Cette thèse traite de certaines propriétés spectrales de deux classes spécifiques des opérateurs périodiques. Nous nous intéressons tout d’abord à un modèle périodique perturbée par un opérateur dépendant d’un petit paramètre semi-classique. Nous obtenons alors le comportement asymptotique de la fonction du comptage des valeurs propres dans les gaps spectrales avec une estimation optimale du reste. Le second modèle étudié dans cette thèse est un modèle elliptique périodique d’ordre deux perturbée par un opérateur dépendant d’une grande constante de couplage. Nous donnons également la description de la fonction de compactage des valeurs propres lorsque la constante de couplage tend vers l’infini. La dernière partie de cette thèse discute l’étude du spectre discret de l’opérateur de Schrödinger avec un potentiel très oscillent dépendant d’un petit paramètre semi-classique. / This Ph.D thesis deals with some spectral properties of two specific classes of two periodic operators. We are firstly interested in the model periodic perturbed by operator depending on a small semi-classical constant. We obtain an asymptotic behavior of the eigenvalue counting function in the spectral gaps with scharp remainder estimate. The second model studied in this thesis is a two-dimensional periodic elliptic second order opera-tor perturbed by operator depending on a large coupling constant. We also give the description of the counting function of eigenvalues when the coupling constant tends to infinity. The last part of this thesis highlights the study the spectrum of a Schrödinger operator perturbed by a fast oscillatingdecaying potential depending on a small parameter.

RESUME GENERAL<p><p>Contexte :une association entre exposition prolongée aux champs magnétiques (CM) d’extrêmement basses fréquences (ELF) et risque sanitaire a été établie pour la leucémie infantile (CM 50/60 Hz de l’électricité, RR = 2,0 pour ≥ 0,4 µT d‘intensité moyennée dans le temps) et est suggérée pour le décès par maladie d’Alzheimer (CM 50/60 Hz, CM 16,7 Hz des voies ferrées pour 21 µT d’intensité moyennée dans le temps) et pour certaines hémopathies chez l’adulte (CM 16,7 Hz). Ces associations restent inexpliquées à ce jour. Sur base d’observations animales (effets des CM ELF sur la sécrétion de mélatonine) d’une part, et de la sensibilité magnétique confirmée des cryptochromes (régulateurs des biorythmes) d’autre part, il a été suggéré que ces associations puissent être dues à une perturbation des biorythmes par les CM ELF. Selon les instances internationales, une intensité > 1 mT est requise pour l’existence d’effets biologiques. <p>Objectifs et méthode :sur base d’une revue exhaustive de la littérature et de modèles théoriques reconnus, le présent travail développe certains mécanismes possibles pour un effet perturbateur des biorythmes par les CM ELF. L’impact en santé publique de cette hypothèse est ensuite évalué. Enfin, des protocoles sont proposés pour sa mise à l’épreuve, tenant compte des mécanismes envisagés. <p>Résultats :la possibilité existe d’une interaction des oscillations ELF de l’intensité et/ou de l’orientation du CM (somme vectorielle du CM ELF et du CM terrestre ou CMT) avec les cryptochromes rétiniens. Chez l’animal magnétosensible (dont le rongeur), une perturbation des biorythmes pourrait être consécutive à un mécanisme non spécifique de perturbation sensorielle. Toute observation animale pourrait donc ne pas être extrapolable à l’Homme. Chez ce dernier, une perturbation des biorythmes pourrait être causée par les oscillations de l’intensité du CM (peut- être dès < 100 µT d’intensité de CM ELF). Une telle perturbation pourrait aussi être causée par les variations spatiales de l’intensité du CMT qui existent dans l’environnement résidentiel (proximité de structures métalliques). Par ailleurs, dans l’éventualité de l’existence, chez l’Homme également, d’une sensibilité directionnelle basée sur les cryptochromes rétiniens, les oscillations de l’orientation du CM pourraient alors aussi interférer avec ces cryptochromes (peut-être dès ≤ 10 µT). Dans l’hypothèse où une telle interférence affecte les biorythmes, seules pourraient alors être concernées les oscillations dont l’amplitude atteint plusieurs degrés d’angle. Un tel mécanisme ne pourrait donc s’appliquer à la relation entre CM ELF et leucémie infantile que dans l’éventualité où les intensités les plus élevées (+ 1 à 2 SD) de CM ELF y jouent un rôle. Au cas où l’hypothèse de la perturbation des biorythmes par les CM ELF se voyait confirmée, d’autres troubles de santé seraient alors concernés et d’autres sources de CM seraient en cause, tels les CM statiques d’intensité variable émis par les lignes de transport électrifié. Les paramètres d’exposition considérés devraient inclure l’orientation relative CM ELF/CMT, mais aussi l’intensité locale du CMT (facteur à la fois déterminant et confondant dans la présente hypothèse). L’expérimentation animale devrait investiguer l’expression des clock genes. L’expérimentation humaine devrait investiguer les biorythmes chez l’enfant. Et l’épidémiologie devrait investiguer l’incidence de troubles liés à une perturbation des biorythmes en relation avec l’exposition aux CM ELF ainsi qu’aux variations locales de l’intensité du CMT. <p>Conclusions :malgré les incertitudes persistantes quant aux fonctions précises des cryptochromes de la rétine humaine et quant à l’exactitude des modèles théoriques qui décrivent les interactions entre CM et cryptochromes, certains mécanismes paraissent possibles pour une interaction entre CM ELF et biorythmes. En l’absence persistante d’alternative valide pour l’explication de l’association entre CM ELF et leucémie infantile, l’hypothèse de la perturbation des biorythmes par ces CM paraît devoir être investiguée plus avant, mais en tenant compte des variations locales d’intensité du CMT. <p>Background: An association between prolonged exposure to extremely low frequency (ELF) magnetic fields (MF) and health risk has been established for childhood leukemia (50/60 Hz MF of electricity, RR = 2.0 for ≥ 0.4 µT of time-averaged intensity) and is suggested for death by Alzheimer's disease (50/60 Hz MF, 16.7 Hz MF of railways at 21 µT of time-averaged intensity) and for some hematologic malignancies in adults (16.7 Hz MF). These associations remain unexplained so far. Based on animal studies (effects of ELF MF on melatonin secretion) on the one hand, and on the confirmed magnetic sensitivity of cryptochromes (regulators of biorhythms) on the other hand, it has been suggested that these associations may be due to a disruption of biorhythms by ELF MF. From current data, however, biological effects seem only possible at > 1 mT of intensity.<p>Objectives and methods: on the basis of an exhaustive literature review and with use of recognized theoretical models, this paper develops some possible mechanisms for disruption of biorhythms by ELF MF. The public health impact of this hypothesis is then evaluated. Finally, protocols are proposed for the testing of it, with taking into account the proposed mechanisms.<p>Results: an interaction seems possible between ELF oscillations of the intensity and/or the orientation of the ambient MF (the vector sum of both the ELF MF and the geomagnetic field or GMF) with retinal cryptochromes. In magnetosensitive animals (including rodents), disruption of biorhythms may then be secondary to a non-specific mechanism of sensory disturbance. All animal observation could therefore not be extrapolated to humans. In the latter, on his turn, a disruption of biorhythms may be caused by the oscillations of the MF intensity (perhaps from <100 µT of ELF MF intensity). Such disruption could also be caused by spatial variations of the intensity of the GMF that exist in residential environment (near steel structures). Moreover, in case of the existence in humans (like in animals) of a directional sensitivity based on retinal cryptochromes, then the oscillations of the MF orientation also could interfere with these cryptochromes (perhaps from ≤ 10 µT). In the event that such interference affects biorhythms, only oscillations of several degrees of amplitude would then be concerned. As a consequence, such a mechanism could apply to the relation between ELF MF and childhood leukemia only in the event that the highest MF intensities (Mean + 1-2 SD) also play a role in that relation. In the event the hypothesis of disruption of biorhythms by ELF MF is confirmed, other health problems would be concerned and other kind of MF would be involved, such as the static MF of variable intensity that are emitted by the lines of electrified transport. The considered exposure parameters should include the relative orientation of ELF MF and GMF, but also the local intensity of GMF (both determining factor and confounder in this case). Animal experiments should investigate the expression of clock genes. Human experimentation should investigate biorhythms in children. And epidemiology should investigate the incidence of disorders related to disruption of biorhythms in relation to exposure to ELF MF as well as to local variations in the intensity of the GMF.<p>Conclusions: Despite the persisting uncertainties about the precise functions of retinal cryptochrome as well as about the accuracy of the theoretical models that describe the interactions between MF and cryptochromes, some mechanisms seem possible for an interaction between ELF MF and biorhythms. In the persisting absence of valid alternative explanation for the association between childhood leukemia and ELF MF, the hypothesis of biorhythm disturbance by ELF MF deserves further investigation, however with taking into account local intensity variations of the GMF.<p> / Doctorat en Sciences de la santé publique / info:eu-repo/semantics/nonPublished

In a synchronous machine, radial, tangential, and axial forces are generated. In this thesis, three different technologies to control them are proposed. The first one, involves the utilization of the radial forces that arise between the rotor and the stator. This is achieved by segmenting the rotor field winding into groups of poles and controlling their corresponding magnetization individually. This technology is particularly useful to achieve magnetic balance and to create controllable radial forces. The second technology, involves the control of the rotor field in order to influence the tangential forces that produce torque. This is achieved by inverting the rotor field winding polarity with respect to the stator field. With this technique, breaking and accelerating torques can be created. It is particularly useful to start a synchronous machine. Finally, the application of axial forces with a magnetic thrust bearing is discussed. The main benefits of this technology are higher efficiency and increased reliability. The work presented in this thesis was carried out within the Division of Electricity in the Department of Engineering Sciences at Uppsala University. It is based on original research supported by analytical calculations, computational simulations and extensive experimental work.

L’étude du paléomagnétisme des roches extraterrestres fournit des informations sur les champs magnétiques présents dans le système solaire il y a plusieurs milliards d’années. A travers ce travail de thèse nous avons exploré deux grands aspects : un champ de dynamo sur un corps différencié, la Lune, et des champs magnétique dans le système solaire primitif avec l’étude de chondrites (CM et les rumurutites). Notre étude des échantillons lunaires a permis de confirmer l’existence d’un champ de dynamo ancien de forte intensité (~50 µT) entre 3.8 et 3.3 Ga. En utilisant l’anisotropie de susceptibilité magnétique comme indicateur de paléohorizontale et en faisant l’hypothèse d’une géométrie de champ dipolaire, nous avons déterminé que l’axe de cette dynamo était centré sur l’axe de rotation actuel de la Lune. Les chondrites CM et les Rumurutites ont enregistré des champs magnétiques anciens quelques millions d’années après la formation du système solaire. L’aimantation rémanente des chondrites CM constitue probablement le plus ancien enregistrement paléomagnétique jamais mis en évidence. L’estimation de la paléointensité (2 µT) et les contraintes chronologiques dont nous disposons ne permettent pas de trancher entre un champ d’origine externe (solaire ou nébulaire) ou d’origine interne (dynamo). Cette dernière hypothèse laisse entrevoir la possibilité de la formation de corps partiellement différenciés dès les premiers millions d’année du système solaire. Dans les rumurutites, les conclusions sont similaires, avec une paléointensité également estimée à 2 µT. Cependant l’aimantation étant plus jeune, une origine interne est favorisée. / Paleomagnetic studies of extraterrestrials rocks provide information on magnetic fields that prevailed in the solar system several million years ago.Through this work we have explored two main aspects: a field dynamo on a differentiated body, the moon, and magnetic fields in the early solar system with the study of two chondrites classes; CM and rumurutites. Our study of lunar samples confirmed that an old dynamo field of high intensity (~50 µT) existed on the Moon between at least 3.8 and 3.3 Ga. Using the anisotropy of magnetic susceptibility as a proxy for paleohorizontale and assuming a dipole field geometry, we determined that the dynamo axis was centered on the actual rotation axis of the Moon.CM chondrites and Rumurutites recorded old magnetic fields acquired few million years after the formation of the solar system. The remanent magnetization of CM chondrites is probably the oldest paleomagnetic record never evidenced. The estimated paleointensity (2 µT) and time constraints not allowed to discriminate between a field of external (solar or nebular) or internal origin (dynamo). This latter hypothesis suggests that formation of partially differentiated body could occur during the first million years of the solar system history. In rumurutites, conclusions are similar, with a paleointensity also estimated at ~2µT. However, the magnetization is younger and an internal origin is favored.

O objetivo desse trabalho é desenvolver métodos numéricos para simular escoamentos de cristais líquidos nemáticos governados pelas equações dinâmicas de Ericksen-Leslie. São apresentados dois métodos numéricos para a simulação de escoamentos de cristais líquidos nemáticos. O primeiro método foi desenvolvido para simular escoamentos tridimensionais de cristais líquidos nemáticos sob efeito de forte campo magnético enquanto que o segundo método foi desenvolvido para a simulação de escoamentos bidimensionais. Utilizando a notação de Einstein, as equações dinâmicas de Ericksen-Leslie são apresentadas. Empregando variáveis primitivas e coordenadas cartesianas, as equações governantes para escoamentos de cristais líquidos nemáticos são derivadas e as formulações matemáticas para a obtenção dos métodos numéricos são apresentadas. As equações descrevendo os métodos numéricos são resolvidas por um método numérico baseado na metodologia GENSMAC3D para o caso tridimensional enquanto que o método bidimensional é baseado na metodologia GENSMAC (GENeralized-Simplified-Marker-And-Cell). Em ambos os métodos, a técnica de diferenças finitas em uma malha deslocada é utilizada. As equações que descrevem as técnicas numéricas desenvolvidas foram incorporadas aos ambientes de simulação Freeflow2D e Freeflow3D. As condições de contorno para cada tipo de contorno são descritas em detalhes. A solução analítica apresentada por Stewart para o escoamento entre duas placas paralelas é utilizada para a validação do método numérico tridimensional. Empregando as hipóteses de escoamento desenvolvido e que o ângulo de orientação do diretor é pequeno, uma solução analítica para o escoamento em um canal bidimensional é encontrada. O método numérico bidimensional é então validado utilizando a solução analítica obtida. Utilizando refinamento de malha, resultados de convergência dos métodos numéricos são apresentados. Os métodos numéricos desenvolvidos nesse trabalho são aplicados para a simulação dos seguintes problemas: escoamento de um cristal líquido nemático em um canal tridimensional; investigação numérica do escoamento em L-canais e escoamento através de uma contração 4:1 e de uma expansão planar 4:1 / The aim of this work is to develop numerical methods capable of simulating nematic liquid crystal flows described by the dynamic Ericksen-Leslie equations. Two numerical techniques have been proposed: a numerical method for solving three-dimensional flows of nematic liquid crystals under strong magnetic fields and a method for simulating flows of nematic liquid crystals in two-dimensional complex geometries. Both methods employ Cartesian coordinates using primitive variables of pressure and velocity. These techniques are based on the GENSMAC (GENeralized-Simplified-Marker-And-Cell) methodology and a detailed description of the equations involved is presented. The resulting governing equations are solved by the finite difference method on a staggered grid. The three-dimensional technique was applied to solve fully developed flow between two paralel plates for which an analytic solution exists. By using this analytic solution, validation and convergence results of the developed numerical technique were obtained. To validate the two-dimensional method developed herein, an analytic solution for steady state flow in a 2D-channel was found which was used to obtain validation and convergence results. The method was then applied to simulate the flow in L-shaped channels, flow through a 4:1 contraction and flow in a 4:1 expansion

O tópico abordado nesta tese é a perda de massa através de ventos coronais magnetizados em estrelas de baixa massa. Ventos estelares têm sido estudados extensivamente há vários anos, tendo inicialmente como foco o vento solar. Atualmente, sabe-se que o campo magnético é essencial na aceleração e aquecimento dos ventos coronais. Apesar do conhecimento detalhado que temos da estrutura magnética do Sol, pouco se sabe sobre a configuração do campo magnético em outras estrelas. Nesta tese, é investigada a estrutura do campo magnético nas coroas de estrelas do tipo solar na Seqüência Principal e de suas predecessoras na pré Seqüência Principal através de simulações numéricas magneto-hidrodinâmicas tri-dimensionais. Aqui, consideramos de forma auto-consistente a interação entre o vento e o campo magnético e vice-versa. Dessa forma, pela interação entre forças magnéticas e forças do vento, consegue-se determinar a configuração do campo magnético e a estrutura dos ventos coronais. Realizamos um estudo de ventos de estrelas do tipo solar e a dependência dos mesmos com o parâmetro beta do plasma (a razão entre as densidades de energia térmica e magnética). Este é o primeiro estudo a realizar tal análise resolvendo as equações tri-dimensionais da magneto-hidrodinâmica ideal. Em nossas simulações, adotamos um parâmetro de aquecimento descrito por gamma, que é responsável pela aceleração térmica do vento. Então, nós analisamos ventos com intensidades de campo magnético nos pólos no intervalo de B0 = 1 a 20 G e mostramos que a estrutura do vento apresenta características que são similares à do vento coronal do Sol. No estado estacionário, a topologia do campo magnético obtida é similar para todos os casos estudados, apresentando uma configuração do tipo helmet streamer, com zonas de linhas fechadas e abertas de campo magnético co-existindo. Intensidades mais altas de campo levam a ventos mais acelerados e mais quentes. O aumento na intensidade do campo gera também uma zona morta maior no vento, i.e., os loops fechados que previnem que a matéria escape da coroa em latitudes menores que ~45 graus se estendem a maiores distâncias da estrela. Além disso, mostramos também que a força de Lorentz gera naturalmente um vento que é dependente da latitude. Ao aumentar a densidade da coroa mantendo B0 = 20 G, mostramos que o sistema volta a apresentar ventos menos acelerados e mais frios. Para um valor fixo de gamma, mostramos que o parâmetro essencial na determinação do perfil de velocidade do vento é o parâmetro beta calculado na base da coroa. Dessa forma, acredita-se que haja um grupo de ventos magnetizados que apresenta a mesma velocidade terminal independentemente das densidades de energia térmica ou magnética, desde que o parâmetro beta seja o mesmo. No entanto, essa degenerescência pode ser removida ao se comparar outros parâmetros físicos do vento, tal como a taxa de perda de massa. Nós também analisamos a influência do gamma nos nossos resultados e mostramos que ele é importante na determinação da estrutura do vento. Além disso, investigamos ventos magnetizados de estrelas de baixa massa da pré Seqüência Principal. Em particular, analisamos sob quais circunstâncias tais estrelas apresentam estruturas magnéticas alongadas (e.g., helmet streamers, proeminências do tipo slingshot, etc). Focamos especialmente em estrelas do tipo T Tauri fracas, uma vez que o tênue disco de acreção, quando presente ao redor de tais estrelas, não deve causar forte influência na estrutura do vento estelar e nem na do campo magnético coronal. Nós mostramos que o parâmetro beta do plasma é um fator decisivo na configuração do campo magnético do vento estelar. Usando parâmetros iniciais adequados ao que se é observado para tais estrelas, nós mostramos que a configuração do campo magnético pode variar entre uma configuração semelhante à de um dipolo e uma configuração com linhas fortemente colimadas em torno do eixo polar e streamers fechados ao redor do equador (configuração de multi-componentes para o campo magnético). Mostramos que as estruturas alongadas do campo magnético somente estão presentes se o parâmetro beta do plasma na base da coroa é beta0 << 1. Usando nossos modelos magneto-hidrodinâmicos, auto-consistentes, tri-dimensionais, estimamos para ventos de estrelas da pré Seqüência Principal a escala temporal de migração planetária devido a forças de arraste exercidas pelo vento em um planeta tipo hot-Jupiter (i.e., um planeta gigante que orbita muito próximo da estrela). Nosso modelo sugere que os ventos estelares de coroas com multi-componentes de campo magnético não têm influências significativas na migração de hot-Jupiters. / The subject of this thesis is the mass loss of low-mass stars through magnetized coronal winds. Stellar winds have been a topic of extensive research in Astrophysics for a long time, and their first investigations focused on the solar wind. Nowadays, we know that the magnetic field plays a crucial role in the acceleration and heating of coronal winds. Despite of the knowledge of the fine structure of the solar magnetic field, much less information is known regarding the configuration of the magnetic field in other stars. In this thesis, we investigate the structure of the magnetic field in the coronae of solar-like stars and young stars by means of three-dimensional magnetohydrodynamical numerical simulations. We self-consistently take into consideration the interaction of the outflowing wind with the magnetic field and vice versa. Hence, from the interplay between magnetic forces and wind forces, we are able to determine the configuration of the magnetic field and the structure of the coronal winds. We investigate solar-like stellar winds and their dependence on the plasma-beta parameter (the ratio between thermal and magnetic energy densities). This is the first study to perform such analysis solving the fully ideal three-dimensional magnetohydrodynamics equations. We adopt in our simulations a heating parameter described by gamma, which is responsible for the thermal acceleration of the wind. We analyze winds with polar magnetic field intensities ranging from B0 = 1 to 20 G and we show that the wind structure presents characteristics that are similar to the solar coronal wind. The steady-state magnetic field topology for all cases is similar, presenting a configuration of helmet streamer-type, with zones of closed field lines and open field lines coexisting. Higher magnetic field intensities lead to faster and hotter winds. The increase of the field intensity generates a larger ``dead zone\'\' in the wind, i.e., the closed loops that inhibit matter to escape from latitudes lower than 45 degrees extend farther away from the star. The Lorentz force leads naturally to a latitude-dependent wind. We show that by increasing the density and maintaining B0 = 20 G, the system recovers to slower and cooler winds. For a fixed gamma, we show that the key parameter in determining the wind velocity profile is the beta-parameter at the coronal base. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-beta parameter is the same. This degeneracy, however, can be removed if we compare other physical parameters of the wind, such as the mass-loss rate. We also analyze the influence of gamma in our results and we show that it is also important in determining the wind structure. We further investigate magnetized stellar winds of low-mass pre-main-sequence stars. In particular we analyze under which circumstances these stars present elongated magnetic features (e.g., helmet streamers, slingshot prominences, etc). We focus on weak-lined T Tauri stars, as the presence of the tenuous accretion disk is not expected to have strong influence on the structure of the stellar wind neither on the coronal magnetic field. We show that the plasma-beta parameter is a decisive factor in defining the magnetic configuration of the stellar wind. Using initial parameters within the observed range for these stars, we show that the coronal magnetic field configuration can vary between a dipole-like configuration and a configuration with strong collimated polar lines and closed streamers at the equator (multicomponent configuration for the magnetic field). We show that elongated magnetic features will only be present if the plasma-beta parameter at the coronal base is beta0 << 1. Using our self-consistent three-dimensional magnetohydrodynamical model, we estimate for the stellar winds of pre-main-sequence stars the timescale of planet migration due to drag forces exerted by the stellar wind on a hot-Jupiter (i.e., on a giant planet that orbits very close to the star). Our model suggests that the stellar wind of these multicomponent coronae are not expected to have significant influence on the migration of hot-Jupiters.

In the present work, the magnetotransport measurement technique was developed and various materials, exhibiting resistances from 1 mOhm up to several tens of kOhm, were investigated in pulsed magnetic fields of up to 60 T. Phase diagrams of irreversibility and upper critical fields for pure and Zn-doped YBa2Cu3O_7-x high-temperature superconductors were measured. A high-field study of the electronic properties of the two semimetals LaBiPt and CeBiPt were presented. Magnetoresistance of La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 thin films were investigated.

No description available.

No description available.

No description available.