formalism in a simple model showing that in principle it is possible to pump heat through a linear chain of atoms without thermal gradient.