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The Global ETD Search service is a free service for researchers
to find electronic theses and dissertations. This service is
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Networked Digital Library of Theses and Dissertations.
Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
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Showing 261 to 265 of 265 (0.017 seconds)| 261 |
Non-Orthogonality and Electron Correlations in Nanotransport : Spin- and Time-Dependent CurrentsFransson, Jonas January 2002 (has links)
<p>The concept of the transfer Hamiltonian formalism has been reconsidered and generalized to include the non-orthogonality between the electron states in an interacting region, e.g. quantum dot (QD), and the states in the conduction bands in the attached contacts. The electron correlations in the QD are described by means of a diagram technique for Hubbard operator Green functions for non-equilibrium states. </p><p>It is shown that the non-orthogonality between the electrons states in the contacts and the QD is reflected in the anti-commutation relations for the field operators of the subsystems. The derived forumla for the current contains corrections from the overlap of the same order as the widely used conventional tunneling coefficients. </p><p>It is also shown that kinematic interactions between the QD states and the electrons in the contacts, renormalizes the QD energies in a spin-dependent fashion. The structure of the renormalization provides an opportunity to include a spin splitting of the QD levels by polarizing the conduction bands in the contacts and/or imposing different hybridizations between the states in the contacts and the QD for the two spin channels. This leads to a substantial amplification of the spin polarization in the current, suggesting applications in magnetic sensors and spin-filters.</p>
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Non-Orthogonality and Electron Correlations in Nanotransport : Spin- and Time-Dependent CurrentsFransson, Jonas January 2002 (has links)
The concept of the transfer Hamiltonian formalism has been reconsidered and generalized to include the non-orthogonality between the electron states in an interacting region, e.g. quantum dot (QD), and the states in the conduction bands in the attached contacts. The electron correlations in the QD are described by means of a diagram technique for Hubbard operator Green functions for non-equilibrium states. It is shown that the non-orthogonality between the electrons states in the contacts and the QD is reflected in the anti-commutation relations for the field operators of the subsystems. The derived forumla for the current contains corrections from the overlap of the same order as the widely used conventional tunneling coefficients. It is also shown that kinematic interactions between the QD states and the electrons in the contacts, renormalizes the QD energies in a spin-dependent fashion. The structure of the renormalization provides an opportunity to include a spin splitting of the QD levels by polarizing the conduction bands in the contacts and/or imposing different hybridizations between the states in the contacts and the QD for the two spin channels. This leads to a substantial amplification of the spin polarization in the current, suggesting applications in magnetic sensors and spin-filters.
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BOSONS DE SPIN-1/2 ET 1 SUR RÉSEAUX OPTIQUES EN UNE ET DEUX DIMENSIONSDe Forges De Parny, Laurent 14 November 2012 (has links) (PDF)
Le piégeage optique d'atomes sur réseaux optiques permet d'étudier leur comportement dans un régime de très basse température, de l'ordre du nanokelvin, sans geler le degré de liberté de leur moment cinétique. Ces méthodes récentes de piégeage offrent la possibilité d'analyser le magnétisme des phases quantiques spontanément adopté par les atomes. Dans cette thèse, nous étudions numériquement des bosons à deux états effectifs de spin, ou bosons de spin-1/2, ainsi que des bosons à trois états de spin, ou bosons de spin-1, avec deux méthodes conceptuellement différentes: une approche simplifiée en champ moyen et une méthode exacte, la méthode de Monte Carlo Quantique. Au delà de l'étude de ces deux systèmes, nous comparons une méthode en champ moyen, parfois abusivement employée, à la méthode de Monte Carlo Quantique. L'étude approfondie du système de bosons de spin-1/2 en une et deux dimensions dans la limite de température nulle montre l'influence de la dimension sur la physique de ce système. Les effets thermiques, non négligeables expérimentalement, sont aussi étudiés. Enfin, le système de bosons de spin-1 piégés sur un réseau bidimensionnel, système plus riche et plus complexe à étudier que le précédent, est étudié dans la limite de température nulle. L'étude de ces deux systèmes révèle différentes organisations magnétiques dans les phases isolantes de Mott ainsi que dans la phase superfluide, telles qu'un superfluide ferromagnétique. Des transitions de phases du premier ordre et des mouvements cohérents anticorrélés, présents dans les phases de Mott, sont aussi observés.
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The attraction of sloppy nonsense: resolving cognitive estrangement in Stargate through the technologising of mythologyWhitelaw, Sandra January 2007 (has links)
The thesis consists of the novel, Stargate Atlantis: Exogenesis (Whitelaw and Christensen, 2006a) and an accompanying exegesis.
The novel is a stand-alone tie-in novel based on the television series Stargate Atlantis (Wright and Glassner), a spin-off series of Stargate SG-1 (Wright and Cooper) derived from the movie Stargate (Devlin and Emmerich, 1994). Set towards the end of the second season, Stargate Atlantis: Exogenesis begins with the discovery of life pods containing the original builders of Atlantis, the Ancients. The mind of one of these Ancients, Ea, escapes the pod and possesses Dr. Carson Beckett. After learning what has transpired in the 10,000 years since her confinement, the traumatised Ea releases an exogenesis machine to destroy Atlantis. Ea dies, leaving Beckett with sufficient of her memories to reveal that a second machine, on the planet Polrusso, could counter the effects of the first device. When the Atlantis team travel to Polrusso, what they discover has staggering implications not only for the future of Atlantis but for all life in the Pegasus Galaxy.
The exegesis argues that both science and science fiction narrate the dissolution of ontological structures, resulting in cognitive estrangement. Fallacy writers engage in the same process and use the same themes and tools as science fiction writers to resolve cognitive estrangement: they technologise mythology. Consequently, the distinction between fact and fiction, history and myth, is blurred.
The exegesis discusses cognitive estrangement, mythology, the process of technologising mythology and its function as a novum that facilitates the resolution of cognitive estrangement in both fallacy and science fiction narratives. These concepts are then considered in three Stargate tie-in novels, with particular reference to the creative work, Stargate Atlantis: Exogenesis.
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Theoretical methods for the electronic structure and magnetism of strongly correlated materialsLocht, Inka L. M. January 2017 (has links)
In this work we study the interesting physics of the rare earths, and the microscopic state after ultrafast magnetization dynamics in iron. Moreover, this work covers the development, examination and application of several methods used in solid state physics. The first and the last part are related to strongly correlated electrons. The second part is related to the field of ultrafast magnetization dynamics. In the first part we apply density functional theory plus dynamical mean field theory within the Hubbard I approximation to describe the interesting physics of the rare-earth metals. These elements are characterized by the localized nature of the 4f electrons and the itinerant character of the other valence electrons. We calculate a wide range of properties of the rare-earth metals and find a good correspondence with experimental data. We argue that this theory can be the basis of future investigations addressing rare-earth based materials in general. In the second part of this thesis we develop a model, based on statistical arguments, to predict the microscopic state after ultrafast magnetization dynamics in iron. We predict that the microscopic state after ultrafast demagnetization is qualitatively different from the state after ultrafast increase of magnetization. This prediction is supported by previously published spectra obtained in magneto-optical experiments. Our model makes it possible to compare the measured data to results that are calculated from microscopic properties. We also investigate the relation between the magnetic asymmetry and the magnetization. In the last part of this work we examine several methods of analytic continuation that are used in many-body physics to obtain physical quantities on real energies from either imaginary time or Matsubara frequency data. In particular, we improve the Padé approximant method of analytic continuation. We compare the reliability and performance of this and other methods for both one and two-particle Green's functions. We also investigate the advantages of implementing a method of analytic continuation based on stochastic sampling on a graphics processing unit (GPU).
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