Browsing by Subject "Hall"
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Item Electronic correlations in few layer graphene(2011-12) Zhang, Fan, 1983-; MacDonald, Allan H.In this thesis we investigate the electronic band structures and the correlations in chirally (ABC) stacked N-layer graphene with N ≥ 2. We use ab initio density-functional theory and k · p theory to fit the parameters of a p-band tightbinding model. External potential differences between top and bottom layers are strongly screened by charge transfer but still open an energy gap at overall neutrality. Perpendicular magnetic field drives the system into the quantum Hall region with 4N-fold zero energy Landau levels. We predict that Coulomb interactions spontaneously break the SU(4N) symmetry and drive quantum Hall effects at all integer fillings n from −2N to 2N with exotic spin and pseudospin polarizations. Based on mean-field theory and perturbative renormalization group analysis, we predict that the ground state of bilayer graphene spontaneously breaks inversion symmetry for arbitrarily weak electron-electron interactions and conclude that this instability is not suppressed by quantum fluctuations but that, because of trigonal warping, it may occur only in high quality suspended bilayers. Remarkably flat conduction and valence bands that touch at charge neutrality point and Bloch states with large pseudospin chirality combine to make the bilayer graphene gapless band state strongly susceptible to a family of broken symmetry states in which each spinvalley flavor spontaneously transfers charge between layers. We explain how these states are distinguished by their charge, spin, and valley Hall conductivities, by their orbital magnetizations, and by their edge state properties. We further analyze how these competing states are influenced by Zeeman fields that couple to spin and by interlayer electric fields that couple to layer pseudospin, and comment on the possibility of using response and edge state signatures to identify the character of the bilayer ground state experimentally. We demonstrate that similar insulating broken symmetry states and spontaneous topological orders also occur in bilayer’s thicker cousins, chirally stacked multilayer graphene systems.Item Gas Kinetic Study of Magnetic Field Effects on Plasma Plumes(2012-12-07) Ebersohn, Frans 1987-Plasma flow physics in magnetic nozzles must be clearly understood for optimal design of plasma propulsion devices. Toward that end, in this thesis we: i) perform an extensive literature survey of magnetic nozzle physics, ii) assess the validity of magnetohydrodynamics for studying magnetic nozzle physics, and iii) illustrate the effects of the Hall term in simple flows as well as in magnetic nozzle configurations through numerical experiments with the Magneto-Gas Kinetic Method (MGKM). The crucial steps necessary for thrust generation in magnetic nozzles are energy conversion, plasma detachment, and momentum transfer. These three physical phenomena must be understood to optimize magnetic nozzle design. The operating dimensionless parameter ranges of six prominent experiments are considered and the corresponding mechanisms are discussed. An order of magnitude analysis of the governing equations reveal: i) most magnetic nozzles under consideration operate at the edge of the continuum regime rendering continuum-based description and computation valid; ii) in the context of MHD framework, the generalized Ohm?s law must be used to capture all of the relevant physics. This work also continues the development of the Magneto Gas Kinetic Method (MGKM) computational tool. Validation of the solver is performed in shock-tube and Hartmann channel flows in the Hall physics regime. Comparison with theory and available data is made whenever possible. Novel numerical experiments of magnetic nozzle plasma jets in the Hall regime are performed, confirming the theoretically predicted azimuthal rotation of the plasma jet due to Hall physics. The primary conclusion from this work is that the addition of the Hall effect generates helical structures in magnetic nozzle plasma flows. Preliminary results are encouraging for future magnetic nozzle studies and further challenges are identified.Item The Rhetoric Of Writing: A Rhetorical Analysis of Modern Writing Memoirs(2010-01-14) Illich, Lindsay P.This dissertation analyzes concepts of the writing self in works about writing by professional creative writers (writers, poets, and essayists). Through a rhetorical analysis of these texts, I observe that writers view the writing self as a complex structure that is fully conscious as a rhetorical agent, an embodied self that interacts with the world and actively chooses linguistic representations of that experience, and maintains a concept of self that is subject to influences which the writers do not fully understand (such as inspiration and insight). The discourse used by writers to describe their writing processes challenges recent critiques of expressionism and the model of social construction that pervades contemporary composition scholarship. Chapter II examines Virginia Woolf's use of the central metaphor for invention in A Room of One's Own, a river, which sharply calls into question a unified view of the self which is central to critiques of expressivism by composition scholars. Woolf's concept of invention requires a negation of the self and harmony with nature (widely conceived as the entire world, including texts). Chapter III, an analysis of two writing memoirs by contemporary professional creative writers, Annie Dillard's The Writing Life and Donald Hall's Life Work, finds that Dillard and Hall use metaphors that establish freedom (rhetorical agency) and bodily presence as primary characteristics of their writing processes. Chapter IV, an analysis of two collections of essays about writing by professional creative writers, argues that the writers' use of metaphors of inspiration and instrumental metaphors creates a concept of the writing self that maintains a sense of writerly control (rhetorical agency) alternating with a sense of a diminished control; ultimately, the two concepts coexist in the minds of the writers. Chapter V proposes that the rhetorical situation of the contemporary composition classroom affects students' creativity adversely. The chapter also suggests further analyses of writing memoirs can provide new ways of understanding writing processes (as opposed to one writing process model) and therefore contribute substantially to composition scholarship and pedagogy.