Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

In this thesis we study the effect of electron-electron interactions on Chiral two-dimensional electron gas (C2DEGs). C2DEGs are a very good description of the low-energy electronic properties of single layer and multilayer graphene systems. The low-energy properties of single layer and multilayer graphene are described by Chiral Hamiltoninans whose band eigenstates have definite chirality. In this thesis we focus on the effect of electron-electron interactions on two of these systems: monolayer and bilayer graphene. In the first half of this thesis we use the massless Dirac Fermion model and random-phase-approximation to study the effect of interactions in graphene sheets. The interplay of graphene's single particle chiral eigenstates along with electron-electron interactions lead to a peculiar supression of spin susceptibility and compressibility, and also to an unusual velocity renormalization. We also report on a theoretical study of the influence of electron-electron interactions on ARPES spectra in graphene. We find that level repulsion between quasiparticle and plasmaron resonances gives rise to a gap-like feature near the Dirac point. In the second half we anticipate interaction driven integer quantum Hall effects in bilayer graphene because of the near-degeneracy of the eight Landau levels which appear near the neutral system Fermi level. We predict that an intra-Landau-level cyclotron resonance signal will appear at some odd-integer filling factors, accompanied by collective modes which are nearly gapless and have approximate q[superscrit 3/2] dispersion. We speculate on the possibility of unusual localization physics associated with these modes. / text

Introduction An atom or ion in a solid is associated with a given lattice site. Translational motion can however occur in solids and such diffusion processes can be studied by nuclear magnetic resonance, tracer diffusion and other methods. In ionic compounds the diffusion is also a mechanism for charge transport and hence there is an associated ionic conductivity. [continued in text ...] NOTE: The Abstract contains many indistinct letters in mathematical formulas and it is impracticable to reproduce it here.

The production of three gauge bosons in high-energy collisions - in particular in view of a next-linear collider with center of mass energies in the TeV range - offers an unique opportunity to probe the Standard Model (SM) of today's particle physics. In this thesis we pay particular attention to the electroweak sector of the theory. We investigate the gauge structure {i. e. possible deviations from the SM predictions of gauge boson self-interactions manifest e. g. in anomalous quartic gauge boson couplings and Radiation zeros) as well as electroweak radiative corrections in order to improve theoretical predictions for SM processes. Quartic gauge boson couplings can be regarded as a direct window on the sector of electroweak symmetry breaking. We have studied the impact of three such anomalous couplings on the processes e+e(^-) → WWγ, ZZγ and Zγγ at LEP2 and a future linear collider. In certain high-energy scattering processes involving charged particles and the emission of one or more photons, the scattering amplitude vanishes for particular configurations of the final state particles. The fact that gauge symmetry is a vital ingredient for the cancellation to occur means that radiation zeros can be used to probe physics beyond the standard model. For example anomalous electroweak gauge boson couplings destroy the delicate cancellation necessary for the zero to occur. We have studied the process qq → WWγ. To match the expected experimental precision at future linear colliders, improved theoretical predictions beyond next-to-leading order are required. By choosing an appropriate gauge, we have developed a formalism to calculate such corrections for arbitrary electroweak processes. As an example we consider here the processes e(^+)e → f f and e(^+)e(^-) → W(^+)(_T)W(^-)(_T), W(^+)(_L)W(^-)(_L) and study the perturbative structure of the electroweak Sudakov logarithms by means of an explicit two-loop calculation. In this way we investigate how the Standard Model, with its mass gap between the photon and Z boson in the neutral sector, compares to unbroken theories like QED and QCD. We observe that the two-loop corrections are consistent with an exponentiation of the one-loop corrections. In this sense the Standard Model behaves like an unbroken theory at high energies.

Thesis (Ph. D.)--Rutgers University, 2008. / "Graduate Program in Physics and Astronomy." Includes bibliographical references (p. 142-151).

Made available in DSpace on 2014-10-09T12:26:49Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:10Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:26:49Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:10Z (GMT). No. of bitstreams: 0 / O dosímetro alanina gel é um novo material na forma de gel desenvolvido pelo IPEN, que apresenta uma melhora significativa em relação ao sistema a base de Alanina desenvolvido por Costa. A DL-Alanina (C3H7NO2) é um aminoácido tecido equivalente que favorece a produção de íons Férricos na solução. Este trabalho visa analisar as principais características dosimétricas deste novo material na forma de gel para futura aplicação na medida da distribuição da dose. Foi avaliado o desempenho da solução gel de Alanina para radiação gama, de fótons, de elétrons e de nêutrons térmicos utilizando a técnica de espectrofotometria. De acordo com os resultados obtidos para os diferentes tipos de radiação estudados, as reprodutibilidades intra e inter-lotes são melhores que 4% e 5%, respectivamente. A resposta em função da dose apresenta um comportamento linear para o intervalo de dose estudado. A dependência da resposta em função da taxa de dose e energia da radiação incidente é melhor que 2% e 3%, respectivamente. A dose mínima detectável é 0,1 Gy. Os resultados obtidos indicam que o dosímetro Alanina gel apresenta um bom desempenho, e pode ser usado como dosímetro alternativo na área de radioterapia, utilizando a técnica de ressonância magnética para avaliar a distribuição tridimensional de dose. / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Mott insulators are a family of materials in which strong electron-electron interactions induce an unconventional insulating state in the system that would otherwise behave as an electrical conductor according to the non-interacting band theory. In particular, the nature of the Mott metal-insulator transition (MIT) has been the subject of intense research interest because it can involve a complicated interplay between magnetic and electronic properties. In some Mott systems, Mott transitions occur in the one-step process, from an antiferromagnetic insulator (AFI) to a paramagnetic metal phase (PMM), while in other Mott systems a two-step transition with an intermediate antiferromagnetic metal (AFM) phase can be observed. Since 2015, the muon spin relaxation (𝜇SR) group at Columbia University started systematic 𝜇SR studies on a series of Mott systems, including one-step transition Mott systems 𝑅𝐸NiO₃ and V₂O₃, as well as two-step transition Mott systems Ba(Co, Ni)S₂, Ni(S, Se)₂ and (La, Sr)VO₃. This dissertation first introduces the comprehensive 𝜇SR research on multiple families of Mott systems conducted by our 𝜇SR group, including 𝑅𝐸NiO₃, V₂O₃, and BaCoS₂. Then the 𝜇SR experimental findings on the Mott system NiS₂₋ₓSeₓ will be presented, which is the most extensively studied material in this thesis. The NiS₂₋ₓSeₓ system is of particular interest because there is a large region of intermediate AFM state emerging between AFI and PMM states with Se doping, making it an ideal platform to provide information on static magnetism in the AFM state and thus can help us better understand the evolution of magnetic NiS₂₋ₓSeₓ, with our key findings being: (1) The AFM state of the NiS₂₋ₓSeₓ system shows significantly random spin correlations, and the magnetic order is suppressed by a gradual reduction of the ordered moment size, with a nearly full ordered volume fraction until very close to the AFM-PMM boundary. (2) No signature of dynamic critical behavior was observed in the thermal phase transition, indicating a first-order thermal phase transition. The next part of this dissertation presents our computational simulations on the NiS₂₋ₓSeₓ system. Dipolar field simulations have shown that only the combination of easy axis randomization and Ni moment dilution in NiS₂ can lead to the internal field distribution corresponding to the observed 𝜇SR spectrum in the AFM region. Also, this picture could qualitatively explain the neutron and muon results consistently, thus reconciling the seemingly contradicting experimental results by 𝜇SR and the previous neutron scattering studies shown in the AFM region. Furthermore, we propose a percolation model that can capture both the charge and spin connections of the interpenetrating percolating spin and charge networks in the NiS₂₋ₓSeₓsystem, which raises the possibility of "compromising metallicity and magnetic order" in the two-step Mott transition evolving AFI to AFM to PMM states in NiS₂₋ₓSeₓ.

Made available in DSpace on 2014-10-09T12:41:47Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:26Z (GMT). No. of bitstreams: 0 / Materiais poliméricos (plásticos e borrachas) abrangem uma proporção continuamente crescente de resíduos urbanos e industriais descartados em aterros, seus impactos no meio ambiente são cada vez mais preocupantes. A implementação de novas tecnologias em prol da redução dos resíduos poliméricos, aceitáveis do ponto de vista ambiental e a um custo eficaz, provou ser um grande problema, face às complexidades inerentes para a reutilização dos polímeros. A radiação ionizante tem capacidade para alterar a estrutura e propriedades dos materiais poliméricos. As borrachas butílicas e halobutílicas têm sido usadas em larga escala, numa variedade de aplicações tais como partes de pneus e artefatos diversos. O principal efeito do fóton de alta energia, como raios gama nas borrachas butílicas e butílicas halogenadas é a geração de radicais livres, acompanhada por mudanças nas propriedades mecânicas. O objetivo deste trabalho é desenvolver processos de degradação controlada (desvulcanização) de borrachas butílicas e halobutílicas (cloro e bromo), de modo a caracterizar sua disponibilidade para transformação e alteração de suas propriedades. Os resultados experimentais obtidos mostraramm que as borrachas butílica e halobutílicas irradiadas a 25 kGy e posteriormente cisalhadas podem ser usadas como ponto de partida para misturas com borracha virgem. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Scope and Content stated in the place of the abstract. / The principal methods of calculating nuclear spin coupling constants by applying perturbation theory to molecular orbital wavefunctions for the electronic structure of molecules are discussed. A new method employing a self-consistent-field perturbation theory (SCFPT) is then presented and compared with the earlier methods. In self-consistent-field (SCF) methods, the interaction of an electron with other electrons in a molecule is accounted for by treating the other electrons as an average distribution of negative charge. However, this charge distribution cannot be calculated until the electron-electron interactions themselves are known. In the SCF method, an initial charge distribution is assumed and then modified in an iterative calculation until the desired degree of self-consistency is attained. In most previous perturbation methods, these electron interactions are not taken into account in a self consistent manner in calculating the perturbed wavefunction even when SCF wavefunctions are used to describe the unperturbed molecule. The main advantage of the new SCFPT approach is that it treats the interactions between electrons with the same degree of self-consistency in the perturbed wavefunction as in the unperturbed wavefunction. The SCFPT method offers additional advantages due to its computational efficiency and the direct manner in which it treats the perturbations. This permits the theory to be developed for the orbital and dipolar contributions to nuclear spin coupling as well as for the more commonly treated contact interaction. In this study, the SCFPT theory is used with the Intermediate Neglect of Differential Overlap (INDO) molecular orbital approximation to calculate a number of coupling constants involving 13c and 19F. The usually neglected orbital and dipolar terms are found to be very important in FF and CF coupling. They can play a decisive role in explaining the experimental trend of JCF among a series of compounds. The orbital interaction is found to play a significant role in certain CC couplings. Generally good agreement is obtained between theory and experiment except for JCF and JFF in oxalyl fluoride and the incorrect signs obtained for cis JFF in fluorinated ethylenes. The nature of the theory permits the latter discrepancy to be rationalized in terms of computational details. The value of JFF in difluoracetjc acid is predicted to be -235 Hz. The SCFPT method is used with a theory of dπ - pπ bonding to predict in agreement with experiment that JCH in acetylene will decrease when that molecule is bound in a transition metal complex. / Thesis / Doctor of Philosophy (PhD)