Cette thèse propose une stratégie de calcul pour étudier les oxydes des métaux de transition de la première série à l'aide de la théorie de la fonctionnelle de la densité. Elle consiste en la validation d'une méthode de calcul sur un système donné en reproduisant au mieux les observations expérimentales, puis son utilisation pour compléter nos connaissances des propriétés spectroscopiques et de la réactivité de ces systèmes. A partir de l'étude en matrice de gaz rare des produits de la réaction de Ni, Ni2+O2 par spectroscopie d'absorption, nous avons rassemblé un ensemble de données expérimentales pertinentes sur les oxydes NiO2 et Ni2O2. Pour le premier, dont l'état fondamental du complexe eta-2 faisait l'objet de publications contradictoires, nous avons fourni une description générale de la réactivité en accord avec celle de l'expérience. Nous avons notamment mis en évidence les deux chemins réactionnels possibles, thermique (à partir des réactifs dans leurs états fondamentaux) et photochimique, de la formation du composé final, le produit de l'insertion ONiO. Le cas de Ni2O2 nous a poussés à développer une méthode particulière. Ce composé est caractérisé par un couplage antiferromagnétique avec de la densité de spin non nulle portée par les deux centres métalliques. Pour traiter ce cas avec une méthode monoréférence, nous avons conçu une procédure dans laquelle un calcul "Broken Symmetry", suivi de l'analyse de la fonction d'onde contaminée ainsi obtenue et de la valeur moyenne , ont permis d'ajuster la fraction d'échange HF dans la fonctionnelle hybride utilisée pour décrire des observables expérimentaux. Cette procédure a été employée pour traiter les dioxydes de type M2O2 (avec M = Cr, Mn, Fe, Co, Ni, Cu) et observer l'évolution de leurs propriétés en fonction du centre métallique. L'état singulet antiferromagnétique a été trouvé comme état fondamental pour tous ces oxydes, excepté dans le cas de Cu2O2. L'interaction antiferromagnétique entre les deux atomes de cuivre n'est pas suffisamment élevée pour favoriser la structure rhombique antiferromagnétique que l'on observe pour les autres métaux. Nous avons montré que la stabilité de ces oxydes est essentiellement due à l'interaction de superéchange entre les centres métalliques et le ligand diamagnétique (O2−). En accord avec ce schéma, nous avons établi, par une étude topologique des fonctions locales rho(r) et ELF, que les dioxydes bimétalliques doivent être considérés comme (M2+)2(O2−)2.

Thesis (Ph. D.)--University of Oregon, 2001. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 124-127). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (p. 86-93).

Made available in DSpace on 2015-12-10T14:24:26Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-07-20. Added 1 bitstream(s) on 2015-12-10T14:30:36Z : No. of bitstreams: 1 000852052.pdf: 587500 bytes, checksum: 14bfe169c56bff4d399427656df6125b (MD5) / O espalhamento relativístico de bósons de spin 0 e de férmions de spin 1/2 por potenciais coulombianos esfericamente simétricos 'e analisado em detalhes com uma mistura arbitrária de acoplamentos escalar e vetorial. Mostra-se que a série de ondas parciais, seja para bósons ou férmions, se reduz à amplitude de espalhamento de Rutherford exata quando os potenciais escalar e vetorial têm a mesma magnitude. O mesmo acontece na aproximação de potenciais fracos. O comportamento da amplitude de espalhamento próximo das condições que fornecem a forma fechada é também discutido. A partir dos polos complexos da amplitude de onda parcial, soluções exatas de estados ligados para partículas e antipartículas em diferentes circunstâncias são obtidas. A quebra perturbativa da degenerescência acidental aparecendo em um par de casos especiais é relacionado com a não conservação do vetor de Runge-Lenz. No caso de férmions, as amplitudes de ondas parciais têm formas fechadas não apenas para as simetrias de spin e pseudospin mas também quando há uma ligeira quebra das mesmas. Mostra-se que no limite não relativístico, a seção de choque diferencial obtida se reduz para a seção de choque de Rutherford para simetrias de spin e pseudospin exatas e para a seção de choque de Mott quando a simetria é suavemente quebrada / The relativistic scattering of spin-0 bosons and spin-1/2 fermions by spherically symmetric Coulomb potentials is analyzed in detail with an arbitrary mixing of vector and scalar couplings. It is shown that the partial wave series for both bosons and fermions reduces the scattering amplitude to that one resulting in the Rutherford formula exactly when the vector and scalar potentials have the same magnitude. The same happen in the approximation for weak potentials. The behavior of the scattering amplitude near the conditions that furnish its closed form is also discussed. From the complex poles of the partial scattering amplitude for bosons the exact closed form of bound-state solutions for both particles and antiparticles with different scenarios for the coupling constants are obtained. Perturbative breaking of the accidental degeneracy appearing in a pair of special cases is related to the nonconservation of the Runge-Lenz vector. In the case of fermions, the closed form for the partial wave series occurs not only for the spin and pseudospin symmetries but also when there is a slight breaking of the same. It is shown that in the non-relativistic limit, the differential cross section obtained reduces to Rutherford's cross section when there are spin and pseudospin symmetries and to Mott differential cross section when the symmetry is slightly broken

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<p>This thesis investigates aspects of field theories and soliton solutions with nontrivial topology. In particular we explore the following effective models: a limited sector of the scalar Electroweak theory called extended Abelian Higgs model, and a classical mechanics model derived from the low energy SU(2) Yang-Mills theory.</p><p>The extended Abelian Higgs model applied on two-component plasma of charged particles is studied numerically. We find evidence that the model admits straight twisted line vortices. The result is described by an energy function that acquires a minimum value for a non-trivial twist. In addition to the twisted line vortices the result also suggests that stable torus shaped solitons are solutions of the theory. </p><p>Furthermore we construct a classical mechanics model exhibiting some of the key properties of the low-energy Yang-Mills theory. The dynamics of the model is studied numerically. We find that its classical equations of motion support stable periodic orbits. In a three dimensional projection these trajectories are self-linked in a topologically non-trivial manner suggesting the existence of knotted configurations in low energy SU(2) Yang-Mills theory. </p><p>We calculate the one-loop effective action for the Abelian Higgs model with extended Higgs sector. The resulting first order quantum corrected model shows close resemblance to a modified model where texture stabilizing term has been added to the system. In the limit where the gauge field can be entirely expressed by the scalar fields, the both models become identical suggesting that the theories are closely connected. This implies that quantum corrections have stabilising effect on the soliton solutions. </p><p>These studies have contributed to a better understanding of the dynamics of non-linear low energy systems, and brought us a step closer to exploring full scale physically realistic models.</p>

This thesis investigates aspects of field theories and soliton solutions with nontrivial topology. In particular we explore the following effective models: a limited sector of the scalar Electroweak theory called extended Abelian Higgs model, and a classical mechanics model derived from the low energy SU(2) Yang-Mills theory. The extended Abelian Higgs model applied on two-component plasma of charged particles is studied numerically. We find evidence that the model admits straight twisted line vortices. The result is described by an energy function that acquires a minimum value for a non-trivial twist. In addition to the twisted line vortices the result also suggests that stable torus shaped solitons are solutions of the theory. Furthermore we construct a classical mechanics model exhibiting some of the key properties of the low-energy Yang-Mills theory. The dynamics of the model is studied numerically. We find that its classical equations of motion support stable periodic orbits. In a three dimensional projection these trajectories are self-linked in a topologically non-trivial manner suggesting the existence of knotted configurations in low energy SU(2) Yang-Mills theory. We calculate the one-loop effective action for the Abelian Higgs model with extended Higgs sector. The resulting first order quantum corrected model shows close resemblance to a modified model where texture stabilizing term has been added to the system. In the limit where the gauge field can be entirely expressed by the scalar fields, the both models become identical suggesting that the theories are closely connected. This implies that quantum corrections have stabilising effect on the soliton solutions. These studies have contributed to a better understanding of the dynamics of non-linear low energy systems, and brought us a step closer to exploring full scale physically realistic models.

Two graphene layers placed in close proximity offer a unique system to investigate interacting electron physics as well as to test novel electronic device concepts. In this system, the interlayer spacing can be reduced to value much smaller than that achievable in semiconductor heterostructures, and the zero energy band-gap allows the realization of coupled hole-hole, electron-hole, and electron-electron two-dimensional systems in the same sample. Leveraging the fabrication technique and electron transport study in dual-gated graphene field-effect transistors, we realize independently contacted graphene double layers separated by an ultra-thin dielectric. We probe the resistance and density of each layer, and quantitatively explain their dependence on the backgate and interlayer bias. We experimentally measure the Coulomb drag between the two graphene layers for the first time, by flowing current in one layer and measuring the voltage drop in the opposite layer. The drag resistivity gauges the momentum transfer between the two layers, which, in turn, probes the interlayer electron-electron scattering rate. The temperature dependence of the Coulomb drag above temperatures of 50 K reveals that the ground state in each layer is a Fermi liquid. Below 50 K we observe mesoscopic fluctuations of the drag resistivity, as a result of the interplay between coherent intralayer transport and interlayer interaction. In addition, we develop a technique to directly measure the Fermi energy in an electron system as a function of carrier density using double layer structure. We demonstrate this method in the double layer graphene structure and probe the Fermi energy in graphene both at zero and in high magnetic fields. Last, we realize dual-gated bilayer graphene devices, where we investigate quantum Hall effects at zero energy as a function of transverse electric field and perpendicular magnetic field. Here we observe a development of v = 0 quantum Hall state at large electric fields and in high magnetic fields, which is explained by broken spin and valley spin symmetry in the zero energy Landau levels. / text

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A tese de doutorado apresenta uma aplicação de técnicas de teoria de campos em um sistema da matéria condensada. Motivados por experimentos em gases atômicos, apresentamos um estudo sobre misturas binárias de gases atômicos na presença de uma interação do tipo Josephson. O foco principal é o estudo de um modelo de dois campos complexos não-relativisticos com simetria O(2). Esta simetria é quebrada por interações que produzem um desbalanço nas populações das duas espécies bosônicas. Estudamos o modelo na aproximação de campo médio mais flutuações gaussianas, usando o formalismo de teoria de campos a temperatura finita em tempo imaginário. Os resultados mostram que, num certo intervalo de temperaturas, as duas espécies bosônicas condensam à mesma temperatura crítica e a fase relativa do condensado é fixa, determinada pela fase do campo externo aplicado. / The thesis apresents an applications of field theory techniques in a condensed matter system. Motivated by experiments in atomic gases, we present a study of binary mixture of atomic gases in the presence of an interaction type Josephson. The main focus is the study of a model of two complex fields with non-relativistic symmetry O(2). This symmetry is broken by interactions that produce an inbalance in the populations of the two species bosonic. We study the model in the mean-field approximattion more gaussian fluctuations, using the formalism of field theory at finite temperature in imaginary time. The results show that in a certain temperature range, the two bosonic species condense the same critical temperature and the ralative phase of the condensate is fixed and determined by the applied external field phase.