Ce travail porte sur la génération directe de triplets de photons par interaction optique non linéaire du troisième ordre avec la matière solide. Les trois photons constituant l'état triplet proviennent de la scission d'un unique photon, et sont donc étroitement corrélées. Des champs supplémentaires peuvent stimuler le processus, et ainsi augmenter son efficacité de conversion, mais au détriment de la conservation des corrélations de l'état triplet. Deux stratégies sont adoptées pour générer efficacement ces triplets tout en conservant leurs propriétés de cohérence. La première porte sur génération de triplets dans des oxydes massifs cristallins, rendue possible à travers la réalisation d'accords de phase par biréfringence. Ces cristaux peuvent être placés en cavité de manière à augmenter artificiellement la longueur d'interaction. Dans ce contexte, KTP et TiO2 sous sa forme rutile sont étudiés expérimentalement; la configuration en cavité fait l'objet d'une étude théorique. La seconde stratégie se concentre sur la génération de triplets dans des fibres optiques, à travers un accord de phase modal. Leurs longueurs, le confinement du champ électromagnétique, ainsi que la non-existence de processus quadratiques pouvant polluer la génération de triplets sont des avantages importants. Des expériences de génération de troisième harmonique dans des fibres de silice dopées germanium sont réalisées ; et les propriétés d'accord de phase dans des fibres à cristaux photoniques en chalcogénures sont calculées. / This work concentrates on the direct generation of triple photons through third-order nonlinear optical interactions with solid-state matter. The three photons constituting the triplet state arise from the splitting of a single photon, and are therefore highly correlated.The four interacting particles fulfill the energy and linear momentum conservation laws. Additional fields can stimulate this process and thus increase its conversion efficiency, but at the cost of losing the correlations of the triplet states. In order to generate efficiently the triplets while preserving their coherence properties, two strategies are investigated. In the first one, the interaction occurs in oxide bulk crystals, thanks to a birefringent phase matching. These crystals can be put into a cavity so as to artificially increase the interaction length. In this context, KTP and rutile TiO2 are studied experimentally; the cavity configuration is subjected to a theoretical work. The second strategy focuses on the generation in optical fibers, through a modal phase matching. Their length, the confinement of the electromagnetic field, and the non-existence of polluting second-order nonlinear processes are key advantages. Third-harmonic generation experiments on germanium-doped silica fibers are performed; and phase-matching properties in chalcogenide photonic-crystal fibers are calculated.

Dans cette thèse nous avons évalué l'effet de la substitution partielle du cobalt par des éléments non-magnétiques de type p (M) sur les propriétés structurales et magnétiques des composés RCo5. Les échantillons ont été synthétisées par fusion dans un four à induction, puis caractérisés en utilisant des nombreuses techniques expérimentales: diffraction de rayons X et de neutrons, microscopie électronique à balayage, magnétométrie, susceptométrie en courant alternatif et spectroscopie photoélectronique par rayons X.Tous les composés de type RCo5-xMx (R= Pr, Sm, Tb, Er et Tm; M= Si, Ge, Al, Ga; x=0,5 et x=1) étudiés gardent la structure cristalline de type CaCu5 des composés de départ RCo5, mais les paramètres de la maille cristalline sont modifiés par la présence des éléments M. Le domaine de stabilité thermique des phases RCo5-xMx est notablement modifié par la présence d'élément métalloïde en substitution sur les sites du cobalt. Il peut être étendu pour Al et Ga et réduit pour Si et Ge. Les atomes de l'élément M sont localisés préférentiellement sur le site cristallographique Co 3g. La solubilité des éléments Ge et Si dans la structure RCo5 est trouvée être inférieure à celle des métalloïdes ayant un électron de moins tels que Al et Ga.La substitution M/Co a une influence importante sur les propriétés magnétiques des composés RCo5. La température d'ordre et l'aimantation spontanée diminuent significativement après le remplacement partiel du cobalt par l'élément métalloïde. Ces modifications sont induites par l'évolution des interactions d'échange en particulier Co-Co mais aussi par la réduction de l'aimantation du Co liée à sa sensibilité au voisinage atomique et magnétique local. La direction de facile aimantation à la température ambiante est préservée le long de l'axe cristallographique c pour tous les échantillons étudiés. Dans quelques composés RCo5-xMx (R = Sm, Er et Tm) la substitution de M au cobalt change le mécanisme de coercitivité par rapport aux composés de départ RCo5. Une coercitivité élevée a été détectée à basse température, surtout pour les composés SmCo4Al et SmCo4Ga, qui présentent aussi des champs d'anisotropie énormes, bien supérieurs à ceux de phase SmCo5.Les expériences XPS sur les composés de type RCo5-xMx ont détecté la réduction de la densité d'états électroniques au niveau de Fermi par rapport aux composés de type RCo5. La bande 3d du cobalt est remplie lors de la substitution M/Co et conduit à un moment magnétique du cobalt réduit. / In this work we have evaluated the effect of the partial substitution of cobalt with non-magnetic p-type elements (M) on the structural and magnetic properties of RCo5 compounds. The samples were prepared by alloying in an induction furnace and were characterized using various experimental techniques: X-ray and neutron diffraction, scanning electron microscopy, magnetometry, AC susceptometry and X-ray photoelectron spectroscopy.All of the studied RCo5-xMx compounds (R=Pr, Sm, Tb, Er and Tm; M=Si, Ge, Al and Ga; x=0.5 and x=1) maintain the CaCu5 crystal structure of the RCo5 compounds, however the lattice parameters are modified due to the presence of M elements. The thermal stability range of the RCo5-xMx phases is modified significantly due to the Co site substitutions. The thermal stability increases for Al and Ga substitutions and decreases when M is Si or Ge. The M atoms were found to preferentially occupy the Co 3g site. The solubility of Ge and Si in the RCo5 structure is inferior to that of metalloid elements with one less electron, such as Al and Ga. The M/Co substitution has an important influence on the magnetic properties of RCo5 compounds. The ordering temperature and the spontaneous magnetization are significantly reduced after the partial substitution of cobalt by the metalloid elements. These changes are induced in particular by the evolutions of the Co-Co exchange interactions and also by the reduction of the Co magnetization due to the sensitivity of Co to the local atomic and magnetic vicinity. The easy magnetization direction at room temperature is preserved along the c-axis for all of the studied samples. In several RCo5-xMx (R = Sm, Er and Tm) compounds the substitution of Co with M atoms changes the coercivity mechanism compared to RCo5. A higher coercivity was found at low temperatures, especially for SmCo4Al and SmCo4Ga compounds, which also show huge values of the anisotropy field, well above the ones found in SmCo5.The XPS measurements on RCo5-xMx compounds show a reduction of the density of states at the Fermi level compared to the RCo5 compounds. There is a filling of the Co 3d band following the M/Co substitution, leading to a lower Co moment.

Les milieux cristallins fracturés constituent un potentiel géothermique non négligeable. Il est essentiel d'améliorer son exploitation, pour la géothermie basse et haute énergie, afin de répondre au processus de transition énergétique. Pour cela, la compréhension des mécanismes de transport thermique dans les milieux fracturés est fondamentale. Le transport de chaleur est fortement influencé par l'hétérogénéité hydrodynamique des milieux fracturés et par la géométrie des fractures et des blocs matriciels. A travers des travaux basés sur des développements analytiques et numériques ainsi que des expériences sur site, l'objectif de cette thèse est ainsi de mieux évaluer l'impact de la géométrie des fractures, que ce soit à l'échelle d'un réseau de fractures, ou à l’échelle d’une fracture, sur le transport et le stockage d’énergie thermique dans les milieux cristallins fracturés. Des simulations numériques du transport de chaleur dans un réseau simple de fractures planes et bien connectées ont permis de caractériser l'impact de la géométrie du système de fractures sur le stockage thermique. Deux régimes sont mis en évidence. Tout d'abord, à court terme, la densité de fractures ou de chemins préférentiels, caractérisant la surface d'échange, contrôle l'échange thermique. Puis, à long terme, c'est le volume de roche total entre les fractures qui contrôle le stockage thermique. Ce modèle ne prend toutefois pas en compte la variabilité des ouvertures à l'échelle de la fracture qui est particulièrement présente dans les réseaux de fractures naturels. Des tests de traçage thermique et de soluté ont ainsi été réalisés pour caractériser le transport de chaleur dans un milieu fracturé sur le site de Ploemeur (SNO H+). Pour interpréter les traçages, les expressions analytiques du retard et de l'amplitude du pic de la courbe de restitution thermique ont été développées pour différentes géométries de fractures : fractures planes et chenaux. Ces expressions constituent un outil puissant et novateur pour caractériser la géométrie des fractures lors de tests de traçage thermique mais aussi pour prédire le déplacement du front thermique et la durée de vie des systèmes géothermiques à partir de tests de traçage de soluté. La comparaison de ces expressions avec les résultats expérimentaux permet de mettre en évidence l'importante chenalisation des flux, induisant l'arrivée anticipée du traceur thermique. / Fractured crystalline media has a significant geothermal potential. Its exploitation, for low and high enthalpy geothermal power generation, could be enhanced to satisfy the energy transition process. For this, understanding thermal transport processes in fractured media is fundamental. Heat transport is strongly influenced by hydrodynamics heterogeneity of fractured media and by fracture and matrix block geometry. Through analytical and numerical modelling and field site experiments, the aim of this thesis is thus to better assess the impact of fracture geometry on thermal transport and storage in fractured crystalline rock, at fracture and fracture network scale. Numerical simulations of heat transport in a simple network of well connected plane fractures allowed us to characterize the impact of the fracture system geometry on thermal storage. Two regimes are highlighted. First, at short term, the density of fractures, or preferential paths, controls heat exchanges. Then at long term, the total rock volume between the fractures controls thermal storage. This model does not take into account the aperture variability at fracture scale, which is particularly present in natural fracture networks. Thus, thermal and solute tracer tests have been achieved to characterize heat transport in a fractured media at Ploemeur field site (SNO H +). To interpret the tracer tests, analytical expressions of thermal breakthrough peak retardation and amplitude have been developed for different fracture geometries : parallel plate fractures and channels. Those expressions are a powerful and innovative tool to characterize fracture geometries from thermal tracer tests, and also to predict thermal front transit time and lifetime of geothermal systems from solute tracer tests. Confrontation of those expressions to experimental results shows that observed differences between thermal and solute breakthrough can be explained only by channeling flow inducing low thermal transit times.

L’utilisation des combustibles fossiles est responsable de l’augmentation de la concentration en gaz à effet de serre dans l’atmosphère. Parmi les solutions de remplacement envisagées, l’hydrogène constitue un vecteur d’énergie très intéressant. Toutefois, cette solution ne sera envisageable que lorsque les problématiques liées à la production de l’hydrogène et à son stockage seront résolues.Le premier objectif de cette thèse porte sur la synthèse et la caractérisation de composés ternaires à base de magnésium dans le système ternaire TR-M-Mg (avec TR = Terres Rares et M = métaux de transition) qui pourraient être de bons candidats pour le stockage de l’hydrogène. Ces composés pourraient de plus avoir d’autres applications, notamment comme matériaux de structure, du fait de leur très faible densité. La composition NdNiMg15 a fait l’objet d’une étude complète. Cette phase cristallise selon une symétrie quadratique avec a= 10,0602(1) et c= 7,7612(2) Å et un groupe d’espace P4/nmm. Un ordre antiferromagnétique à 9 K est observé et la capacité massique de stockage réversible est de 4 %mass. Cette nouvelle phase a montré un effet durcissant sur le magnésium.Le deuxième objectif de cette thèse concerne la production d’hydrogène par hydrolyse i) des composés ternaires TR-M-Mg qui pourrait être considérée comme une possibilité économique et énergétique pour valoriser les déchets de ces composés et ii) des mélanges ternaires TR-M-Mg élaborés par broyage mécanique. Le broyage a permis la création des défauts favorisant ainsi la corrosion des métaux. De plus, la production d'hydrogène par hydrolyse des composites Mg-NdNiMg15 (70, 80 et 90 %mass. Mg) a été réalisée et comparée à celle du composé NdNiMg15 (64 %mass. en Mg). Le mécanisme de corrosion principal déduit des essais électrochimiques sur les composites est la corrosion galvanique. / The use of fossil fuels (non-renewable energy) is responsible for the increase of the concentration of greenhouse gases in the atmosphere. Among the considered alternatives, hydrogen is seen as the most attractive energy vector. Production and storage of hydrogen is one of the key challenges in developing the hydrogen economy.The first objective of this thesis deal with the synthesis and characterization of magnesium-based ternary compounds in the RE-TM-Mg ternary system (with RE = Rare Earth and TM = transition metals) which could be good candidates for hydrogen storage. These compounds could also have other applications than the hydrogen storage in the future such as light structured material. The NdNiMg15 compound has been the subject of a completed study. This phase crystallizes with a tetragonal symmetry (a= 10.0602(1) and c= 7.7612(2) Å and a space group P4/nmm). It showed an antiferromagnetic ordering at 9 K and a reversible hydrogen storage capacity of 4 %mass. This phase exhibited a hardening effect respect to magnesium compound.The second objective of this thesis concerns the hydrogen production by hydrolysis of i) RE-TM-Mg ternary compounds, which could be considered as an economic and energetic possibility to valorize the waste of these compounds and ii) RE-TM-Mg ternary mixtures prepared by ball milling. The grinding creates defects thus promoting the corrosion of the metals. In addition, the hydrogen production by hydrolysis of the Mg-NdNiMg15 composites (70, 80 and 90 %mass. Mg) was carried out and compared with that of the NdNiMg15 compound (64 %mass. Mg). The main corrosion mechanism determined from the electrochemical measurements of the composites is the galvanic corrosion.

Made available in DSpace on 2015-02-24T19:48:40Z (GMT). No. of bitstreams: 1 AnaCF_ate cap6.pdf: 4452178 bytes, checksum: 27b2ccb7e47f9a71ccabd7ab319267a6 (MD5) Previous issue date: 2002-09-13 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Hydrogeological prospecting in Northeast Brazil and in other crystalline terrains has been developed on the basis of structural and regional geology concepts that date back to the 50-60 decades and, as such, demand a natural re-evaluation and update. In this kind of terrain, the percolation and accumulation of ground water are controlled by fractures and other types of discontinuities, such as foliations and geological contacts that, through weathering, impart porosity and permeability to the rocks, allowing water flow and storage. Several factors should be considered in the process of locating water wells, as discussed in the literature. Among these, the kind of structures, fracture geometry (including aperture and connectivity) and their geological and chronological context. It is important to correlate fracture systems with the regional neotectonic framework. Fractures at low angle (sub parallel) with the principal stress axis (s1) are those which tend to open (actually they work as tension joints) and, in principle, would present major hydric potential; in the opposite side, fractures at high angle to s1 would behave as closed by a compressional component. Fractures diagonal to the compression and tension axes correspond to shear fractures and, due to their connectivity with second fractures, are also important in terms of hydric potential. Uplift followed by terrain denudation leads to decompression and a general tendency to open (aided by weathering processes) fractures and other rock discontinuities, at different orientations. Low angle fractures, formed in this context, are equally important to increase connectivity, collection of water and recharge of the aquifer systems. In a general way, an opening component (neotectonic or by terrain decompression) and several models to increase fracture connectivity correlate with a greater hydric potential of these structures. Together with parallel research, this thesis addresses models of ground water occurrence in crystalline terrains, either improving well established concepts like the (Riacho-Fenda model), but also stressing other possibilities, like the role of alluvium and paleo-regoliths (the Calha El?vio-Aluvionar model) and of strongly altered, permo-porous zones placed at variable depths below the present surface, flanking several types of discontinuities, especially interconnected fracture arrays (the Bols?es de Intemperismo model). Different methodological approaches are also discussed in order to improve success rates in the location of water wells in crystalline terrains. In this methodological review, a number of case studies were selected in the eastern domain of the State of Rio Grande do Norte, involving the localities of Santa Cruz, Santo Ant?nio, Serrinha, Nova Cruz, Montanhas, Lagoa de Pedras and Lagoa Salgada. Besides the neotectonic analysis of brittle structures, this Thesis addresses the validation of remote sensing as a tool for ground water prospecting. Several techniques were tested in order to detect and select areas with higher potential for ground water accumulation, using Landsat 5-TM and RADARSAT images, besides conventional aerial photos. A number of filters were tested to emphasize lineaments in the images, improving their discrimination, to identify areas with higher overburden humidity, which could reflect subsurface water accumulation, as well as alluvium and other sedimentary covers that might act as recharge zones. The work started with a regional analysis with the orbital images, followed by analysis of aerial photos, up to a detailed structural study of rock exposures in the terrain. This last step involved the analysis of outcrops surrounding wells (in a ray of approximately 10 to 100 m) with distinct productivities, including dry examples. At the level required for detail, it was not possible to accomplish a statistical approach using the available well data catalogs, which lack the desired specific information. The methodology worked out in this Thesis must undergo a testing phase through location of new water wells. An increase in the success rates as desired will led to a further consolidation step with wider divulgation of the methodology to private companies and governmental agencies involved in ground water prospecting in crystalline terrains / A prospec??o hidrogeol?gica no Nordeste do Brasil e em outros terrenos cristalinos tem sido baseada em conhecimentos de Geologia Estrutural e Regional que, pelo tempo decorrido, demandam uma natural reavalia??o. Neste tipo de terreno, a percola??o e ac?mulo de ?gua subterr?nea s?o controlados por fraturas e outros tipos de descontinuidades, tais como folia??es e contatos geol?gicos, que intemperizados adquirem certa porosidade e permeabilidade, permitindo o fluxo e/ou reten??o de ?gua. Diversos fatores devem ser considerados no processo de loca??o de um po?o para ?gua, conforme amplamente discutido na literatura. Dentre estes, o tipo de estrutura observada, a geometria do fraturamento (incluindo abertura e conectividade) e seu contexto geol?gico e cronol?gico. Neste ?ltimo sentido, ? importante correlacionar o fraturamento com o arcabou?o neotect?nico conhecido na regi?o. Fraturas orientadas em baixo ?ngulo (subparalelas) com o eixo de tens?o principal (s1) s?o aquelas que tendem a abrir (funcionam atualmente como juntas de distens?o) e apresentam, em princ?pio, maior potencial h?drico; em situa??o oposta situam-se as fraturas em forte ?ngulo com s1 (corresponderiam a fraturas fechadas , com um componente de compress?o). As fraturas diagonais ?s dire??es de compress?o e distens?o s?o equivalentes ?s fraturas de cisalhamento e, pela conectividade com planos de segunda ordem, s?o tamb?m importantes em termos de produtividade h?drica. A denuda??o do terreno tamb?m enseja descompress?o e uma tend?ncia geral de abertura (inclusive pelo efeito do intemperismo) de fraturas e outras descontinuidades, em orienta??es quaisquer. Fraturas de baixo ?ngulo, formadas nesse contexto, s?o igualmente importantes para definir a conectividade, coleta de ?gua e a recarga dos sistemas aq??feros. De um modo geral, um componente de abertura (neotect?nico ou pela descompress?o) e os v?rios modelos de interconex?o de fraturas resultam no aumento de sua potencialidade h?drica. Em conjunto com pesquisas em paralelo, a presente Tese aborda modelos de ocorr?ncia de ?gua subterr?nea no cristalino, procurando aperfei?oar conceitos j? estabelecidos (como o modelo Riacho-Fenda) e enfatizando outras possibilidades, a exemplo do papel de aluvi?es e paleo-regolitos (o modelo Calha El?vio-Aluvionar) e de zonas fortemente alteradas (permo-porosas) em subsuperf?cie, ladeando v?rios tipos de descontinuidades, em especial fraturas interconectadas (o modelo Bols?es de Intemperismo). S?o tamb?m discutidos diferentes aspectos metodol?gicos atualmente utilizados na loca??o de po?os, fazendo uma reavalia??o destes procedimentos com vistas a aumentar o ?ndice de acerto nas loca??es de po?os em terrenos cristalinos. Nessa metodologia, foram estudadas ?reas selecionadas, no interior do Rio Grande do Norte, envolvendo os munic?pios de Santa Cruz, Santo Ant?nio, Serrinha, Nova Cruz, Montanhas, Lagoa de Pedras e Lagoa Salgada, todos na regi?o oriental do Estado. Al?m de fazer uma an?lise neotect?nica do fraturamento, esta Tese aborda a validade da utiliza??o de sensoriamento remoto como ferramenta na prospec??o de ?gua subterr?nea. Foram testadas v?rias t?cnicas que pudessem facilitar a detec??o e sele??o de ?reas com maior potencial para o ac?mulo de ?gua subterr?nea, utilizando imagens Landsat 5-TM, RADARSAT e fotografias a?reas. Assim, foram utilizados filtros que melhor real?assem os lineamentos observados nas imagens, facilitando a sua discrimina??o, e destacassem ?reas com maior umidade no terreno, que pudessem refletir o ac?mulo de ?gua em subsuperf?cie, bem como coberturas sedimentares e aluvi?es que servem como zonas de recarga. O trabalho partiu de uma an?lise regional com as imagens orbitais, passando pela an?lise de fotografias a?reas, at? um estudo de detalhe, com estudo estrutural de afloramentos em campo. Esta ?ltima envolveu a an?lise de afloramentos pr?ximos a po?os pr?-existentes (em um raio de aproximadamente 10 a 100m), com produtividade distinta, inclusive secos. Ao n?vel de detalhamento requerido, n?o foi poss?vel realizar um trabalho estat?stico utilizando os dados (fichas) de po?os, pela falta das informa??es espec?ficas requeridas. Todo este acervo de conhecimentos gerados deve agora passar por uma fase de testes atrav?s de loca??es espec?ficas. O aumento no ?ndice de acertos, assim pretendido, pode ent?o conduzir a uma etapa posterior de consolida??o e divulga??o da metodologia, para empresas e ?rg?os envolvidos na prospec??o de ?gua subterr?nea em terrenos cristalinos

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Nesta dissertação, foram investigadas as propriedades magnéticas e magnetocalóricas nos compostos intermetálicos de terras-raras Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) usando abordagens teórica e experimental. Do ponto de vista teórico, a série Gd1-xDyxAl2 foi descrita através de um modelo para o hamiltoniano magnético, incluindo o efeito Zeeman, interação de troca e a anisotropia de campo elétrico cristalino. As entropias da rede e eletrônica foram consideradas nas aproximações de Debye e de gás de elétrons livres, respectivamente. A parte experimental inclui a preparação do material, sua caracterização e medidas das quantidades magnéticas e magnetocalóricas. Os resultados experimentais e os cálculos teóricos da variação adiabática da temperatura (ΔTad) e da variação isotérmica da entropia (ΔS T), sob variações de campo magnético ao longo da direção de fácil magnetização, estão de bom acordo. O efeito da aplicação do campo magnético ao longo de uma direção de difícil magnetização foi estudado e as componentes da magnetização em função da temperatura foram investigadas. Também foi observado que a temperatura de reorientação de spin, TR, diminui quando a intensidade do campo magnético aumenta. Além disso, as concentrações molares ótimas de um material híbrido formado pelos compostos Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) foram simuladas usando um método numérico de matriz proposto por Smaili e Chahine. O compósito apresenta um bom intervalo de temperatura para um refrigerador magnético de 60 até 170 K. / In this work, the magnetic and magnetocaloric properties were investigated in the rare earth intermetallic compounds Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) using both theoretical and experimental approaches. From theoretical point of view, the Gd1-xDyxAl2 series was described through a magnetic model Hamiltonian including the Zeeman effect, exchange interaction and the crystalline electrical field anisotropy. The crystalline and electronic entropies were considered in Debye and free electrons gas approximations, respectively. The experimental work included the material preparation, characterization and measurements of magnetic and magnetocaloric quantities. Good agreement between experimental data and calculations were obtained for the adiabatic temperature change (ΔTad) and the isothermal entropy change (ΔS T), upon applied magnetic field changes along the easy magnetic direction. The effect of magnetic field applied along non easy magnetic direction was studied and the temperature dependence of the magnetization components was full investigated. Also, we observed that the spin reorientation temperature, TR, decreases when the magnetic field intensity increases. Besides, the optimal molar concentrations of the hybrid material formed by compounds Gd1-xDyxAl2 (x = 0, 0.25, 0.50, 0.75 e 1.00) were simulated using numerical matrix method proposed by Smaïli and Chahine. The obtained composite presents a good temperature interval for magnetic refrigeration from 60 to 170 K.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O efeito magnetocalórico, base da refrigeração magnética, é caracterizado por duas quantidades: a variação isotérmica da entropia (ΔST) e a variação adiabática da temperatura (ΔTad) as quais podem ser obtidas sob variações na intensidade de um campo magnético aplicado. Em sistemas que apresentam anisotropia magnética, pode‐se definir o efeito magnetocalórico anisotrópico, o qual, por definição, é calculado através da variação na direção de aplicação de um campo magnético cuja intensidade se mantém fixa. Nos materiais de nosso interesse, o efeito magnetocalórico é estudado teoricamente partindo de um hamiltoniano modelo que leva em conta a rede magnética (que pode ser composta por diversas sub-redes magnéticas acopladas), rede cristalina e a dinâmica dos elétrons de condução. No hamiltoniano magnético são consideradas as interações de troca, Zeeman e campo cristalino (esta ultima responsável pela anisotropia magnética). Recentemente, estudamos o efeito magnetocalórico convencional e o efeito magnetocalórico anisotrópico nos compostos mononitretos com terras-raras, a saber: Ho(y)Er(1-y)N para as concentrações y= 0,1,0.5 e 0.75. Comparações entre nossos resultados teóricos e os dados experimentais para o EMC foram bastante satisfatórias [3,9]. Além disso, diversas predições teóricas como a existência de uma fase ferrimagnética no sistema Ho(y)Er(1-y)N (para a concentração y=0.5) e reorientações de spin nas sub-redes do Ho e Er foram feitas [25]. / The magnetocaloric effect, magnetic refrigeration base, is characterized by two quantities: the isothermal entropy change (ΔST) and the adiabatic temperature change (ΔTad) which can be obtained through variations in the intensity of a magnetic field applied. In systems which present magnetic anisotropy, one can define anisotropic magnetocaloric effect, which, by definition, is calculated through the variation the direction of application of a magnetic field whose intensity remains fixed. In the materials of our interest, the magnetocaloric effect is studied theoretically starting from a model Hamiltonian which takes into account the magnetic lattice (that can be composed of several magnetic sublattices coupled), crystalline lattice and the dynamics of the conduction electrons. In the magnetic hamiltonian are considered the exchange interactions, Zeeman and crystalline electrical field (this latter responsible for the magnetic anisotropy). Recently, we studied the conventional magnetocaloric effect and anisotropic magnetocaloric effect in mononitrides compounds with rare earths, namely: o(Y)Er(1-Y)N for concentrations y= 0,1,0.5 e 0.75 . Comparisons between our theoretical results and experimental data for EMC were quite satisfactory [26].Furthermore, several theoretical predictions how to the existence of a phase ferrimagnetic in the system Ho(y)Er(1-y)N (for concentration ) and spin reorientations in the sublattices of Ho and Er were made [25].

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho foram estudadas as propriedades estruturais e termomagnéticas dos pseudobinários Ho1-yGdyAl2, através de abordagens experimentais e teóricas. A parte experimental envolveu a preparação de cinco amostras, com as concentrações y = 0, 0,25, 0,5, 0,75 e 1, assim como medidas de magnetização, calor especifico e da variação adiabática da temperatura. Na parte teórica usamos um hamiltoniano modelo que leva em consideração a interação dos íons com o campo magnético aplicado, com o campo elétrico cristalino e a troca entre os íons magnéticos. A entropia da rede foi considerada na aproximação de Debye e a entropia eletrônica na aproximação do gás de elétrons livres. A influência das reorientações de spin, espontâneas e induzidas pelo campo magnético, na magnetização e no calor especifico foram investigadas sistematicamente tanto a partir de dados experimentais quanto teoricamente. Também obtemos resultados teóricos para a variação de entropia e variação adiabática da temperatura alterando a intensidade ou a direção do campo magnético. / In this work the structural and thermomagnetic properties of the pseudobinaries Ho1-yGdyAl2 have been studied by experimental and theoretical approaches. The experimental part included the preparation of five samples, with concentrations y = 0, 0,25, 0,5, 0,75 and 1, as well as magnetization, specific heat and adiabatic temperature change measurements. In the theoretical part we used a model Hamiltonian which includes the interaction of the ions with the applied magnetic field, the crystalline electrical field and exchange interactions with others magnetic ions. The lattice entropy has been considered within Debyes approach and the electronic entropy as that of a free electron gas. The spin reorientation influence, spontaneous and magnetic field induced ones, in magnetization and specific heat has been systematically investigated either from experimental data as well as theoretically. We have also obtained theoretical results for the isothermal entropy change and the adiabatic temperature change by modifying either the magnetic field strength or its direction.

Cette thèse est constituée de deux parties indépendantes, étudiant deux aspects de la théorie des (φ,Γ)-modules en caractéristique p. La première partie porte sur l'étude de la réduction modulo p des représentations cristallines irréductibles de dimension deux. Nous donnons, pour des poids k ≤ p², un calcul explicite de la réduction de V(k,a) pour a dans un disque fermé centré en zéro, généralisant ainsi des résultats déjà connus pour k ≤ 2p. En particulier, nous calculons le plus grand rayon possible pour ce disque, et montrons que dans certains cas, la réduction qui est constante à l'intérieur du disque change sur son bord. Dans la seconde partie, nous nous intéressons aux représentations d'un sous-groupe de Borel de GL[indice]2(Q[indice]p) sur un corps de caractéristique p, et en particulier à celles qui sont lisses, irréductibles et admettent un caractère central. Une méthode pour construire de telles représentations à partir de (φ,Γ)-modules irréductibles a été décrite par Colmez dans sa construction de la correspondance de Langlands p-adique. Après avoir donné un cadre un peu plus général dans lequel la construction de Colmez fonctionne encore, nous classifions les représentations irréductibles du Borel, prouvant que la construction précédente permet d'obtenir toutes les représentations de dimension infinie. Lorsque le corps des coefficients est fini, ou algébriquement clos, nous disposons d'une interprétation galoisienne des (φ,Γ)-modules irréductibles, et la classification précédente permet alors d'obtenir une correspondance entre ces représentations du Borel et des représentations galoisiennes modulaires. / This thesis is made of two independent parts, dealing with two different aspects of characteristic p (φ,Γ)-modules. In the first part we study the reduction modulo p of -2-dimensional irreducible crystalline representations. For weights k ≤ p2, we give an explicit description of the reduction V(k,a) for a belonging to a closed disk centered at zero, generalizing results already known for k ≤ 2p. We explicitely compute the biggest possible radius for this disk, and prove that in some cases, the reduction which is constant on the interior of the disk is different for a belonging to the border of the disk. In the second part, we study the smooth, irreducible representations of a Borel subgroup of GL[indice]2(Q[indice]p) over a field of characteristic p and admitting a central character. One way of constructing such representations from irreducible (φ,Γ)-modules was described by Colmez in his construction of the p-adic Langlands correspondence. After giving a more general framework for Colmez's construction, we classify the irreducible representations of the Borel subgroup, proving that the previous construction already gives all the infinite dimensional representations. When the coefficient field is finite, Fontaine's equivalence combined with the previous classification gives a correspondence between these representations of a Borel subgroup of GL[indice]2(Q[indice]p) and modular galois representations.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / A caracterizaÃÃo geomorfolÃgica dos maciÃos cristalinos costeiros do municÃpio de Caucaia, integrante da RegiÃo Metropolitana de Fortaleza, CearÃ, bem como a apropriaÃÃo das paisagens naturais por variados segmentos sociais, representa o principal objetivo desta pesquisa. Dividido em quatro partes, este trabalho discute, no primeiro momento, a base metodolÃgica que contempla a evoluÃÃo geomorfolÃgica do setor de pesquisa, baseada no PrincÃpio do Atualismo. A discussÃo acerca da apropriaÃÃo dos aspectos geomorfolÃgicos foi realizada atravÃs da anÃlise ambiental, na tentativa de compreender a complexa relaÃÃo sociedade-natureza e variada formas de uso e ocupaÃÃo. Ao longo da dissertaÃÃo, o meio fÃsico e suas interconexÃes à abordado integrando dados geolÃgicos, geomorfolÃgicos, climÃticos, hidrolÃgicos, pedolÃgicos e fitogeogrÃficos, caracterizando um quadro ambiental marcado, sobretudo, pelas precipitaÃÃes pluviais concentradas no primeiro semestre e dÃficit hÃdrico no perÃodo que se estende de agosto a dezembro. Os maciÃos cristalinos, com altitudes mÃximas da ordem de 600m, possuem importÃncia Ãmpar na organizaÃÃo hidrogrÃfica do municÃpio de Caucaia, pois constituem interflÃvios de todas as sub-bacias existentes no municÃpio. Os solos e cobertura vegetal associados possuem variaÃÃes verificadas diante da mudanÃa das altitudes e grau de declividade das vertentes. Esses elevados relevos, situados à vanguarda da planÃcie costeira, representam o resultado da aÃÃo, por dezenas de milhÃes de anos, de processos naturais como os tectÃnicos (soerguimento no CretÃceo e flexura marginal, ao longo do TerciÃrio, sobretudo), climÃticos (aÃÃo externa e mudanÃas nos padrÃes climÃticos) e eustÃticos (com mudanÃas de nÃvel de base). A evoluÃÃo dos relevos à contextualizada na divisÃo do super-continente Gondwana, impulsionada pelos processos que culminaram com a geraÃÃo do Oceano AtlÃntico e formaÃÃo da margem continental passiva transformante do Nordeste brasileiro. Os dados obtidos possibilitaram identificar as unidades geomorfolÃgicas como os maciÃos costeiros modelados em estruturas dobradas, apresentando feiÃÃes como hog-backs e combe. Trata-se da primeira indicaÃÃo, descriÃÃo e anÃlise de relevo em estrutura dobrada no Estado do CearÃ. No contato com a estrutura dobrada dissecada, ocorrem ainda pÃes-de-aÃÃcar, relevos mamelonizados e inselbergs. AlÃm da evoluÃÃo dos relevos elevados, discute-se a relaÃÃo entre esses relevos e as superfÃcies de piso adjacentes, no caso a SuperfÃcie de Aplainamento Sertaneja (DepressÃo Sertaneja) e Tabuleiros Costeiros. Para finalizar, apresentamos os relevos como recurso natural, que constituiu a base para organizaÃÃo da sociedade local. A apropriaÃÃo dos aspectos geomorfolÃgicos iniciou antes da colonizaÃÃo do CearÃ, pela presenÃa de comunidades indÃgenas. A transformaÃÃo das paisagens vem sendo progressiva, atravÃs de urbanizaÃÃo complexa, expondo formas diversas de uso e ocupaÃÃo, como Ãreas agrÃcolas, zona residencial destinada ao veraneio ou à moradia fixa, exploraÃÃo do potencial ecoturÃstico, religioso e mineraÃÃo. A mercantilizaÃÃo da natureza à apresentada, sobretudo, diante da atuaÃÃo do mercado imobiliÃrio sobre a Ãrea de pesquisa. A ocupaÃÃo crescente de todos esses espaÃos naturais hoje resulta em degradaÃÃo ambiental, e em escassez de natureza para os menos favorecidos. / The geomorphological characterization of the 600m high crystalline coastal massifs of Caucaia City, that integrates the Metropolitan Region of Fortaleza, CearÃ, and the appropriation of the nature by various social sectors, are the main objective of this research. Divided into four parts, this research first discusses the methodological basis which contemplates the geomorphologic evolution of the sector of research, based on the âUniformitarianism Principleâ and the discussions about the appropriation of nature, through the environmental analysis, in an attempt to understand the complex society-nature relationship and varied forms of use and occupation. Throughout the dissertation, the physical environment and their interconnections is discussed integrating geological, geomorphologic, climatic, hydrological, pedological and biogeographical data, featuring an environmental framework, mainly marked by rain precipitation, which is concentrated in the first semester of the year, with a water deficit in the period extending from August to December. The crystalline mountains type geomorphology has great importance in the organization of the drainage system, as it is the water divisor for all the hydrological basins in the area. The soils and associated vegetation show physiological changes with altitude and slope declivity. As a result of the action on ten ten of millions of years of natural processes such as tectonic (uplift of basement in the Cretaceous time and marginal flexure especially along the Tertiary), climate action (external action and changes in weather patterns) and eustatic (with changes in the erosion base level). The evolution of the relief is contextualized in the division of the super-continent Gondwana, driven by processes that culminated in the generation of the Atlantic Ocean and formation of passive transform continental margin of Brazilian Northeast. The data obtained allowed to identify the geomorphological units that have been developed in a folded geological structure, presenting features such hog-backs and comb. This is the first time that folded controlled morphology is indicated, described and analyzed in Cearà territory. Other features as sugar-loafs, multi-convex relief type and inselbergs also occur in the vicinities of the hog-backs. The relationship between these high slopes with the adjacent planation surface (Sertaneja) and the coastal plain are also discussed. Finally, we discuss the ânature x societyâ relationship in the area. The appropriation of the geomorphologic features began before the colonization of Cearà State by Portuguese, with the presence of indigenous communities. The gradual transformation of the landscape was made through a complex urbanization process, exhibiting various forms of use and occupation as agricultural areas, residential areas, for vacations and living, as well as ecological and religious tourism activities, besides mining economical activity. The mercantilization of nature is mainly linked to real estate market, highly increasing in the area. The development and appropriation of nature today results in environmental degradation and lack of nature to less favored population.