The topic of the master thesis are the numerical simulations of magnetoacoustic waves in the solar corona in relation to the reconnection of magnetic field. The first part is devoted to the basic description of the Sun, its atmosphere and the processes that occur in it. Other sections are devoted to the description of reconnection of magnetic field and the description of solar flares taking place in the solar corona in relation to the mechanisms of magnetic field reconnection. Then the attention is focused on plasma waves and oscilations. The last section aims to simulate the startup options of the reconnection of magnetic field by oscillations and waves in the Harris current-sheet. Numerical simulations are implemented in FLASH 3.3.

Cette thèse vise à étudier le phénomène dit de " cascades électromagnétiques cosmologiques ". Ces cascades sont typiquement générées dans le milieu intergalactique par l'absorption de rayons gamma sur les photons du fond optique / UV et par la production de paires électron / positron associés. Ces leptons eux-mêmes interagissent avec les photons du fond diffus cosmologique via diffusion inverse Compton pour produire de nouveaux rayons gamma qui eux même peuvent s'annihiler, générant à partir d'un unique photon primaire toute une gerbe de photons et de particules secondaires. D'un point de vue observationnel, le développement de cette cascade introduit trois effets : une déformation du spectre à haute énergie, un retard temporel dans l'arrivée des rayons gamma et une extension de la taille apparente de la source. Les cascades électromagnétiques cosmologiques ont commencé à être étudiées dans les années soixante. Mais ce n'est qu'à partir des années 2010 avec l'arrivée du satellite Fermi (entre autres) et des observations dans la bande au GeV et au TeV que la discipline a explosé. Le phénomène est particulièrement important. D'une part il altère le spectre observé des sources rendant difficile la compréhension de la physique de ces dernières. D'autre part les cascades se développant dans le milieu extragalactique, elles sont très sensibles à la composition de ce dernier (fond diffus de photons, champ magnétique). Or ce milieu étant très ténu, il est difficile à étudier. Les cascades deviennent alors une formidable sonde pour accéder à sa compréhension et pouvoir en comprendre l'origine qui remonte au commencement de l'Univers. Pourtant les cascades cosmologiques sont un phénomène complexe faisant intervenir des interactions difficiles à modéliser (sections efficaces complexes) et le transport de particules dans un Univers en expansion (cosmologie). Face à cette complexité les expressions analytiques sont vite limitées et le passage au numérique devient inévitable. Dans le cadre de cette thèse un code de simulation Monte Carlo a donc été développé visant à reproduire aussi précisément que possible le phénomène des cascades. Ce code a été testé et validé en le confrontant aux expressions analytiques. Grâce à ce code, le rôle des différents paramètres physiques impactant le développement de la cascade a été étudié de manière systématique. Cette étude a permis de mieux comprendre la physique du phénomène. En particulier, l'impact des propriétés du milieu extragalactique (fond diffus extragalactique, champ magnétique extragalactique) sur les observables a été mis en évidence. Finalement, une seconde étude a été menée pour mesurer la contribution des cascades au fond gamma extragalactique. Des travaux récents montrent qu'une grande partie de l'émission diffuse à très haute énergie provient de sources ponctuelles non résolues (blazars en particulier). Ces sources gamma (résolues et non résolues) doivent en principe initier des cascades qui peuvent contribuer au fond diffus. En partant d'une modélisation de l'émission des blazars à différents redshifts, l'absorption et la contribution des cascades ont alors été calculées à l'aide du code Monte Carlo. Les résultats montrent que la contribution des cascades au fond gamma extragalactique pourrait violer les limites Fermi mais l'excès doit encore être confirmé. / This thesis aims at studying "cosmological electromagnetic cascades". These cascades are initiated by the absorption of very high energy gamma-rays through gamma-gamma annihilation with optical / UV background photons of the intergalactic medium. In this interaction, electron/positron pairs are produced. The newly created leptons interact with photons of the Cosmological Microwave Background producing new gamma-rays through inverse Compton scattering which can also annihilate producing a cascade of secondary particles from a single primary photon. Observationally, the development of this cascade has three effects : the observed high energy spectrum is altered, observed photons arrive with a time delay with respect to primary photons and the source appears extended. Cosmological electromagnetic cascades start to being studied in the early sixties. But it is during the 2010's with the Fermi satellite and GeV to TeV observations that the field has really started to being explored. In the fast evolving backgound of gamma-ray astronomy, understanding the cascade physics has become a crucial stake. First the observed spectrum from a distant source is altered, which directly affects the modelling of high energy sources. Secondly, the cascades develop in the extragalactic medium and are very sensitive to its composition (background light, magnetic field). This medium is hard to study because it is extremely thin. Hence the cosmological cascades are a formidable probe to access its comprehension and its origin coming from the very beginning of our Universe. Yet the cosmological cascades are a complex phenomenon which involves complicated interactions (complex cross sections) and transport of particles in an expanding Universe. Analytical expressions are rapidly limited and numerical computations are required. In this thesis a Monte Carlo simulation code has been developed aiming at reproducing the cosmological cascades. This code has been tested and validated against analytical expressions. With the simulation code, a systematic study of the parameters impacting the development of the cascade has been led. This study allows a better understanding of the cascade physics. Especially, the impact of the intergalactic medium properties (extragalactic background light, extragalactic magnetic field) on the observables has been highlighted. Finally, a second study has been done to measure the contribution of cascades to the extragalactic gamma ray background. Recent works show that a great part of the diffuse emission at very high energy is explained by unresolved sources (blazars in particular). These gamma sources (resolved and unresolved) must in principle initiate cosmological cascades which can also contribute to the extragalactic gamma ray background. Starting from a modeling of the blazars at different redshifts, absorption and contribution of the cascades have been estimated with the simulation code. The results show that the contribution of the cascades might violate the Fermi limits but the excess must be confirmed.

Lorsqu'un champ magnétique est appliqué au cours de la solidification directionnelle, une convection dans la phase liquide peut être induite par l'effet thermoélectrique. En effet la présence d'un gradient de température le long du front de solidification peut provoquer la circulation du courant thermoélectrique, qui interagit avec le champ magnétique appliqué pour créer un écoulement (convection thermo électromagnétique-TEMC). Les conditions de transport de soluté et de l'énergie sont affectées par cette convection, donc il y a influence sur l'espacement des dendrites et la macro-ségrégation des composants de l'alliage. Dans ce travail, l'influence du champ magnétique sur la solidification directionnelle d'alliages métalliques est étudiée. Des travaux expérimentaux de la solidification directionnelle de Sn-Pb et Sn-Bi alliages sont réalisés. La solidification directionnelle dans la configuration Bridgman est effectuée avec ou sans champ magnétique appliqué. L'influence, sur la solidification, du champ magnétique et d'un courant électrique (AC et DC) appliqués est étudiée. Les mouvements du liquide provoquent de fortes macro-ségrégations ainsi qu'un modification des espacements interdendritiques. Les résultats expérimentaux sont interprétés à la lumière d'une modélisation heuristique. Le cas d'un champ magnétique tournant a été aussi étudié. Ainsi, la valeur de la rotation du champ est choisie pour ralentir assez brassage électromagnétique sans pour autant supprimer les effets de TEMC. À faible vitesse de tirage et faible vitesse de rotation faible champ une macro-ségrégation en forme de spirale a pu être obtenue. / If magnetic field is applied during directional solidification, liquid phase convection can be induced by means of thermoelectromagnetic effect. Temperature gradient at the solidification front can cause thermoelectric current circulation, which then interacts with field and creates convection (Thermoelectromagnetic convection-TEMC). Solute and energy transport conditions are affected by this convection, thus it influences dendrite spacing and macrosegregation of the alloys. In this work magnetic field influence on the directional solidification of metallic alloys is studied. Experimental work of directional solidification of Sn-Pb and Sn-Bi alloys is done. Alloys are directionally solidified in Bridgman setup without or with applied magnetic field. Influence on the structure by magnetic field and applied electric current (AC and DC) is studied in this work. Analytical and experimental results are compared and interpreted. Bridgman solidification under rotating transverse magnetic field is studied as well, field rotation value is chosen to be slow enough that electromagnetic stirring does not fully suppress effects of TEMC. At low pulling velocity and low field rotation velocity spiral shaped component macrosegregation can be achieved.

Les piles à combustible à membrane échangeuse de protons (PEMFC) constituent une alternative aux moteurs thermiques utilisés dans le cadre d’applications transport ou dans le cadre d’applications stationnaires. Cependant, une large commercialisation des PEMFC dépend des progrès qui peuvent être réalisés pour améliorer leur fiabilité et leur durabilité. La PEMFC est sujette à plusieurs types de dégradations complexes et non entièrement maitrisées qui varient en fonction des conditions de fonctionnement. Cependant, il est admis qu’il est souhaitable de faire fonctionner la PEMFC à distributions de courant uniformes car des distributions de courant hétérogènes entraînent une mauvaise utilisation des réactifs et des catalyseurs, une diminution des performances globales et une possible dégradation des matériaux constitutifs du coeur de la pile. De nouvelles stratégies de diagnostic doivent donc être proposées en s’appuyant sur les distributions de courant. Mes travaux de recherche consistent à développer un nouvel outil de diagnostic s’appuyant sur une mesure du champ électromagnétique externe (non invasive) rayonné par la pile PEMFC. Le champ magnétique possède l’intérêt d’être corrélé à la distribution locale du courant circulant à l’intérieur de la pile, et permet d’avoir des informations sur les performances locales. Cette distribution est liée aux conditions opératoires de la pile. Il est alors possible, à partir d’une signature magnétique de remonter à une information locale et à la cause des distributions de courant non uniformes. Des bases (vecteurs) qui contiennent les données des champs magnétiques issues des 30 capteurs disposés autour de la PAC sont construites à partir de distributions de courant spécifiques. Ces bases constituent un espace de représentation du comportement anormal de la PEMFC et permettront de d’élaborer des signatures caractérisant les fonctionnements indésirables. Ainsi, deux méthodes ont été développées pour permettre : (i) d’extraire des paramètres pertinents sur la répartition de la densité de courant traduisant les performances locales de la PAC, (ii) de classifier les différents modes de fonctionnements indésirables. La première méthode consiste à générer des résidus vectoriels en comparant le comportement réel du système (caractérisé par un vecteur mesure) avec les bases générées. Des variables qualitatives ont été élaborées pour classifier les modes de fonctionnement indésirables de la pile. La deuxième méthode consiste à extraire des paramètres à partir de la projection du vecteur mesure dans la direction des bases. La classification est réalisée dans des espaces 2D. Une validation des deux méthodes proposées a été effectuée à partir de mesures expérimentales sur une PEMFC de taille industrielle (stack GENEPAC de 40 cellules construit par le CEA et PSA). La pertinence des paramètres extraits a été vérifiée en s’appuyant sur des distributions de courant mesurées directement. Les modes de fonctionnement indésirables prédéfinis permettent de localiser les paramètres opératoires ayant conduit à l’évolution de la distribution de courant. Les outils ainsi réalisés sont très facilement transposables à d’autres piles PEMFC. / Proton exchange membrane fuel cell (PEMFC) is a potential alternative energy conversion device for stationary and automotive applications. Wide commercialization of PEMFC depends on progress that can be achieved to enhance its reliability and durability. The PEM fuel cell is subject to several types of complex and not fully mastered degradations which vary with operating conditions. It is desirable to operate the PEMFC at uniform current distribution because non uniform current distribution over the MEA could result in poor reactant and catalyst utilization, overall cell performance degradation as well as corrosion processes inside the PEM fuel cell. Therefore, new diagnostic strategies must be proposed, including choice of information gathered on the system and the fuel cell operation representation. My research is to develop a new diagnostic tool based on a measure of the external electromagnetic field (non-invasive) radiated by the fuel cell. The magnetic field has the advantage of being correlated to the local distribution of the current flowing inside the fuel cell (a physical indicator to obtain information on local performance of a fuel cell); it is linked to the local operating conditions: relative humidity level, temperature etc. It is then possible, from a magnetic signature to trace local information. Baselines (vectors) which contain the magnetic fields data generated by specific current distribution are built to characterize the magnetic field generated by the undesirable operation of the fuel cell. Baselines constitute a representation space of abnormal system behavior. Two methods have been developed to enable: (i) to extract the relevant parameters on the distribution of the current density resulting from PEM fuel cell stack local performance, (ii) to classify different types of undesirables operations. The first method is to generate vector residuals by comparing the actual behavior of the system (characterized by a measurement vector) with the baselines generated. Qualitative variables were created to classify the undesirable modes of PEM fuel cell stack operation. The second method is to extract parameters from the projection of the vector in the direction of measurement baselines. The classification is performed in 2D space. Validation from experimental measurements of the two proposed methods has been carried out on a commercial scale PEMFC (GENEPAC stack of 40 cells built by the CEA and PSA). The relevance of the extracted parameters was verified based on current density distributions measured directly. The undesirable predefined operating modes were used to locate the operating conditions parameters that led to the evolution of the current density distribution. The tools are made easily transferable to other PEMFC stack.

Es werden die Ergebnisse der Untersuchungen von Stromverteilungen an Solarzellen und Solarmaterial durch magnetische, elektrische und optische Messmethoden gezeigt. Die neue magnetfeldtopographische Messmethode CAIC wird hierbei erläutert und deren Stromverteilungen mit Ergebnissen der IR-Durchlichtmikroskopie, der LBIC- und der LIT-Methode verglichen und ausgewertet. Auf Basis der durchgeführten Untersuchungen und der Annahme des Vorhandenseins einer Korngrenzendekoration werden die Stromverlaufsmodelle einer elektrisch aktiven Korngrenze für eine Probe mit pn-Übergang sowie für eine Probe ohne pn-Übergang gezeigt. Anhand von CAIC-Messungen können die Position und die Orientierung leitfähiger und oberflächennaher SiC- und Si₃N₄-Ausscheidungen in mc-Silizium ohne pn-Übergang ermittelt werden. Hierfür wird ein Stromverlaufsmodell gezeigt. Weiterhin werden Zellmikrobrüche, Fehler in der Kontaktstruktur und Layoutunterschiede der Kontaktstruktur durch CAIC-Messungen an Solarzellen eindeutig nachgewiesen.

Les sources de rayonnement synchrotron sont en constante évolution afin de fournir aux scientifiques, un faisceau de rayons-X plus intense et plus cohérent. Ces évolutions requièrent une augmentation du champ fournit par les structures multipolaires utilisés dans la maille de l'accélérateur pour focaliser le faisceau d'électrons. Ceci conduit à un intérêt grandissant pour la technologie à aimants permanent car elle permettrait d'obtenir des structures multipolaires plus performantes et plus compactes. De plus cette solution est attractive car elle ne consomme pas d'énergie électrique contrairement à la technologie résistive conventionnelle. Néanmoins la réalisation des structures à aimant permanent est plus complexe lorsqu'une bonne qualité de champ est nécessaire à cause des erreurs d'assemblage de la structure.Une structure innovante d'un quadripôle à aimant permanent à été développé pour la future machine de l'ESRF. Cette structure est composée de pièces ferromagnétiques et d'aimants permanents. Malgré sa simplicité et sa compacité, celle-ci est très performante car elle fournit une composante multipolaire intense ainsi qu'une qualité de champ conforme aux spécifications rigoureuses. Un logiciel de calcul magnétostatique à permis d'optimiser la structure et un prototype à été construit et caractérisé par notre équipe. L'impact des erreurs d'assemblage sur la qualité du champ à été corrigé grâce à des pièces ferromagnétiques permettant de modifier localement le champ du prototype. Les résultats obtenus en accord avec les prévisions du modèle démontrent la faisabilité d'une telle structure pour les futures sources de lumière synchrotron. / Storage ring light sources are in constant improvement in order to increase the intensity and the coherence of the X-rays beams. These improvements require stronger multipoles magnet in the accelerator lattice. To reach such strength, the magnets apertures are significantly reduced. The reduction of magnet apertures makes the permanent magnet technology very attractive for some magnets provided that they are limited field tuning. Indeed, introduction of PMs structures may lead to better performances for a more compact design. Furthermore, with this technology running cost can be very low.A simple PM based high gradient quadrupole prototype has been built and magnetically characterized. This magnetic structure results from a compromise between performance and technological complexity. It includes simple soft iron parts and PMs blocks. A suitable field quality was obtained with an optimized pole shape. Mechanicals errors during assembly are corrected with shims to reach the required field quality.

Made available in DSpace on 2014-06-11T19:27:13Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-02-29Bitstream added on 2014-06-13T19:35:06Z : No. of bitstreams: 1 mesquitaneto_c_me_ilha.pdf: 1673109 bytes, checksum: 0f8131abf5fc45715c92abece81e6a7a (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Neste trabalho é apresentado uma proposta de absorvedor de vibrações ajustável tipo viga sanduíche utilizando fluido Magnetoreológico no centro. Para o desenvolvimento deste projeto foi realizada uma revisão sobre os vários tipos de absorvedores e algumas aplicações. Em seguida foi realizado um estudo sobre o comportamento do fluido magnetoreológico, mostrando como este material inteligente varia suas propriedades quando submetido a um campo magnético. O objetivo do estudo foi verificar as propriedades do sistema para realização de um futuro controle, que é realizado através da variação do campo magnético. Avaliou-se, também, a relação com a corrente elétrica, quais os parâmetros que o influenciam e como podemos produzir um campo magnético com a intensidade desejada. Para avaliar as características do sistema foi utilizado o modelo no programa Ansys, com o objetivo de se verificar o comportamento do sistema. Para encontrar as características reais do sistema foi utilizado o modelo na forma de espaço de estados modais, identificado através do método PEM, Método de Predição de Erros (do inglês Prediction Error Methods PEM). Os testes experimentais foram realizados para se adquirir conhecimento do comportamento dinâmico deste tipo de fluido e, verificar se há repetibilidade nas medidas / This work presents a proposal of a tunable vibrations absorber type sandwich beam, using the Magnetorheologic fluid in the intermediate layer. For the development of this study a revision of some types of absorber with some applications was carried out. After that, a study of the behavior of the magnetorheologic fluid was carried through, showing as this intelligent material tunable its properties when submitted to a magnetic field. The objective of this analysis was to verify the properties of the system for implementation of a future control, which is based on the variation of the magnetic field. It was realized an analysis of the relation of the electric current and the parameters that influence it, in order to produce a magnetic field with the desired intensity. The characteristics of the system were verified through a mathematical model obtained with the software Ansys. The real characteristics of the system were found through the identification method PEM, Prediction Error Methods, using modal space states formulation. Experimental tests were carried out in order to obtain know how of the dynamic behavior of this type of material

Submitted by Izabel Franco (izabel-franco@ufscar.br) on 2016-09-19T18:31:46Z No. of bitstreams: 1 TeseMGS.pdf: 2043252 bytes, checksum: a70365d13804598ad8c94cb75631f81d (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-20T14:01:52Z (GMT) No. of bitstreams: 1 TeseMGS.pdf: 2043252 bytes, checksum: a70365d13804598ad8c94cb75631f81d (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-20T14:01:58Z (GMT) No. of bitstreams: 1 TeseMGS.pdf: 2043252 bytes, checksum: a70365d13804598ad8c94cb75631f81d (MD5) / Made available in DSpace on 2016-09-20T14:02:06Z (GMT). No. of bitstreams: 1 TeseMGS.pdf: 2043252 bytes, checksum: a70365d13804598ad8c94cb75631f81d (MD5) Previous issue date: 2015-02-11 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Superconductivity one of the most intriguing physical phenomena has been extensively studied due to their potential for technological applications. Despite advances in research in physics there is still no complete theory to explain the phenomenon. We study the behavior of a superconducting system described by a multiband model. We consider the interband interaction, i.e., the interaction is between the fermions with dierent masses at dierent bands, under the inuence hybridization. Also, we study the intraband interaction with hybridization and external magnetic eld. Using the method of Green's functions of Zubarev, we calculate the superconducting order parameters with intra-and interbanda interaction at zero temperature, ie, T = 0, and for dierent temperatures of zero. Interband interaction obtained in the superconducting order parameter as a function of temperature and hybridization V. The phase diagram T versus V was also obtained. The superconducting properties, such as free energy, entropy and specic heat, were also determined in the intraband interaction. Initially, we calculate the superconducting properties in zero eld, ie, for H = 0, then we consider the presence of external magnetic eld H nonzero. obtained an implicit equation that relates the magnetic field, temperature and hybridization, which allowed us to obtain the phase diagram T versus T and T versus V . Our results show that both the hybridization V , as the external magnetic field H acts at the expense of superconductivity by suppressing it. We noticed also, and conrmed by other studies, that the intraband interaction, we have only second order transition, since the interaction interbanda, we obtained a superconducting tricritico point where we encounter a line first with a second order. Based on these results, we conclude that left a signicant contribution to research in superconductivity. / A supercondutividade, um dos fenômenos mais intrigantes da fisica, vem sendo extensivamente estudada, devido ao seu potencial para aplicações tecnológicas. Apesar do avanço nessa área de pesquisa, como a teoria BCS, teoria microscópica que explica os supercondutores convencionais, ainda não há uma teoria completa que explique os não convencionais. Neste trabalho estudamos o comportamento de um sistema supercondutor descrito por um modelo multibandas, pois é uma extensão da teoria BCS a qual descreve bem os supercondutores de baixas temperaturas. Consideramos a interação interbanda, isto é, a interação se da entre férmions que apresentam propriedades diferentes, tais como, massas distintas, ou mesma massa, mas comportamentos diferentes que se encontram em diferentes bandas, sob a influência da hibridização. Tambem, estudamos a interação intrabanda com hibridização e campo magnético externo. Usando o método das funções de Green de Zubarev, calculamos os parâmetros de ordem supercondutor para as interações intra e interbanda a temperatura nula, isto é, T = 0 (transições quânticas), e para temperaturas finitas (transições térmicas ou clássicas) afim de obtermos um estudo completo do sistema. Na interação interbanda obtivemos o parâmetro de ordem supercondutor em função da temperatura e da hibridização V . O diagrama de fase T versus V também foi obtido. As propriedades supercondutoras, tais como energia livre, entropia e calor especifico, também foram determinadas na interação intrabanda. Inicialmente, calculamos as propriedades Supercondutoras a campo nulo, isto é, para H = 0, em seguida, consideramos a presença do campo magnético externo H diferente de zero. Obtivemos uma equação implícita que relaciona o campo magnético, temperatura e hibridização, o que nos permitiu obter o diagrama de fase T versus H e T versus V . Nossos resultados mostram que tanto a hibridização V , quanto o campo magnético externo H atuam em detrimento da supercondutividade, suprimindo-a, concordando com outros trabalhos, apesar de não ser isso geral, uma vez que alguns trabalhos mostram que V contribui para a supercondutividade até um determinado valor e para outros valores, suprime-a. Percebemos também que tanto na interação interbanda, quanto na interação intrabanda temos transições de primeira e segunda ordem e um ponto tricritico, no qual temos o encontro de uma linha de primeira ordem com uma de segunda ordem.

Submitted by Vasti Diniz (vastijpa@hotmail.com) on 2017-09-11T13:09:04Z No. of bitstreams: 1 arquivototal.pdf: 1583885 bytes, checksum: 1b7650a627b47ca275b6bc5af085bd27 (MD5) / Made available in DSpace on 2017-09-11T13:09:04Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 1583885 bytes, checksum: 1b7650a627b47ca275b6bc5af085bd27 (MD5) Previous issue date: 2017-02-20 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this thesis, we investigate the e ect of the topology and the magnetic eld on the vacuum uctuations associated to bosonic and fermionic charged elds. Firstly, we analize the vacuum induced current densities for a bosonic eld in a (D+1)-dimensional cosmic string spacetime. For this analysis, we consider the presence of a magnetic eld along of the azimuthal and axial directions and that the z-axis is compacti ed to a circle of lenght L. This analysis is performed using the positive frequencies Wightman function, that is necessary to calculate the vacuum expectation values of the induced current densities. Our second investigation is related with fermionic elds. We investigate the expectation values of the fermionic condensate, h i, charge and current densities for a massive fermionic eld in thermal equilibrium with T temperature, with a nonzero chemical potential, . We consider the background of a (2+1)-dimensional conical spacetime and the presence of a magnetic eld in the cone apex. In this analysis, we consider two separeted cases. Firstly, we consider the case where j j 6 m and after that the case where j j > m. / Nesta tese, investigamos o efeito da topologia e de um campo magnético nas flu­tuações do vácuo associadas a campos bosônicos e fermiônicos carregados. Primeiramente, analisamos as densidades de correntes induzidas no vácuo para um campo bosônico em um espaço-tempo de uma corda cósmica com (D+1) dimensões. Para esta analise, consideramos a presença de um fluxo magnético ao longo das direções axial e azimutal e que o eixo z é com­pactificado em um circulo de comprimento L. Esta analise é realizada fazendo use da função de Wightman de frequências positivas, que é necessária para calcular os valores esperados no vácuo das densidades de correntes induzidas. Nossa segunda investigação é relacionada a campos fermiônicos. Investigamos os valores esperados do condensado fermiônico, (N), carga e densidades de corrente para u campo fermiônico massivo em equilíbrio térmico a uma temperatura T, e com um potencial químico não-nulo, µ. Consideramos um espaço-­tempo cônico em (2+1) dimensões na presença de um campo magnético no vértice do cone. Para esta analise, dois casos foram considerados separadamente. Primeiramente, considera­mos o caso em que |µ| < m, e ern seguida, o caso ern que 1p,1 > m.

Submitted by Vasti Diniz (vastijpa@hotmail.com) on 2017-09-19T13:02:13Z No. of bitstreams: 1 arquivototal.pdf: 1631315 bytes, checksum: 9ca68dbec4c6690247c6d2507322f997 (MD5) / Made available in DSpace on 2017-09-19T13:02:13Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 1631315 bytes, checksum: 9ca68dbec4c6690247c6d2507322f997 (MD5) Previous issue date: 2014-07-07 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this thesis, we investigate quantum properties associated with massive fields near a cosmic string within two distinct scenarios: a Minkowski flat background, and a maximally symmetric anti-de Sitter spacetime (AdS). Initially, we present a brief review of the formalism needed in our contribution. In Minkowski spacetime, we analyze the relativistic quantum dynamics of bosonic and fermionic charged particles in the presence of a magnetic field also considering the presence of scalar potentials. In order to develop this analysis, we assume that the magnetic field is parallel to the string axis, and the scalar potentials present a cylindrical symmetry with their center on the string. Two distinct configurations for the scalar potential are considered: (i) the potential proportional to the inverse of the polar distance, and (ii) the potential linearly proportional to this distance. The energy spectra are explicitly computed for different physical situations, and their dependence on the magnetic field strength, and scalar coupling constants are presented. In the context of a string in AdS, we investigate the quantum properties of a massive fermionic field. More specifically, we calculate the fermionic condensate (FC), (4RP), and the vacuum expectation value (VEV) of the energy-momentum tensor, (T"), induced by the presence of a cosmic string. In order to develop this analysis, we consider the MIT bag boundary condition on the boundary of AdS. The FC and the VEV of the energy-momentum tensor are decomposed into pure AdS and cosmic string induced parts. The string-induced part in the VEV of the energy-momentum tensor is diagonal, and the axial, and radial stresses are equal to the energy density. For points near the cosmic string, the effects of the curvature are sub-dominant and to leading order, the VEVs coincide with the corresponding VEVs for the cosmic string in the Minkowski bulk. At large proper distances from the string, the decay of the VEVs show a power-law dependence on the distance for both massless and massive fields, in contrast to the case of a Minkowski bulk where, for a massive field, the string-induced parts decay exponentially. / Nesta tese, investigamos aspectos quanticos associados a campos de materia na vizinhanca de uma corda cosmica em dois contextos distintos: a corda cosmica no espayo-tempo piano de Minkowski e a corda cosmica no espayo-tempo maximamente simetrico de anti-de Sitter (AdS). Inicialmente, fazemos uma breve revisao do formalismo utilizado nas nossas contribuicOes. No contexto da corda em Minkowski, analisamos a dinamica quantica relativistica de particulas carregadas, bosonicas e fermionicas, na pre­senca de um campo magnetico, considerando tambem a presenca de potenciais escalares. Para desenvol­vermos esta andlise, admitimos que o campo magnetico é uniforme e paralelo a corda e que os potenciais escalares apresentam simetria cilIndrica, cujos centros estao sobre a corda cosmica. Duas configuracOes distintas para o potencial escalar sao consideradas: (i) o potencial proporcional ao inverso da distancia polar e (ii) o potencial linearmente proporcional a esta distancia. Nesse sentido os espectros de ener­gia sao obtidos exatamentes em diferentes cendrios ffsicos e sua relayao com a intensidade do campo magnetico e com as constantes de acoplamento escalares é apresentada. No contexto da corda em AdS, analisamos o comportamento quantico de um campo fermi8nico massivo. Especificamente, investiga­mos o condensado fermionico, (4RP), e valor esperado no vacuo (VEV) do tensor energia-momento, (T"), induzidos pela presenca de uma corda cosmica. Para esta andlise, impusemos a condicao de contorno da sacola do MIT, na fronteira de AdS. 0 condensado fermionico e o VEV do tensor energia­momento sao decompostos, respectivamente, em duas contribuicOes: uma induzida pelo espayo de AdS e outra induzida pela presenca da corda. No que se refere ao VEV do tensor energia-momento, a parte induzida pela corda é diagonal e as press6es radial e axial sao iguais a densidade de energia. Para pontos proximos a corda, os efeitos da curvatura sao subdominantes e o VEV do tensor energia-momento coin­cide com o correspondente na geometria de Minkowski. ia para grandes distancias proprias, os VEVs decaem de acordo com uma lei de potencia, em contraste com o caso no espayo-tempo de Minkowski onde, para um campo massivo, o termo induzido pela corda decai exponencialmente.