Resumen (Castellano) El aumento del consumo de energía global junto con las preocupaciones ambientales ha generado mucho interés por las fuentes de energía alternativas y limpias, como la energía solar fotovoltaica. Los investigadores en la comunidad fotovoltaica han estado buscando formas de reducir costos mientras mantienen o aumentan las eficiencias. Una mejor comprensión de los materiales implicados es esencial para el rápido desarrollo de nuevas tecnologías. Las películas delgadas I-III-VI2 ofrecen sistemas prometedores para lograr células solares de alta eficiencia a un costo menor. De hecho, al adaptar la composición de los compuestos, es posible cambiar la banda prohibida del material para captar la luz solar de manera más eficiente. Esta tesis se centra en la preparación y caracterización del material de la capa absorbente, especialmente las películas delgadas nanocristalinas y la consideración de las características estructurales y eléctricas de dicha capa principal absorbente de células. La tesis examina cómo las diferentes técnicas de preparación y uso del material podrían afectar las propiedades del películas delgadas sintetizadas. Películas delgadas CuInSe2 y CuInS2 se depositaron sobre sustratos de vidrio ITO usando la técnica de electrodeposición en solución acuosa. Las películas electrodepositadas se caracterizaron por difracción de rayos X (XRD), microscopía electrónica de barrido (SEM) y análisis de rayos X de energía dispersiva (EDS). Se investigaron los efectos de recocido sobre los precursores electrodepositados. La estructura de calcopirita de CuInSe2/CuInS2 mostró una mejora de la cristalinidad después del tratamiento posterior de selenización/sulfurización en atmósfera Se/S, respectivamente. Los estudios de XRD y SEM revelaron una mejora de la calidad cristalina de las películas de CIS después de los tratamientos térmicos. Las propiedades ópticas de las películas delgadas recocidas CuInSe2-Se y CuInSe2-S se han estudiado para determinar el efecto del proceso de recocido en diferentes ambientes de selenio y azufre. Además, modificamos el CuInxCryGa1-x-ySe2 de cobre indio, donde x = 0.4, y = (0.0, 0.1, 0.2, 0.3) la capa de superestrato por el proceso de recubrimiento por centrifugado. CuInxCryGa1-xySe2 donde x = 0.4, y = (0.0, 0.1, 0.2, 0.3) nanopartículas han sido sintetizadas en primer lugar usando un método hidrotermal químico húmedo que se basa en un proceso térmico sin vacío sin ningún proceso de selenización adicional. Introduciendo diferentes fuentes de metal en un autoclave con etilenamina como solvente, se obtuvieron nanopartículas de CIGS a diferentes temperaturas en un rango de 190-230 °C. Los resultados de la difracción de rayos X (XRD) confirmaron la formación de una estructura de calcopirita CuInxCryGa1-x-ySe2 tetragonal. Finalmente, se estudió el efecto de la temperatura de recocido en los materiales tipo Kesterita (como el Cu2ZnSnS4) que son materiales de muy bajo costo y que no dañan el medio ambiente. Estudiamos el crecimiento de las películas delgadas cuaternarias Cu2ZnSnS4 (CZTS) de kesterita mediante un depósito electroquímico de un solo paso seguido de un recocido a baja temperatura. La influencia de diferentes atmósferas de recocido a tiempos de recocido constantes (t = 45 min) y parámetros de control de preparación fijos; es decir, concentración de la solución de materiales de partida (sales de metales precursores), tiempo de deposición y potencial de electrodeposición. Se estudiaron las propiedades estructurales, de composición, morfológicas y ópticas, así como las propiedades fotoelectroquímicas. / Abstract Increasing global energy consumption together with environmental concerns has led to much interest in alternative, cleaner sources of energy such as solar photovoltaic. Researchers in the solar cell community have been looking for ways to reduce costs while maintaining or increasing already high efficiencies. A fundamental understanding of the materials under consideration is essential to rapid development of new technologies. The I-III-VI2 thin films offer promising systems for achieving high efficiency solar cells at lower costs. In fact, by tailoring the chemistry of the compounds it is possible to change the bandgap of the material in order to collect sunlight more efficiently. First of all, this thesis focuses on absorber layer material preparation and characterization, especially nanocrystalline thin films and consideration of both structural and electrical characteristics of such main cell absorber layer.The thesis examines how different preparation techniques and material usage could affect the properties of the synthesized thin films (absorber layer). In this study CuInSe2 and CuInS2 thin films were deposited onto ITO glass substrate using the electrodeposition technique in aqueous solution. The electrodeposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). The annealing effects on the electrodeposited precursors were investigated. The chalcopyrite structure of CuInSe2/CuInS2 showed an enhancement of crystallinity after subsequent selenization/sulfurization treatment in Se/S atmosphere, respectively. XRD and SEM studies revealed a dramatic improvement of the crystalline quality of CIS films after annealing treatments. The optical properties of annealed CuInSe2-Se and CuInSe2-S thin films have been studied in order to determine the effect of annealing process in different selenium and sulfur atmosphere. In the second step we modified copper indium CuInxCryGa1-x-ySe2 where x=0.4, y= (0.0, 0.1, 0.2, 0.3)superstrate layer by spin coating process. CuInxCryGa1-x-ySe2 where x=0.4, y= (0.0, 0.1, 0.2, 0.3) nanoparticles have been synthesized firstly using a wet chemical hydrothermal method that is based on a non-vacuum thermal process without any additional selenization process. Introducing different metal sources in an autoclave with ethylenediamine as solvent, CIGS nanoparticles were obtained at different temperatures range 190-230°C. The X-ray diffraction (XRD) results confirmed the formation of a tetragonal CuInxCryGa1-x-ySe2 chalcopyrite structure. Finally, we turned again to the study of the annealing temperature effect onKesterite materials but this time in those of very low-cost materials and environmentally friendly Cu2ZnSnS4. We studied the growth of quaternary Cu2ZnSnS4 (CZTS) kesterite thin films by a single step electrochemical deposition followed by annealing at low temperature. The influence of different annealing atmospheres at constant annealing times (t = 45 min) and fixed preparation controlling parameters; i.e., starting materials (precursor metal salts) solution concentration, time of deposition and electrodeposition potential. Structural, compositional, morphological, and optical properties, as well as photoelectrochemical properties were studied. / Resum (Valencià) L'augment del consum d'energia global juntament amb les preocupacions ambientals ha generat molt d'interès per les fonts d'energia alternatives i netes, com ara l'energia solar fotovoltaica. Els investigadors de la comunitat fotovoltaica han estat buscant formes de reduir costos mentre mantenen o augmenten les eficiències. Una millor comprensió dels materials implicats és essencial per al ràpid desenvolupament de noves tecnologies. Les pel·lícules primes I-III-VI2 ofereixen sistemes prometedors per aconseguir cèl·lules solars d'alta eficiència a un cost menor. De fet, en adaptar la composició dels compostos, és possible canviar la banda prohibida del material per captar la llum solar de manera més eficient. Aquesta tesi se centra en la preparació i caracterització del material de la capa absorbent, especialment les pel·lícules primes nanocristal·lines i la consideració de les característiques estructurals i elèctriques d'aquesta capa principal absorbent de cèl·lules. La tesi examina com les diferents tècniques de preparació i ús del material podrien afectar les propietats del pel·lícules primes sintetitzades. Pel·lícules primes CuInSe2 i CuInS2 es van dipositar sobre substrats de vidre ITO usant la tècnica d'electrodeposició en solució aquosa. Les pel·lícules electrodepositadas es van caracteritzar per difracció de raigs X (XRD), microscòpia electrònica de rastreig (SEM) i anàlisi de raigs X d'energia dispersiva (EDS). Es van investigar els efectes de recuit sobre els precursors electrodepositados. L'estructura de calcopirita de CuInSe2/CuInS2 va mostrar una millora de la cristal·linitat després del tractament posterior de selenització/sulfurització en atmosfera de Se o S, respectivament. Els estudis de XRD i SEM van revelar una millora de la qualitat cristal·lina de les pel·lícules de CIS després dels tractaments tèrmics. Les propietats òptiques de les pel·lícules primes recuites CuInSe2-Es i CuInSe2-S s'han estudiat per determinar l'efecte del procés de recuit en diferents ambients de seleni i sofre. A més, modifiquem el CuInxCryGa1-x-ySe2 de coure indi, on x = 0.4, i = (0.0, 0.1, 0.2, 0.3) la capa d'superstrat pel procés de recobriment per centrifugat. CuInxCryGa1-x-ySe2 on x = 0.4, i = (0.0, 0.1, 0.2, 0.3) nanopartícules han estat sintetitzades en primer lloc fent servir un mètode hidrotermal químic humit que es basa en un procés tèrmic sense buit sense cap procés de selenización addicional. Introduint diferents fonts de metall en un autoclau amb etilenamina com solvent, es van obtenir nanopartícules de CIGS a diferents temperatures en un rang de 190- 230 °C. Els resultats de la difracció de raigs X (XRD) van confirmar la formació d'una estructura de calcopirita CuInxCryGa1-x-ySe2 tetragonal. Finalment, es va estudiar l'efecte de la temperatura de recuit en els materials tipus kesterita (com el Cu2ZnSnS4) que són materials de molt baix cost i que no danyen el medi ambient. Vam estudiar el creixement de les pel·lícules primes quaternàries Cu2ZnSnS4 (CZTS) de kesterita mitjançant un dipòsit electroquímic d'un sol pas seguit d'un recuit a baixa temperatura. La influència de diferents atmosferes de recuit a temps de recuit constants (t = 45 min) i paràmetres de control de preparació fixos; és a dir, concentració de la solució de materials de partida (sals de metalls precursors), temps de deposició i potencial d'electrodeposició. Es van estudiar les propietats estructurals, de composició, morfològiques i òptiques, així com les propietats fotoelectroquímiques / Kamal Abdelhamied Saber, S. (2018). Synthesis and characterization of some nano-selenides and their applications in solar cells [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/107389

This thesis covers most of my work in the field of ultracold atoms loaded in optical lattices. This thesis can be divided into five different parts. In Chapter 1, after a brief introduction to the field of optical lattices I review the fundamental aspects pertaining to the physics of systems in periodic potentials and a short overview of the experiments on ultracold atoms in an optical lattice. In Chapter 2 we develop an inhomogeneous mean-field theory for the extended Bose-Hubbard model with a quadratic, confining potential. In the absence of this poten¬tial, our mean-field theory yields the phase diagram of the homogeneous extended Bose-Hubbard model. This phase diagram shows a superfluid (SF) phase and lobes of Mott-insulator(MI), density-wave(DW), and supersolid (SS) phases in the plane of the chemical potential and on-site repulsion ; we present phase diagrams for representative values of , the repulsive energy for bosons on nearest-neighbor sites. We demonstrate that, when the confining potential is present, superfluid and density-wave order parameters are nonuniform; in particular, we obtain, for a few representative values of parameters, spherical shells of SF, MI ,DW ,and SSphases. We explore the implications of our study for experiments on cold-atom dipolar con¬densates in optical lattices in a confining potential. In Chapter3 we present an extensive study of Mottinsulator( MI) and superfluid (SF) shells in Bose-Hubbard (BH) models for bosons in optical lattices with har¬monic traps. For this we develop an inhomogeneous mean-field theory. Our results for the BH model with one type of spinless bosons agrees quantitatively with quan¬tum Monte Carlo(QMC) simulations. Our approach is numerically less intensive than such simulations, so we are able to perform calculations on experimentally realistic, large three-dimensional(3D) systems, explore a wide range of parameter values, and make direct contact with a variety of experimental measurements. We also generalize our inhomogeneous mean-field theory to study BH models with har¬monic traps and(a) two species of bosons or(b) spin-1bosons. With two species of bosons we obtain rich phase diagrams with a variety of SF and MI phases and as¬sociated shells, when we include a quadratic confining potential. For the spin-1BH model we show, in a representative case, that the system can display alternating shells of polar SF and MI phases; and we make interesting predictions for experi¬ments in such systems. . In Chapter 4 we carry out an extensive study of the phase diagrams of the ex-tended Bose Hubbard model, with a mean filling of one boson per site, in one dimension by using the density matrix renormalization group and show that it contains Superfluid (SF), Mott-insulator (MI), density-wave (DW) and Haldane ¬insulator(HI) phases. We show that the critical exponents and central charge for the HI-DW,MI-HI and SF-MI transitions are consistent with those for models in the two-dimensional Ising, Gaussian, and Berezinskii-Kosterlitz-Thouless (BKT) uni¬versality classes, respectively; and we suggest that the SF-HI transition may be more exotic than a simple BKT transition. We show explicitly that different bound¬ary conditions lead to different phase diagrams.. In Chapter 5 we obtain the excitation spectra of the following three generalized of Bose-Hubbard(BH) models:(1) a two-species generalization of the spinless BH model, (2) a single-species, spin-1 BH model, and (3) the extended Bose-Hubbard model (EBH) for spinless interacting bosons of one species. In all the phases of these models we show how to obtain excitation spectra by using the random phase approximation (RPA). We compare the results of our work with earlier studies of related models and discuss implications for experiments.

Quantum spin liquids (QSLs) are strongly correlated systems displaying fascinating phenomena like long-range entanglement and fractionalized excitations. The research on these states has since its beginning followed trends generated by the synthesis of new compounds and the construction of new theoretical tools. In coherence with this history, a manifold of new results about QSLs were established during the past decade due to studies on the integrable Kitaev model on the honeycomb lattice. This j = 1/2 model displays bond-dependent and anisotropic exchanges that are essential to stabilize its QSL ground state with Majorana fermion excitations and emergent Z2 gauge field. Even more interestingly, this model is relevant to understand the magnetism of a certain class of 4/5d5 Mott insulators with specific lattice constraints, t2g orbital degeneracy and strong spin-orbit coupling (SOC). This mechanism defining these so-called Kitaev materials can be applied to similar compounds based on transition metal ions in different electronic configurations. In this thesis, I investigate minimal models for two types of 4/5d1 Mott insulators: the ones on the ordered double perovskite structure (ODP) and the ones isostructural to the Kitaev materials. Their effective models generically show bond-dependent and anisotropic interactions involving multipoles of an effective j = 3/2 angular momentum. Such degrees of freedom are conveniently written in terms of pseudospin s and pseudo-orbital τ operators resembling spin and orbital operators of Kugel-Khomskii models with twofold orbital degeneracy. Despite their anisotropy, the two realistic models display continuous global symmetries in the limit of vanishing Hund\'s coupling enhancing quantum fluctuations and possibly stabilizing a QSL phase. Parton mean-field theory was used to propose fermionic QSLs that will be called quantum spin-orbital liquids (QSOLs) due their dependence with s and τ. On ODPs, I studied a chiral QSOL with Majorana fermion excitations and a gapless spectrum characterized by nodal lines along the edges of the Brillouin zone. These nodal lines are topological defects of a non-Abelian Berry connection and the system exhibits dispersing surface states. Several experimental responses of the chiral QSOL within the mean-field approximation are compared with the experimental data available for the spin liquid candidate Ba2YMoO6. Moreover, based on a symmetry analysis, I discuss the operators involved in resonant inelastic X-ray scattering (RIXS) amplitudes for 4/5d1 Mott insulators and show that the RIXS cross sections allow one to selectively probe pseudospin and pseudo-orbital degrees of freedom. For the chiral spin-orbital liquid in particular, these cross sections provide information about the spectrum for different flavors of Majorana fermions. The model for materials isostructural to the Kitaev materials has an emergent SU(4) symmetry that is made explicit by means of a Klein transformation on pseudospin degrees of freedom. The model is known to stabilize a QSOL on the honeycomb lattice and instigated the investigation of QSOLs on a generalization of this lattice to three dimensions. Parton mean-field theory was used once again to propose the liquid states, and a variational Monte Carlo (VMC) method was used to compute the energies of the projected wave functions. The numerical results show that the lowest-energy QSOL corresponds to a zero-flux state with a Fermi surface of four-color fermionic partons. Further VMC computations also revealed that this state is stable against formation of plaquette ordering (tetramerization). The energy of this QSOL is highly competitive even when Hund\'s coupling induced perturbations are included, as shown by comparison with simple ordered states. Extensions and perspectives for future work are discussed in the end of this thesis. / Líquidos de spin quânticos (QSLs) são sistemas fortemente correlacionados que apresentam fenômenos fascinantes como emaranhamento de longo alcance e excitações fracionárias. A pesquisa a respeito destes estados seguiu tendências geradas pela síntese de novos compostos e construção de novas técnicas teóricas desde seu princípio. Coerentemente com essa história, uma variedade de novos resultados a respeito de líquidos de spin foram estabelecidos na última década graças a estudos feitos sobre o modelo integrável de Kitaev na rede colmeia. Este modelo de spins j = 1/2 apresenta interações de troca anisotrópicas e direcionalmente dependentes que são essenciais para estabilizar um estado fundamental do tipo QSL com férmions de Majorana e campo de gauge Z2 emergente. Ainda mais interessante, este modelo é relevante para se entender o magnetismo de uma certa classe de isolantes de Mott baseados em metais de transição na configuração 4/5d5 em redes específicas, degenerescência orbital t2g e acoplamento spin-órbita forte (SOC). Esse mecanismo que define os chamados materiais do tipo Kitaev podem ser aplicados a compostos baseados em metais de transição em configurações eletrônicas diferentes. Nesta tese, eu investigo modelos mínimos para dois tipos de isolantes de Mott do tipo 4/5d1: os que se apresentam na estrutura perovskita dupla ordenada (ODP) e os isostruturais aos materiais do tipo Kitaev. Seus modelos efetivos genericamente apresentam interações multipolares anisotrópicas e direcionalmente dependentes de um momento angular efetivo j = 3/2. Estes graus de liberdade são convenientemente escritos em termos de operadores de pseudospin s e pseudo-orbital τ semelhantes a operadores de spin e orbital de modelos do tipo Kugel-Khomskii com orbitais duplamente degenerados. A despeito da anisotropia, esses dois modelos realísticos apresentam simetrias globais contínuas no limite de acoplamento de Hund nulo que incrementam flutuações quânticas e possivelmente estabilizam uma fase do tipo QSL. A teoria de campo médio com partons foi usada para propor QSLs fermiônicos que serão chamados de líquidos spin-orbitais quânticos (QSOLs) devido à dependência deles com s e τ. Em ODPs, eu estudei um líquido de spin quiral com excitações do tipo férmion de Majorana e um espectro sem gap caracterizado por linhas nodais ao longo das arestas da zona de Brillouin. Essas linhas nodais são defeitos topológicos de uma conexão de Berry não-abeliana e o sistema apresenta estados de superfície dispersivos. Várias respostas experimentais foram calculadas para o QSOL quiral dentro da aproximação de campo médio e comparadas com os dados experimentais disponíveis para o candidato a líquido de spin Ba2YMoO6. Além disso, baseado em uma análise de simetria, discuto os operadores envolvidos nas amplitudes de espalhamento de raios-x ressonante para isolantes de Mott na configuração 4/5d1 e mostro que seções de choque de RIXS permitem estudar seletivamente os graus de liberdade de pseudospins e pseudo-orbitais. Para o caso particular do líquido spin-orbital quiral, essas seções de choque nos fornecem informações sobre o espectro de diferentes sabores de férmions de Majorana. Esse modelo possui uma simetria SU(4) emergente que é tornada explícita através de uma transformações de Klein nos graus de liberdade de pseudospin. Sabe-se que este modelo estabiliza um QSOL na rede colmeia, o que instigou uma investigação de QSOLs na generalização desta rede em três dimensões. A teoria de campo médio com partons foi usada novamente para propor estes líquidos quânticos, e o método de Monte Carlo Variacional (VMC) foi usado para calcular as energias das funções de onda projetadas. Os resultados numéricos mostraram que o QSOL de menor energia corresponde a um estado de fluxo-zero com superfície de Fermi envolvendo partons fermiônicos de quatro cores. Cálculos adicionais com VMC também demonstraram que este estado é estável à formação de ordem de plaquetas (tetramerização). A energia deste QSOL é altamente competitiva mesmo quando perturbações induzidas pelo acoplamento de Hund são incluídas, o que é mostrado através da comparação com estados ordenados simples. Extensões e perspectivas para trabalhos futuros são discutidas no final desta tese.

Le développement de la plaque bipolaire - PB -métallique nécessite une amélioration des propriétés du matériau qui la constitue. L'utilisation de l'acier implique une meilleure compréhension du film passif - FP. En effet, le cahier des charges de la PB demande une bonne conduction électronique ainsi qu'une longue durée de vie. L'amélioration de ces paramètres passe par une meilleure corrélation et compréhension entre les propriétés semi-conductrices et la résistance à la corrosion. La difficulté liée à l'étude de la passivation réside dans les nombreux phénomènes modifiant le comportement du film passif. De nombreux modèles présentés dans la littérature peuvent être utilisés pour caractériser la passivation. L'un des principaux, le Modèle de Défauts Ponctuels de D. D. Macdonald (Point Defect Model - PDM), décrit le FP à partir d'une série de réactions électrochimiques se déroulant à l'interface métal/oxyde et à l'interface oxyde/électrolyte. La réactivité est limitée par le transport de matière à travers l'oxyde. Après une étude bibliographique, ce phénomène semble être un paramètre discriminant dans le choix des modèles. Une méthode de sélection permettant une utilisation de chacun de ces modèles en fonction de leurs spécificités est proposée. Ainsi, cette méthode est basée sur la corrélation entre la mesure du transport de matière à partir des équations du PDM et l'analyse des Spectres d'Impédance Electrochimique (SIE). Le PDM caractérisant le transport de matière indépendamment de la SIE, il devient possible de déterminer le bien-fondé de la prise en compte de celui-ci dans les mesures de spectroscopie d'impédance électrochimique. L'évolution de la densité du porteur de charge majoritaire avec le potentiel de formation de l'oxyde permet d'accéder au coefficient de transport à partir des équations du PDM. Connaissant l'épaisseur de l'oxyde par des mesures XPS, la constante de temps et la fréquence caractéristique peuvent être déterminées. Sur la base de ces deux valeurs, un modèle descriptif d'analyse des spectres est sélectionné en évitant le sur-paramétrage. Cette méthode est appliquée d’abord à un matériau modèle, le chrome pur exposé à un milieu acide (pH 2), à différentes températures (30°C et 80°C). Elle valide la nécessité de la prise en compte du transport de matière à 80°C ainsi que la présence d'une surface composée d'une couche d'oxyde interne et d'une couche d'hydroxyde externe. Dans un second temps, cette méthode est utilisée sur un matériau industriel, un acier de type AISI 316L, à différents pH (1,2 et 3) et à différentes températures (30°C et 80°C). Elle a permis de décrire l'oxyde en surface comme une jonction p-n prenant en compte une couche riche en chrome interne avec un gradient de concentration de fer. Cette méthode a permis de caractériser de manière approfondie l'acier de type AISI 316L. Bien que ne concernant que le substrat, cette étape est déterminante dans l'amélioration des performances des PB métalliques. / Developments in metallic bipolar plate, to apply more widely fuel cells, require an improved of the constitutive material. The use of stainless steel calls for a good understanding of the passive film. The required specifications are for good electrical conductivity and a long life-time. Those two parameters correspond to a correlation between the semiconductive properties and the good corrosion behavior. Nevertheless, the main problems of the passivity lie on the multiplicity of the phenomena that alter the passive film behavior. Numerous models described in the literature can be used to characterize the passivation. The Point Defect Model (PDM) describes the passivation through electrochemical reactions at the metal / oxide and at the oxide / electrolyte interfaces. The reactivity is limited by mass transport through the oxide. From the literature, those phenomena seems to be a discriminating parameter in the choice of a model. The selective method proposed allows us to use each model taking into account their specifics. This methodology is based on the correlation between the mass transport characterization, thanks to the PDM, and the analysis of the Electrochemical Impedance Spectroscopy (EIS). The PDM determines the transport coefficient apart from EIS measurements, so to validate the consideration of the mass transport during the analysis of the electrochemical impedance spectra. The evolution of the main charge carrier density as a function of the oxide formation potential allows us to calculate the transport coefficient from PDM equations. Thanks to the thickness of the oxide (determined by X-ray Photoelectron Spectroscopy), the time constant of the mass transport is determined. Based on this value, a descriptive model is used to analyze the EIS data, avoiding overparametrization. This method is applied first on a model material, pure chromium exposed to acidic solution (pH 2) at several temperatures (30°C and 80°C). It shows that the mass transport has to be taken into account at 80°C and the EIS model considers an inner chromium oxide layer and an outer chromium hydroxide. Secondly, the method is used to characterize an industrial material, AISI 316L stainless steel, at several pH (1, 2 and 3) and at several temperatures (30°C and 80°C). In this case, the oxide is describe as a p-n semiconductor junction with an chromium rich inner layer and an outer iron rich layer. The present methodology permits to deeply characterize the AISI 316L stainless steel. Even if this study concerns the substrate, this step is decisive to improve the performances of the metallic bipolar plates

A trial was conducted to evaluate forage production potential, forage intake and milk production in two perennial tropical grasses under continuous grazing of lactating cows, in the growth seasons of 2003/04 and 2004/05, in Palmeira das Missões/RS. Twelve tester animaIs were distributed in two paddocks per treatment, using "put-and-take" technique to adjust stocking rates. Evaluated treatments were Dwarf Elephantgrass (Pennisetum purpureum cv. Mott) and Tifton 85 (Cynodon dactylon x C. nlemfuensis) pastures. Under the proposed management criteria (4 % of leaf lamina allowance), Tifton 85 demonstrates to be less sensible to forage production and stocking rate reduction under adverse climate conditions, with relative stability of forage mass available. Its superiority in individual animal performance, in more favourable conditions (2003/04 growth season), when compared to Dwarf Elephantgrass, should be related to the allowance used. Maintaining low levels of leaf lamina on offer could be limiting to cows' individual performance, rebounding negatively over persistency of the pastures. The evaluated species presented different structures and forage densities in the most accessible strata to grazing by the animaIs. However, those differences didn't affect the chemical composition of these layers nor of its respective of its respective "hand-plucking" samples, causing similar forage intake. Higher individual animal production indices were found, indicating a little known potential for perennial tropical grasses in milk from pasture systems. / Para avaliar o potencial forrageiro, o consumo de forragem e a produção de leite em duas gramíneas perenes tropicais sob pastejo contínuo de vacas em lactação, foi conduzido um experimento nos anos agrícolas de 2003/04 e 2004/05, em Palmeira das Missões/RS. Foram utilizados 12 animais testes, distribuídos em dois piquetes por tratamento, adotando-se a técnica dos animais reguladores para ajustes da carga animal. Os tratamentos avaliados foram pastagens de Capim Elefante Anão (Pennisetum purpureum cv. Mott) e Tifton 85 (Cynodon dactylon x C. nlemfuensis). Sob o critério de manejo proposto (oferta de 4 % de lâminas foliares), o Tifton 85 demonstra ser menos sensível à redução da produção forrageira e capacidade de carga sob condições adversas de clima, com relativa manutenção da massa de forragem disponível. Sua superioridade em produção individual por animal, em condições mais favoráveis (ano agrícola 2003/04), quando comparado ao Capim Elefante Anão, deve estar relacionada à oferta utilizada. A manutenção de níveis reduzidos de oferta de lâminas foliares pode ter sido limitante ao desempenho individual das vacas, repercutindo também de forma negativa na persistência produtiva das pastagens. As espécies estudadas apresentaram distintas estruturas e densidades de forragem nos estratos mais acessíveis ao pastejo pelos animais. Contudo, tais diferenças estruturais não afetaram a composição química destas camadas ou das respectivas simulações de pastejo, acarretando em consumo de forragem similar. Foram registrados elevados índices de produção animal individual, indicando um potencial pouco conhecido das gramíneas tropicais em sistemas de produção de leite a pasto.

Made available in DSpace on 2014-12-17T15:15:02Z (GMT). No. of bitstreams: 1 FabioFM.pdf: 1459636 bytes, checksum: 4f2bb557c6a5bddb80e46f325bd5b4dd (MD5) Previous issue date: 2004-12-03 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work we study the spectrum (bulk and surface modes) of exciton-polaritons in infinite and semi-infinite binary superlattices (such as, ???ABABA???), where the semiconductor medium (A), whose dielectric function depends on the frequency and the wavevector, alternating with a standard dielectric medium B. Here the medium A will be modeled by a nitride III-V semiconductor whose main characteristic is a wide-direct energy gap Eg. In particular, we consider the numerical values of gallium nitride (GaN) with a crystal structure wurtzite type. The transfer-matrix formalism is used to find the exciton-polariton dispersion relation. The results are obtained for both s (TE mode: transverse electric) and p (TM mode: transverse magnetic) polarizations, using three diferent kind of additional boundary conditions (ABC1, 2 e 3) besides the standard Maxwell's boundary conditions. Moreover, we investigate the behavior of the exciton-polariton modes for diferent ratios of the thickness of the two alternating materials forming the superlattice. The spectrums shows a confinement of the exciton-polariton modes due to the geometry of the superlattice. The method of Attenuated Total Reflection (ATR) and Raman scattering are the most adequate for probing this excitations / Neste trabalho estudamos o espectro (modos de volume e de superf?cie) dos polaritons de exciton em uma super-rede bin?ria infinita e semi-infinita (tal como, ???ABABA???), onde um meio semicondutor (A), cuja fun??o diel?trica depende da frequ?ncia e do vetor de onda, alterna-se com um diel?trico comum (B). Aqui, o meio A ser? modelado por um semicondutor da fam?lia dos nitretos (semicondutor III-V) que tem como caracter?stica principal um gap de energia (Eg) direto e largo. Em particular, consideramos os valores num?ricos para o nitreto de g?lio (GaN) com uma estrutura cristalina tipo wurtzite. A t?cnica da matriz de transfer?ncia ? utilizada para encontrarmos a rela??o de dispers?o do polariton de exciton. Os resultados s?o obtidos para os modos de polariza??o s (ou modo TE: transversal el?trico) e p (ou modo TM: transversal magn?tico), usando tr?s diferentes condi??es de contorno adicionais (ABC1, 2 e 3), mais as condi??es de contorno padr?es de Maxwell. Al?m disso, investigamos o comportamento dos modos do polariton de exciton para diferentes raz?es entre as espessuras das camadas dos dois materiais que comp?em a super-rede Os espectros encontrados evidenciam um comportamento de confinamento dos polaritons de exciton devido ?s geometrias empregadas. As t?cnicas experimentais ATR ("Attenuated Total Reflection") e o espalhamento Raman s?o as mais adequadas para a caracteriza??o dessas excita??es

Quantum spin liquids (QSLs) are strongly correlated systems displaying fascinating phenomena like long-range entanglement and fractionalized excitations. The research on these states has since its beginning followed trends generated by the synthesis of new compounds and the construction of new theoretical tools. In coherence with this history, a manifold of new results about QSLs were established during the past decade due to studies on the integrable Kitaev model on the honeycomb lattice. This j = 1/2 model displays bond-dependent and anisotropic exchanges that are essential to stabilize its QSL ground state with Majorana fermion excitations and emergent Z2 gauge field. Even more interestingly, this model is relevant to understand the magnetism of a certain class of 4/5d5 Mott insulators with specific lattice constraints, t2g orbital degeneracy and strong spin-orbit coupling (SOC). This mechanism defining these so-called Kitaev materials can be applied to similar compounds based on transition metal ions in different electronic configurations. In this thesis, I investigate minimal models for two types of 4/5d1 Mott insulators: the ones on the ordered double perovskite structure (ODP) and the ones isostructural to the Kitaev materials. Their effective models generically show bond-dependent and anisotropic interactions involving multipoles of an effective j = 3/2 angular momentum. Such degrees of freedom are conveniently written in terms of pseudospin s and pseudo-orbital τ operators resembling spin and orbital operators of Kugel-Khomskii models with twofold orbital degeneracy. Despite their anisotropy, the two realistic models display continuous global symmetries in the limit of vanishing Hund\'s coupling enhancing quantum fluctuations and possibly stabilizing a QSL phase. Parton mean-field theory was used to propose fermionic QSLs that will be called quantum spin-orbital liquids (QSOLs) due their dependence with s and τ. On ODPs, I studied a chiral QSOL with Majorana fermion excitations and a gapless spectrum characterized by nodal lines along the edges of the Brillouin zone. These nodal lines are topological defects of a non-Abelian Berry connection and the system exhibits dispersing surface states. Several experimental responses of the chiral QSOL within the mean-field approximation are compared with the experimental data available for the spin liquid candidate Ba2YMoO6. Moreover, based on a symmetry analysis, I discuss the operators involved in resonant inelastic X-ray scattering (RIXS) amplitudes for 4/5d1 Mott insulators and show that the RIXS cross sections allow one to selectively probe pseudospin and pseudo-orbital degrees of freedom. For the chiral spin-orbital liquid in particular, these cross sections provide information about the spectrum for different flavors of Majorana fermions. The model for materials isostructural to the Kitaev materials has an emergent SU(4) symmetry that is made explicit by means of a Klein transformation on pseudospin degrees of freedom. The model is known to stabilize a QSOL on the honeycomb lattice and instigated the investigation of QSOLs on a generalization of this lattice to three dimensions. Parton mean-field theory was used once again to propose the liquid states, and a variational Monte Carlo (VMC) method was used to compute the energies of the projected wave functions. The numerical results show that the lowest-energy QSOL corresponds to a zero-flux state with a Fermi surface of four-color fermionic partons. Further VMC computations also revealed that this state is stable against formation of plaquette ordering (tetramerization). The energy of this QSOL is highly competitive even when Hund\'s coupling induced perturbations are included, as shown by comparison with simple ordered states. Extensions and perspectives for future work are discussed in the end of this thesis. / Líquidos de spin quânticos (QSLs) são sistemas fortemente correlacionados que apresentam fenômenos fascinantes como emaranhamento de longo alcance e excitações fracionárias. A pesquisa a respeito destes estados seguiu tendências geradas pela síntese de novos compostos e construção de novas técnicas teóricas desde seu princípio. Coerentemente com essa história, uma variedade de novos resultados a respeito de líquidos de spin foram estabelecidos na última década graças a estudos feitos sobre o modelo integrável de Kitaev na rede colmeia. Este modelo de spins j = 1/2 apresenta interações de troca anisotrópicas e direcionalmente dependentes que são essenciais para estabilizar um estado fundamental do tipo QSL com férmions de Majorana e campo de gauge Z2 emergente. Ainda mais interessante, este modelo é relevante para se entender o magnetismo de uma certa classe de isolantes de Mott baseados em metais de transição na configuração 4/5d5 em redes específicas, degenerescência orbital t2g e acoplamento spin-órbita forte (SOC). Esse mecanismo que define os chamados materiais do tipo Kitaev podem ser aplicados a compostos baseados em metais de transição em configurações eletrônicas diferentes. Nesta tese, eu investigo modelos mínimos para dois tipos de isolantes de Mott do tipo 4/5d1: os que se apresentam na estrutura perovskita dupla ordenada (ODP) e os isostruturais aos materiais do tipo Kitaev. Seus modelos efetivos genericamente apresentam interações multipolares anisotrópicas e direcionalmente dependentes de um momento angular efetivo j = 3/2. Estes graus de liberdade são convenientemente escritos em termos de operadores de pseudospin s e pseudo-orbital τ semelhantes a operadores de spin e orbital de modelos do tipo Kugel-Khomskii com orbitais duplamente degenerados. A despeito da anisotropia, esses dois modelos realísticos apresentam simetrias globais contínuas no limite de acoplamento de Hund nulo que incrementam flutuações quânticas e possivelmente estabilizam uma fase do tipo QSL. A teoria de campo médio com partons foi usada para propor QSLs fermiônicos que serão chamados de líquidos spin-orbitais quânticos (QSOLs) devido à dependência deles com s e τ. Em ODPs, eu estudei um líquido de spin quiral com excitações do tipo férmion de Majorana e um espectro sem gap caracterizado por linhas nodais ao longo das arestas da zona de Brillouin. Essas linhas nodais são defeitos topológicos de uma conexão de Berry não-abeliana e o sistema apresenta estados de superfície dispersivos. Várias respostas experimentais foram calculadas para o QSOL quiral dentro da aproximação de campo médio e comparadas com os dados experimentais disponíveis para o candidato a líquido de spin Ba2YMoO6. Além disso, baseado em uma análise de simetria, discuto os operadores envolvidos nas amplitudes de espalhamento de raios-x ressonante para isolantes de Mott na configuração 4/5d1 e mostro que seções de choque de RIXS permitem estudar seletivamente os graus de liberdade de pseudospins e pseudo-orbitais. Para o caso particular do líquido spin-orbital quiral, essas seções de choque nos fornecem informações sobre o espectro de diferentes sabores de férmions de Majorana. Esse modelo possui uma simetria SU(4) emergente que é tornada explícita através de uma transformações de Klein nos graus de liberdade de pseudospin. Sabe-se que este modelo estabiliza um QSOL na rede colmeia, o que instigou uma investigação de QSOLs na generalização desta rede em três dimensões. A teoria de campo médio com partons foi usada novamente para propor estes líquidos quânticos, e o método de Monte Carlo Variacional (VMC) foi usado para calcular as energias das funções de onda projetadas. Os resultados numéricos mostraram que o QSOL de menor energia corresponde a um estado de fluxo-zero com superfície de Fermi envolvendo partons fermiônicos de quatro cores. Cálculos adicionais com VMC também demonstraram que este estado é estável à formação de ordem de plaquetas (tetramerização). A energia deste QSOL é altamente competitiva mesmo quando perturbações induzidas pelo acoplamento de Hund são incluídas, o que é mostrado através da comparação com estados ordenados simples. Extensões e perspectivas para trabalhos futuros são discutidas no final desta tese.

Indiana University-Purdue University Indianapolis (IUPUI) / The collapse of communism and the fall of the Soviet Union offered an unprecedented opportunity for the international community to support transitions to democracy in a region that had long known only totalitarian rule. Among the key players engaged in supporting efforts were U.S. grantmaking institutions, including both non-state and quasi-state aid providers. This thesis explores the motivations and evolving strategies of three different types of grantmaking institutions in a single country, Russia, with a particular focus on democracy aid provision from 1988-2002. The three types of grantmaking organizations examined through case studies include: the Ford Foundation, a private foundation with a history of international grantmaking spanning several decades; the Charles Stewart Mott Foundation, a private foundation known primarily for its domestic focus with a much shorter history of international grantmaking; and, finally, the National Endowment for Democracy, a U.S. government-created and heavily taxpayer-funded organization established as a private nonprofit organization to make grants specifically for democracy promotion. Motivating factors for initiating or expanding grantmaking in Russia in the late 1980s included a previous history of grantmaking in the region, a previously established institutional commitment to democracy promotion, international peace and security concerns, and interest from a top institutional leader. Over the course of the fourteen year period studied, five grantmaking features are identified as influencing the development of grantmaking strategies: professional grantmaking staff; organizational habit; global political, social, and economic environments; market and other funding source influences; and physical presence. Though subject to constraints, the non-state and quasi-state grantmaking institutions included in this study were able to avoid weaknesses identified with private philanthropy in other research and demonstrated a willingness to experiment and take risks, an ability to operate at the non-governmental level, and a commitment to long-term grantmaking, informed by expertise.

Les couches organiques greffées de façon covalente sur le silicium apparaissent comme des systèmes prometteurs tant sur le plan fondamental qu'en raison des nombreuses applications envisageables : couches minces diélectriques, capteurs chimiques ou biochimiques. Ce travail de thèse est consacré à l'étude de ces systèmes par diverses techniques. La stabilité chimique et électrique de l'interface au cours du temps et au sein d'un électrolyte de pH variable sont décrites. La morphologie de la surface est contrôlée par microscopie AFM. La nature chimique du système est déterminée par spectroscopie infrarouge en mode ATR et par spectroscopie de photoélectrons X. La qualité électrique de l'interface est étudiée par des mesures de photoluminescence, photopotentiel et Mott-Schottky. L'influence de la préparation sur les diverses caractéristiques du système est examinée. L'effet de charges adsorbées en surface a également été exploré par une titration quantitative de l'ionisation de groupements carboxyles greffés en surface. Enfin, une méthode originale de fonctionnalisation de couches alkyles greffées de compacité maximum par traitement dans un plasma doux oxydant est présentée. Le suivi du système, in situ par spectroscopie IR et ex situ par XPS, ainsi que la connaissance de la nature du plasma d'oxygène ont permis de mettre en évidence une destruction de la couche aux plus fortes puissances et une intégrité de celle-ci aux plus faibles puissances : un mécanisme de fonctionnalisation des chaînes dans ce régime est proposé.

This thesis explores prevailing and changing attitudes among Protestant Christians as manifested in the World Missionary Conference of 1910, held in Edinburgh, Scotland. It compares the conference to missionary literature to demonstrate how well it fit the context of the missionary endeavor during the Edwardian era. It examines the issues of race and empire in the thinking of conference participants. It pays particular attention to the position of West Africa and West Africans in conference deliberations. It suggests that the conference, which took place soon after the scramble for empire and just before World War I and the subsequent upsurge of nationalism and anti-colonialism, offers a valuable historical perspective on the uneven nature of globalizing Christianity.