Nous étudions plusieurs problémes reliés aux propriétés de transport dans les systèmes corrélés. La thèse contient 3 parties distinctes, chacune d'entre elles décrivant un aspect particulier. Nous avons obtenu dans chacun des cas des résultats qui permettent une meilleure compréhension du transport. Nous étudions l'effet de la dissipation et d'une perturbation extérieure dépendant du temps sur le diagramme de phases d'un systèmes à N corps à température nulle et à température finie. En présence de perturbation dépendant du temps, la dissipation joue un rôle important dans l'évolution vers un état stable indépendant du temps. Nous utilisons le formalisme de Keldysh dans l'approximation adiabatique qui permet d'étudier le diagramme de phases du système en fonction de parameter et de la température. Dans la 2ième partie, nous étudions un concept important pour la physique des systèmes métalliques à plusieurs bandes, le concept d'hybridation, et la façon dont l'hybridation affecte la supraconductivité du métal. De façon générale, une hybridation dépendante ou non du vecteur d'onde k a tendance à détruire la supraconductivité. Nous montrons dans ce chapitre qu'une hybridation antisymétrique a l'effet inverse et renforce la supraconductivité. Nous montrons que si l'hybridation est antisymétrique, la supraconductivité a des propriétés non-triviales. Nous proposons que dans un tel système, il puisse exister des fermions de Majorana, même en l'absence de couplage spin-orbite. Le dernier chapitre de la thèse porte sur les effets du couplage spin-orbite sur le transport dans les nanostructures magnétiques. Dans les nanostructures, le couplage spin-orbite joue un rôle important en raison de la brisure de symmétrie à la surface ou aux interfaces. En particulier, nous étudions l'effet de l'interaction Dzyaloshinskii-Moriya (DM) sur le transport de spin dans un système tri-couche. Nous montrons qu'il existe une interaction DM entre les moments des couches et les électrons de conduction, et l'influence de cette interaction sur le transport est étudiée dans un modèle simplifié ou chaque couche est représentée par un point. / We study some important problems related to the transport properties of many body systems. It is divided in three parts, each one focusing in a specific topic. We obtain relevant results that improve our understanding of these systems. We investigate the effect of dissipation and time-dependent external sources, in the phase diagram of a many body system at zero and finite temperature. In the presence of time-dependent perturbations, dissipation is essential for the system to attain a steady, time independent state. In order to treat this time dependent problem, we use a Keldysh approach within an adiabatic approximation that allows us to study the phase diagram of this system as a function of the parameters of the system and temperature. We also discuss the nature of the quantum phase transitions of the system. Next, we study an important concept in the physics of metallic multi-band systems, that of hybridization, and how it affects the superconducting properties of a material. A constant or symmetric $k$-dependent hybridization in general act in detriment of superconductivity. We show here that when hybridization between orbitals in different sites assumes an anti-symmetric character having odd-parity it {it{enhances}} superconductivity. The antisymmetric hybridization in a problem study in this thesis (present in Chapter 3) allow us to propose a new system where it is possible to investigate Majorana fermions, even in absence of spin-orbit interactions. In the last part of this thesis we study the effect of spin-orbit coupling (SOC) on transport properties in magnetic nanostructures. In this system SOC plays an important role, because surfaces (or interfaces) introduce symmetry breaking which is a source of spin-orbit interaction. We study the role of Dzyaloshinshkii-Moriya (DM) interaction on spin-transport in a 3 layer system. We show that there is a DM interaction between magnetics ions in the layers and spin of conduction electrons. We study the influence of this DM interaction on transport within a simple model where each layer is represented by a point.

Dans cette thèse nous étudions deux problèmes dans le domaine de la géométrie arithmétique, concernant respectivement les points de torsion des variétés abéliennes et le polylogarithme motivique sur les schémas abéliens. La conjecture de Manin-Mumford (démontrée par Raynaud en 1983) affirme que si A est une variété abélienne et X est une sous-variété de A ne contenant aucune translatée d'une sous-variété abélienne de A, alors X ne contient qu'un nombre fini de points de torsion de A. En 1996, Buium présenta une forme effective de la conjecture dans le cas des courbes. Dans cette thèse, nous montrons que l'argument de Buium peut être utilisé aussi en dimension supérieure pour prouver une version quantitative de la conjecture pour une classe de sous-variétés avec fibré cotangent ample étudiée par Debarre. Nous généralisons aussi à toute dimension un résultat sur la dispersion des relèvements p-divisibles non ramifiés obtenu par Raynaud dans le cas des courbes. En 2014, Kings and Roessler ont montré que la réalisation en cohomologie de Deligne analytique de la part de degré zéro du polylogarithme motivique sur les schémas abéliens peut être reliée aux formes de torsion analytique de Bismut-Koehler du fibré de Poincaré. Dans cette thèse, nous utilisons la théorie de l'intersection arithmétique dans la version de Burgos pour raffiner ce résultat dans le cas où la base du schéma abélien est propre. / In this thesis we approach two independent problems in the field of arithmetic geometry, one regarding the torsion points of abelian varieties and the other the motivic polylogarithm on abelian schemes. The Manin-Mumford conjecture (proved by Raynaud in 1983) states that if A is an abelian variety and X is a subvariety of A not containing any translate of an abelian subvariety of A, then X can only have a finite number of points that are of finite order in A. In 1996, Buium presented an effective form of the conjecture in the case of curves. In this thesis, we show that Buium's argument can be made applicable in higher dimensions to prove a quantitative version of the conjecture for a class of subvarieties with ample cotangent studied by Debarre. Our proof also generalizes to any dimension a result on the sparsity of p-divisible unramified liftings obtained by Raynaud in the case of curves. In 2014, Kings and Roessler showed that the realisation in analytic Deligne cohomology of the degree zero part of the motivic polylogarithm on abelian schemes can be described in terms of the Bismut-Koehler higher analytic torsion form of the Poincaré bundle. In this thesis, using the arithmetic intersection theory in the sense of Burgos, we give a refinement of Kings and Roessler's result in the case in which the base of the abelian scheme is proper.

In the present work the Roth s two-pole approximation (Phys. Rev. 184, 451 (1969)) has been used to investigate the role of d− p hybridization in the normal and superconducting states of an extended d− p Hubbard model. Superconductivity with singlet dx2−y2 -wave pairing is treated by following Beenen and Edwards formalism (Phys. Rev. B 52, 13636 (1995)). In the first part of this work, the effects of the hybridization on the superconductivity, in the hole-doped regime, have been studied treating Roth s band shift within two different approximations. In the first one, the band shift has been calculated in the limit U →¥ (U is the Coulomb interaction), with zero temperature and without consider the superconducting effects. These regards, are restrict to the band shift. In the other parts of the problem, U, the temperature and the superconducting effects have been considered finites. In the second approximation, the Coulomb interaction, the temperature and the superconductivity have been considered in the calculation of some relevant correlation functions present in the Roth s band shift. The obtained results show that the hybridization acts in the sense of to suppresses the superconductivity. Also, it has been verified that the first approximation overstimates the effects of the hybridization on the superconductivity. In the second part of these work, hoppings to second-nearest-neighbors have been included in the model with the purpose of reproduces adequately the asymetries (mainly those related with the Fermi surface, band structure and phase diagram) between the hole- and electron-doped systems. Particularly, it is shown that the crossover from hole-like to electronlike Fermi surface is deeply affected by the d − p hybridization in the hole doping case. It has been verified that the effect of the hybridization is most pronounced around the saddle-points, where the superconducting gap is maximum in the particular case of dx2−y2 -wave symmetry. As a consequence, the critical temperature Tc is directly affected by the hibridization. Moreover, the obtained results suggest that in the hole doped regime, the hybridization may act on the transport properties of the system due to the sign changes of the Hall coefficient when the crossover of the Fermi surface occurs. In the electron doped case, the crossover in the Fermi surface is not verified. Nevertheless, as the hybridization suppresses the density of states near the Fermi level, the superconductivity is affected. The topology of the Fermi surface in the hole and electron doping regime agree with recent experimental ARPES results for La2−xSrxCuO4 (hole doping) and Nd2−xCexCuO4 (electron doping). / Neste trabalho, foi usada a aproximação de dois-pólos proposta por L. Roth (Phys. Rev. 184, 451 (1969)), para investigar os efeitos da hibridização no estado normal e no estado supercondutor de um modelo de Hubbard d − p. Para tratar supercondutividade com simetria dx2−y2 , usou-se o procedimento de fatorização proposto por Beenen e Edwards (Phys. Rev. B 52, 13636 (1995)). Na primeira parte do trabalho, os efeitos da hibridização sobre a upercondutividade, foram investigados considerando-se duas aproximações diferentes para calcular o deslocamento de banda. O deslocamento de banda surge quando tratamos as equações de movimento das funções de Green através do método de L. Roth. Na primeira aproximação o deslocamento foi calculado para U →¥ (U ´e a interação coulombiana), temperatura igual a zero, e sem incluir os efeitos da supercondutividade. É importante destacar que essas considerações foram feitas apenas no deslocamento, as outras partes do problema foram tratadas considerando finitas, as quantidades citadas acima. Na segunda aproximação, o deslocamento de banda foi estudado incluíndo-se os efeitos de U, da temperatura e da supercondutividade. Nos dois casos observou-se que a hibridização atua no sentido de suprimir a supercondutividade. Verificou-se também que na primeira aproximação considerada no cálculo do deslocamento de banda, os efeitos da hibridização sobre a supercondutividade, são superestimados. Isso ocorre porque certas funções correlação presentes no deslocamento de banda desaparecem no limite U →¥. Na segunda parte deste trabalho, incluiu-se no modelo, um termo de salto ( hopping ) para os segundos vizinhos de um sítio i. Esse termo foi ncluído com o objetivo de reproduzir adequadamente as assimetrias entre o regime de dopagem por buracos e o regime de dopagem por elétrons. No caso particular de dopagem por buracos, observou-se que a mudança na natureza da superfície de Fermi de buraco para elétron, profundamente afetada pela hibridização. Além disso, verificou-se que o efeito da hibridização ´e mais intenso nas regiões dos pontos de sela, nas quais o gap supercondutor ´e máximo devido à simetria dx2−y2 . Com isso, a temperatura crítica de transiçãoo (Tc) do estado normal para o estado supercondutor, também é afetada pela hibridização. Os resultados sugerem também, que no caso de dopagem por buracos, a hibridização interfere no valor de dopagem em que ocorre a mudança no sinal do coeficiente Hall, portanto deve afetar também as propriedades de transportes do sistema. No caso de dopagem por elétrons, não foi verificado nenhuma mudança na natureza da superf´ıcie de Fermi. No entanto, como a hibridização aumenta a largura das bandas e dimimui a densidade de estados no nível de Fermi, neste caso a supercondutividade também é afetada. A topologia da superfície de Fermi nos regimes de dopagen por buracos e por elétrons concorda bem com resultados experimentais de ARPES obtidos recentemente para o composto La2−xSrxCuO4 (dopado por buracos) e para o composto Nd2−xCexCuO4 (dopado por elétrons).

Este trabalho consiste na análise de um método de reconstrução/tomografia de estado quântico em ressonância magnética nuclear utilizando pulsos de radiofreqüência não-seletivos, que possuem a propriedade de promover rotações globais do sistema de spins 7/2. Tal método foi aplicado para reconstruir estados relacionados à computação quântica. As operações lógicas e os estados iniciais envolvidos nas operações quânticas foram construídos através de pulsos modulados optimizados numericamente; o processo de optimização, em particular, não foi tratado nesse trabalho. Foram elaborados programas que simulam: a construção dos estados e portas lógicas utilizando os parâmetros dos pulsos modulados; a aplicação dos pulsos de tomografia e a geração dos dados necessários à reconstrução (amplitudes espectrais); construção de estados utilizando pulsos simples para testes das circunstâncias experimentais; o efeitos de possíveis problemas relacionados à amostra ou ao equipamento. Finalmente, foi elaborado um programa para reconstrução do estado a partir da leitura das amplitudes espectrais, que podem ser obtidas a partir dos programas relacionados no segundo item, ou experimentalmente. As implementações experimentais foram realizadas medindo sinais de RMN de núcleos de 133Cs, localizados em um cristal líquido, que, por possuírem spin 7/2, devido às interações Zeeman e quadrupolar elétrica, apresentam sete linhas espectrais distintas para transições entre níveis energéticos adjacentes; logo, é possível tratar esses núcleos como sistemas de 3 q-bits. Foram construídos estados pseudo-puros e aplicada uma das portas Toffoli. Além disso, uma discussão do algoritmo quântico de busca de Grover no contexto da Ressonância Magnética Nuclear é apresentada para uma futura implementação. / This work describes a quantum state tomography method in nuclear magnetic resonance using nonselective radiofrequency pulses that cause global rotations of spin 7/2 systems. This method was applied to tomograph states related to quantum computation. Numerically optimized modulated pulses allowed building the initial states and the logical operations involved in the quantum operations; particularly, the optimization process was not treated in this work. Several programs were constructed that simulate: o the construction of the quantum states and the logical operations by means of the modulated pulses parameters; o the application of the tomography pulses and the generation of the necessary data for tomography (spectral amplitudes); o the construction of the states using simple pulses for experimental condition tests; o the effects of possible problems related to the samples or equipments. Finally, a quantum state tomography program was elaborated to read the spectral amplitudes, which can be obtained from the programs related to the second item, or experimentally. The experimental implementations were performed measuring the NMR signals from spin 7/2 133Cs nuclei located in a liquid crystal under Zeeman and quadrupolar electric interactions. The NMR spectrum of these nuclei, under these interactions and located in an oriented sample, present 7 spectral lines for transitions between adjacent energetic levels; with this, it is possible to treat it like a 3 q-bits system. Pseudo-pure states were constructed and one Toffoli gate was applied. Furthermore, a discussion about the Grover\'s quantum search algorithm in the nuclear magnetic resonance context was presented for future implementation.

Orientador: Eduardo Miranda / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-16T08:19:56Z (GMT). No. of bitstreams: 1 Andrade_EricdeCastroe_D.pdf: 5537554 bytes, checksum: 1391d5fcc710b5e471f0814a4a6d484f (MD5) Previous issue date: 2010 / Resumo: Sistemas eletrônicos fortemente correlacionados desordenados possuem dois mecanismos básicos para a localização eletrônica e a subsequente destruição do estado metálico: o de Mott (causado pela interação elétron-elétron) e o de Anderson (causado pela desordem). Nesta tese, estudamos como estes mecanismos competem dentro da fase metálica e também como afetam o comportamento crítico do sistema, empregando uma generalização para o caso desordenado do cenário de Brinkman-Rice para a transição de Mott. Investigamos os efeitos de desordem fraca e moderada sobre a transição metal-isolante de Mott a T = 0 em duas dimensões. Para desordem sucientemente baixa, a transição mantém sua característica do tipo Mott, na qual temos os pesos de quasipartícula Zi indo a zero na transição e uma forte blindagem da desordem na região crítica. Em contraste com o comportamento encontrado para d = 8 , no nosso caso as flutuações espaciais dos pesos de quasipartícula são fortemente amplificadas próximo à transição de Mott de tal forma que eles adquirem uma distribuição do tipo lei de potência P (Z) ~ Z a-1 ,com a --> 0 na transição. Tal comportamento altera completamente as características desta transição com relação ao caso limpo, e é um indício robusto da emergência de uma fase de Griffiths eletrônica precedendo a transição metal-isolante de Mott, com uma fenomenologia surpreendentemente similar àquela do "ponto fixo de desordem infinita" encontrada em magnetos quânticos. Uma consequência imediata dessas novas características introduzidas pela desordem é que estados eletrônicos próximos à superfície de Fermi tornam-se mais homogêneos na região crítica, ao passo que estados com maiores energias têm o comportamento oposto: eles apresentam uma grande inomogeneidade precisamente nas vizinhanças da transição de Mott. Sugerimos que uma desordem efetiva dependente da interação é uma característica comum a todos os sistemas de Mott desordenados. Estudamos também como os efeitos bem conhecidos das oscilações de longo alcance de Friedel são afetados por fortes correlações eletrônicas. Primeiramente, mostramos que sua amplitude e alcance são consideravelmente suprimidos em líquidos de Fermi fortemente renormalizados. Posteriormente, investigamos o papel dos espalhamentos elásticos e inelásticos na presença dessas oscilações. Em geral, nossos resultados analíticos mostram que um papel proeminente das oscilações de Friedel é relegado a sistemas fracamente interagentes. Abordamos, por m, os efeitos das interações sobre o isolante de Anderson em uma dimensão. Construímos a função de escala ß (g) e mostramos que a escala de "crossover" g *, que marca a transição entre o regime ôhmico e o localizado da condutância, é renormalizada pelas interações. Como consequência, embora não haja a emergência de estados verdadeiramente estendidos, o regime ôhmico de g estende-se agora por uma região consideravelmente maior do espaço de parâmetros. / Abstract: Disordered strongly correlated electronic systems have two basic routes towards localization underlying the destruction of the metallic state: the Mott route (driven by electron-electron interaction) and the Anderson route (driven by disorder). In this thesis, we study how these two mechanisms compete in the metallic phase, and also how they change the critical behavior of the system, within a generalization to the disordered case of the Brinkman-Rice scenario for the Mott transition. We investigate the effects of weak to moderate disorder on the Mott metal-insulator transition at T = 0 in two dimensions. For sufficiently weak disorder, the transition retains the Mott character, as signaled by the vanishing of the local quasiparticle weights Zi and strong disorder screening at criticality. In contrast to the behavior in d = 8, here the local spatial fluctuations of quasiparticle parameters are strongly enhanced in the critical regime, with a distribution function P(Z) ~ Z a-1 and a --> 0 at the transition. This behavior indicates the robust emergence of an electronic Griffiths phase preceding the MIT, in a fashion surprisingly reminiscent of the " Infinite Randomness Fixed Point" scenario for disordered quantum magnets. As an immediate consequence of these new features introduced by disorder, we have that the electronic states close to the Fermi energy become more spatially homogeneous in the critical region, whereas the higher energy states show the opposite behavior: they display enhanced spatial inhomogeneity precisely in the close vicinity to the Mott transition. We suggest that such energy-resolved disorder screening is a generic property of disordered Mott systems. We also study how well-known effects of the long-ranged Friedel oscillations are affected by strong electronic correlations. We first show that their range and amplitude are signifficantly suppressed in strongly renormalized Fermi liquids. We then investigate the interplay of elastic and inelastic scattering in the presence of these oscillations. In the singular case of two-dimensional systems, we show how the anomalous ballistic scattering rate is conned to a very restricted temperature range even for moderate correlations. In general, our analytical results indicate that a prominent role of Friedel oscillations is relegated to weakly interacting systems. Finally, we discuss the effects of correlations on the Anderson insulator in one dimension. We construct the scaling function ß(g) and we show that the crossover scaling g*, which marks the transition between the ohmic and the localized regimes of the conductance, is renormalized by the interactions. As a consequence, we show that, although truly extend states do not emerge, the ohmic regime covers now a considerably larger region in the parameter space. / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Cette thèse porte principalement sur l'étude de modèles de fermions en interactions contenant un couplage spin-orbite. Ces modèles (i) peuvent décrire une classe de matériaux composés d'iridates sur le réseau en nid d'abeille ou (ii) pourraient être réalisés artificiellement dans des systèmes d’atomes froids. Nous avons étudié, dans un premier temps, le système à demi-remplissage avec l'interaction de Hubbard et un couplage spin-orbite anisotrope. Nous avons trouvé plusieurs phases: la phase isolant topologique pour de faibles corrélations, et deux phases avec des ordres magnétiques frustrés, l'ordre de Néel et l'ordre spiral, dans la limite de très fortes corrélations. La transition entre les régimes de faibles et de fortes corrélations est une transition de Mott dans laquelle les excitations électroniques se fractionnent en excitations de charge et de spin. Les charges sont localisées par l'interaction. Le secteur de spin présente de fortes fluctuations qui sont modélisées par un gaz d’instantons. Nous avons ensuite exploré la physique d'un système régi au demi-remplissage par le modèle de Kitaev-Heisenberg, qui présente une phase magnétique de type zig-zag. En dopant le système, autour du quart remplissage, la structure de bande présente de nouveaux centres de symétrie en plus de la symétrie d'inversion. Le couplage de spin de Kitaev-Heisenberg favorise alors la formation de paires de Cooper dans un état triplet autour de ces centres de symétrie. La condensation de ces paires de Cooper autour de ces vecteurs d'onde non triviaux se manifeste par une modulation spatiale du paramètre d'ordre supraconducteur, comme dans la supraconductivité de Fulde–Ferrell–Larkin–Ovchinnikov (FFLO). La dernière partie de la thèse propose et étudie une implémentation des phases topologiques dite de Haldane et de Kane-Mele dans un système avec deux espèces de fermions sur le réseau en nid d'abeille, stabilisée grâce à l’interaction RKKY médiée par l’espèce rapide et qui agit sur l’espèce lente. / This thesis is dedicated largely to the study of theoretical models describing interacting fermions with a spin-orbit coupling. These models (i) can describe a class of 2D iridate materials on the honeycomb lattice or (ii) could be realized artificially in ultra-cold gases in optical lattices. We have studied, in the first part, the half-filled honeycomb lattice model with on-site Hubbard interaction and anisotropic spin-orbit coupling. We find several different phases: the topological insulator phase at weak coupling, and two frustrated magnetic phases, the Néel order and spiral order, in the limit of strong correlations. The transition between the weak and strong correlation regimes is a Mott transition, through which electrons are fractionalized into spins and charges. Charges are localized by the interactions. The spin sector exhibits strong fluctuations which are modeled by an instanton gas. Then, we have explored a system described by the Kitaev-Heisenberg spin Hamiltonian at half-filling, which exhibits a zig-zag magnetic order. While doping the system around the quarter filling, the band structure presents novel symmetry centers apart from the inversion symmetry point. The Kitaev-Heisenberg coupling favors the formation of triplet Cooper pairs around these new symmetry centers. The condensation of these pairs around these non-trivial wave vectors is manifested by the spatial modulation of the superconducting order parameter, by analogy to the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconductivity. The last part of the thesis is dedicated to an implementation of the Haldane and Kane-Mele topological phases in a system composed of two fermionic species on the honeycomb lattice. The driving mechanism is the RKKY interaction induced by the fast fermion species on the slower one.

Wideband, wide-scan antenna arrays are a promising candidate for the future wireless networks and as well as an essential part of experimental radio astrophysics. Understanding the underline physics of the element performance in the array environment is paramount to develop and improve the performance of array systems. The focus of this thesis is to develop novel wideband antenna array technologies and develop new theoretical insights of the fundamental limits of antenna arrays. The developed methodologies have also been extended to include a radio astrophysics application for the global 21cm experiment.   Investigating the fundamental antenna array limits and extracting general performance measures can provide a priori estimates for any application of arrays. In this thesis, a general measure for antenna arrays, the array figure of merit is proposed. This measure couples bandwidth, height from the ground plane and reflection coefficient in a bounded quantity. An extension of the array figure of merit that is able to provide matching, bandwidth and directivity/gain limits is also introduced. The soft Vivaldi array is introduced as a novel wideband, wide-scan angle array technology. Periodic structure loading has been utilized to improve the array's performance and mold the electromagnetic wave behavior to our benefit. The soft condition has been utilized in the same manner as the conventional soft-horn antenna at the Vivaldi element. An integrated matching layer in the form of periodic strip loading is introduced. A single polarized soft Vivaldi array prototype has been developed fabricated and measured. The developed finite array has been loaded with a soft condition in the periphery to mitigate edge effects. The results indicated improved cross-polarization and side-lobe levels. A new class of wideband antenna arrays, the Strongly Coupled Asymmetric Dipole Array (SCADA) was also proposed in this thesis. Exploiting asymmetry in the array element introduces an additional degree of freedom that improves bandwidth and scanning performance. A novel methodology for terminating finite arrays is also proposed. The theory and an experimental antenna array is presented with good agreement between measured and simulated results. An effort to integrate a vertical wide angle matching layer was also addressed and a prototype array with this concept is presented. In the last part of this thesis, a methodology for the detection of the global cosmological 21cm signal from the Epoch of Reionization (EoR) is developed. The main sources of errors in this experiment, the foregrounds and the antenna chromaticity are evaluated. A new algorithmic methodology for extracting the global EoR signal is proposed. The method is based on piecewise polynomial fitting and has successfully been applied and evaluated. An antenna array that is based on the methodologies described in this thesis has been developed and evaluated with the proposed algorithm. / Bredbandiga gruppantenner med stor utstyrningsvinkel är en av de lovande kandidaterna för nästa generations trådlösa kommunikationsnätverk samt en väsentlig del av experimentell radioastrofysik. Att förstå de bakomliggande fysikaliska principerna hos gruppantennens element är avgörande för att kunna utveckla och förbättra prestandan hos ett gruppantennsystem. Denna avhandling är fokuserad på att utveckla nya bredbandstekniker samt nya teoretiska insikter om de grundläggande gränserna för gruppantenner. De här utvecklade metoderna har förutom kommunikationstillämpningar också tillämpats på en radioastrofysik tillämpning i det globala 21cm experimentet. Att undersöka de fundamentala gränserna för gruppantenner och att utröna allmängiltiga mått på deras prestandaegenskaper kan möjliggöra a priori uppskattningar om gruppantenns tillämpbarhet för dess planerade användning. I den här avhandlingen föreslås ett allmänt kvalitetsmått på gruppantenner: gruppantennkvaliten. Detta mått kopplar samman främst bandbredd, reflektionskoefficienten med antennens tjocklek över ett jordplanet. En utvidgning av begreppet gruppantennkvaliten, presenters också i avhandlingen det kopplar samman bandbredd, matchning med antennens direktivitet/förstärkningsfaktor. En Vivaldi-gruppantenn med mjuka ytor introduceras här som en ny sorts bredbandig gruppantenn med stor utstyrningsvinkel. I antennen har en periodisk belastning inkluderats för att förbättra dess egenskaper, och för att forma antennens elektromagnetiska utstrålning till vår fördel. Den mjuka ytan på elementet har används på ett liknande sätt som det välkända korrigerade Vivaldihornets design, och har integrerats direkt i elementets design. Den här utvecklade ändliga gruppantennen har också en mjuk yta på dess yttre delar för att minska kanteffekternas påverkan av antennprestandan. Resultaten indikerade både förbättrad korspolarisations och lägre sidlobsnivåer hos antennen. En ny klass av bredbandiga gruppantenner har utvecklas i denna avhandling, den kallas en Starkt Kopplad Asymmetrisk Dipol-gruppAntennen - SCADA. Genom att utnyttja geometrisk asymmetri i antennelementet introduceras ytterligare en frihetsgrad som möjliggör förbättrad bandbredd och utstyrning. Vidare presenteras här en ny metod för impedansterminering av ändliga gruppantenner. Både SCADA-teorin samt dess verifiering i forma av en experimentell gruppantenn presenteras här. Teori, simulering och experiment visar god överenskommelse, vilket validerar idéerna. En prototyp av ett matchande skikt som stöder stor utstyrbarhet har integrerats med gruppantennprototypen och presenteras i avhandlingen. I den sista delen av avhandling utvecklas också en metod för detektering av den globala kosmologiska 21 cm-signalen från universums rejoniseringsepok - EoR. Huvudkällorna för mätfel i detta experiment utvärderas, de är antennens kromaticiteten och förgrundsstrålningen. En ny algoritmbaserad metod för att extrahera den globala EoR-signalen föreslås. Metoden är baserad på anpassning med multipla polynom och har med framgång tillämpats och utvärderats. En gruppantenn som baseras på de metoder som beskrivs i avhandling har också föreslagits och dess prestanda har utvärderats med den föreslagna metoden. / <p>QC 20171121</p>

Les systèmes multiferroïques magnéto-électriques sont des matériaux multifonctionnels très importants du point de vue des applications dans le domaine de l’électronique ou de la spintronique puisqu’ils présentent simultanément des ordres électriques et magnétiques généralement couplés. Ils peuvent donc répondre à la fois à l’application d’un champ magnétique et d’un champ électrique. L’une des familles de multiferroïques magnéto-électriques les plus étudiées est la série RMn₂O₅ où R est une terre rare. Ma thèse porte sur le rôle de la terre rare sur le caractère multiferroïque et sur les propriétés magnétiques de ces composés. De nombreux travaux ont déjà été publiés mais très peu concernent les composés SmMn₂O₅ et GdMn₂O₅, difficiles à étudier aux neutrons. Pourtant ce sont des composés clés, à la frontière entre des composés à terres rares légères qui ne sont pas multiferroïques et ceux à terres rares lourdes qui le sont. Je me suis donc intéressé à ces composés. Grâce à une étude théorique basée sur une analyse de symétrie et des calculs numériques ab initio tenant compte du fort couplage spin-orbite, nous avons pu prévoir un modèle pour l’ordre magnétique stabilisé dans Sm et Gd. Nous avons en parallèle étudié expérimentalement les structures magnétiques pour ces deux composés par le biais d’une analyse des donnés de diffraction de neutrons sur poudre utilisant des matériaux isotopes de Sm et Gd. Ma thèse a permis d’une part de valider le mécanisme d’échange striction comme origine du couplage magnéto-électrique dans cette série importante de multiferroïques. Elle a permis d’autre part de mettre en évidence l’existence d’une interaction d’échange supplémentaire dans GdMn₂O₅, à l’origine de la forte polarisation électrique dans ce membre de la série. Ces résultats amènent plus de clarté à la compréhension de la multiferroïcité dans ces systèmes. / The magneto-electric multiferroic systems are multifunctional materials very important for applications in the field of electronics or spintronics since they present simultaneously electrical and magnetic orders, which are generally coupled. They can thus respond to both the application of a magnetic field and an electric field. One of the most studied magneto-electric multiferroic families is the RMn₂O₅ series where R is a rare earth. My thesis deals with the role of rare earth on the multiferroic and the magnetic properties of these compounds. Numerous works have already been published, but very few concern the compounds SmMn₂O₅ and GdMn₂O₅, difficult to study with neutrons. Yet these are key compounds, on the border between light rare earth compounds that are not multiferroic and heavy rare earths that are. This explains my interest for these compounds. Using a theoretical study based on a symmetry analysis and numerical calculations ab initio taking into account the strong spin-orbit coupling, we were able to predict a model for the stabilized magnetic order in Sm and Gd. We have experimentally studied the magnetic structures for these two compounds by means of an analysis of the powder neutron diffraction data using Sm and Gd isotopes. On one hand, my thesis allowed to validate the mechanism of exchange striction as origin of the magnetoelectric coupling in this important series of multiferroics. On the other hand, it has made it possible to demonstrate the existence of an additional exchange interaction in GdMn₂O₅, at the origin of the strong electrical polarization in this member of the series. These results provide greater clarity to the understanding of multiferroicity in these systems.

Les matériaux corrélés offrent une richesse de nouveaux phénomènes, dont beaucoup ne sont pas encore - ou seulement partiellement - compris. Au centre de cette thèse sont des modèles multi-orbitalaires que j'etudie à travers une palette de méthodes, dont la théorie du champ moyen dynamique. Dans le modèle de Hubbard multi-orbitalaire proche de la transition de Mott, je mets en évidence un régime de mauvais métal induit par le couplage de Hund. Les propriétés de la transition de Mott dans ce système sont analysées. Dans un deuxèime temps, je traite un modèle élargi pour inclure des degrés de liberté des ligands dans les oxydes. Plus spécifiquement, cette thèse étudie les effets des interactions inter-couches entre orbitales corrélés d'un atome de métal de transition et les orbitales p des ligands. Une partie du travail est dédiée au développement de nouvelles méthodes dont une approche de rotateurs esclaves à ce problème. Le dernier chapitre concerne le domaine de la spintronique moléculaire, où j'étudie la physique du "spin-state switching" en fonction de l'hybridation d'un ion de métal de transition avec ses ligands dans les molecules organométalliques du type porphyrine de nickel. / The physics of correlated materials offers a wealth of new phenomena, many of which are not yet - or only partially - understood. In this thesis, we focus on multi-orbital models, which we study using various methods, including dynamical mean-field theory. We show that in the multi-orbital Hubbard model close to the Mott transition, Hund's coupling gives rise to a bad metal regime the properties of which we analyze. Furthermore, we consider a more general class of models that include oxygen ligands. More specifically, we study the effect of inter-shell interactions between correlated metal- and ligand p-orbitals. In this context, we develop and test a new slave-rotor approach to treat such interactions in an effective manner. The final chapter constitutes an excursion to the field of molecular spintronics, where we study the physics of the hybridization-induced spin-state switching in organometallic nickel porphyrin molecules.

In vorliegendem Artikel wird eine Verallgemeinerung eines Approximationsteorems von Chidume /6/ für streng pseudokontraktive Operatoren in Lp beziehungsweise lp-Räumen (mit P =2) angegeben. Es wird ein Verfahren betrachtet, welches von MANN /14/ für reelle Funktionen eingeführt würde. / This article gives a generalization of an approximation theorem by Chidume / 6 / for strictly pseudocontractive operators in Lp or lp spaces (with P = 2). We consider a method, which MANN / 14 / had introduced for real functions.