637 research outputs found
Nonequilibrium spin phenomena in quantum dots induced by periodic optical excitation
The coherent control of a charge carrier spin that is localized in a semiconductor quantum dot and the generation of long-lived states for information storage are of particular interest for quantum information processing. This spin interacts predominantly with the surrounding nuclear spins in the quantum dot, which can be described by the central spin model. The periodic application of circularly polarized laser pulses induces nonequilibrium spin dynamics in the quantum dot, giving rise to various phenomena that can be observed in experiments. In this thesis, models and semiclassical approaches are developed to simulate the driven spin dynamics in this system under experimental conditions. For the case where a transverse magnetic field is applied, it is found that the part of the spin mode locking effect stemming from nuclei-induced frequency focusing depends nonmonotonically on the strength of the magnetic field, with strong similarities to experimental observations. The complex behavior is related to various nuclear magnetic resonances with respect to the repetition rate of the laser pulses, which can be exploited for novel kind of nuclear magnetic resonance spectroscopy of the emerging nonequilibrium steady states. For the case where a longitudinal magnetic field is applied, the influence of the pump pulse power on the spin inertia and on the polarization recovery effect is analyzed. With the help of the developed model, the related experiments can be understood and described quantitatively. In this context, a novel effect termed resonant spin amplification in Faraday geometry is predicted, which enables the direct measurement of the longitudinal g factor of the resident charge carriers. Model calculations are used to find the optimal conditions for its detection and ways to improve its visibility are pointed out. The comparison with recent experiments that demonstrate the realization of the effect shows a remarkable agreement.Die kohĂ€rente Kontrolle eines LadungstrĂ€gerspins, der in einem Halbleiterquantenpunkt lokalisiert ist, sowie die Erzeugung langlebiger ZustĂ€nde zur Informationsspeicherung sind von besonderem Interesse fĂŒr die Quanteninformationsverarbeitung. Solch ein Spin wechselwirkt hauptsĂ€chlich mit den ihn umgebenden Kernspins im Quantenpunkt, was durch das Zentralspinmodell beschrieben werden kann. Durch periodische Anregung mit zirkular polarisierten Laserpulsen lĂ€sst sich die Spindynamik in Quantenpunkten in ein Nichtgleichgewicht treiben, wodurch verschiedene PhĂ€nomene auftreten können, die sich in Experimenten beobachten lassen. In dieser Arbeit werden theoretische Modelle und semiklassische Methoden entwickelt, um die getriebene Spindynamik unter experimentellen Bedingungen zu simulieren. Im Falle eines angelegten transversalen Magnetfelds zeigt sich, dass der Teil des âSpin Mode Lockingâ Effekts (Synchronisation von Spin-Moden), welcher aufgrund einer durch die Kernspins induzierten Frequenzfokussierung entsteht, eine nicht-monotone AbhĂ€ngigkeit von der StĂ€rke des Magnetfelds aufweist, mit starken Parallelen zu experimentellen Beobachtungen. Verantwortlich fĂŒr das komplexe Verhalten sind verschiedene Kernspinresonanzen bezogen auf die Wiederholungsrate der Laserpulse. Hieraus ergibt sich eine neue Art von Kernspinresonanzspektroskopie, durch welche die langlebigen NichtgleichgewichtszustĂ€nde untersucht werden können. Im Falle eines angelegten longitudinalen Magnetfelds wird der Einfluss der Pulsleistung auf den âSpin Inertiaâ (SpintrĂ€gheit) und den âPolarization Recoveryâ (Wiederherstellung der Spinpolarisation) Effekt untersucht. Die zugehörigen Experimente lassen sich durch das entwickelte Modell quantitativ verstehen und beschreiben. In diesem Zusammenhang wird ein neuer Effekt vorhergesagt, welcher als âResonant Spin Amplification in Faraday Geometryâ (Resonante SpinverstĂ€rkung in Faraday Geometrie) bezeichnet wird und die direkte Bestimmung des longitudinalen g-Faktors der LadungstrĂ€ger ermöglicht. Optimale Bedingungen fĂŒr dessen Beobachtung und Möglichkeiten zu Verbesserung seiner Sichtbarkeit werden aufgezeigt. Der Vergleich mit kĂŒrzlich durchgefĂŒhrten Experimenten, welche die Existenz des Effekts bestĂ€tigen, zeigt eine hervorragende Ăbereinstimmung
The Writing is the Wall: Expanding the Means of Communication with Multimodal Approaches to Teaching Composition
As the paradigm of communication shifts into the digital realm, it seems only logical that instructorsâ pedagogical approaches to teaching writing should shift as well. Though there is still much merit to teaching tradition approaches to composition, are there more modern methods that could be employed to teach communication in a contemporary setting? This thesis shall examine the role that new media can play in a multimodal composition course, as new media seems to be the most effective way to teach rhetorical communication skills in a modern setting. By looking at new media elements, such as podcasts, wikis, and images, this thesis shall attempt to understand how these media come into rhetorical meaning, and examine how instructors can effectively integrate new media into their curriculum for their composition courses. The research of this thesis suggest that while implementing multimodal approaches can be an effective way to teach communication strategies, instructors must careful craft their pedagogies to ensure that these multimodal approaches are fully understood by students
The Perennial Quest: Working Towards Equitable Assessment With Student-Centric Learning Contracts
This dissertation examines the role of writing assessment in college composition courses, and how issues including learning aversion, access to technology, and white-language supremacy can harm our assessment practices and pedagogy. Writing assessment is one of the most important aspects of teaching and is a significant factor in shaping studentsâ attitudes about their abilities as writers. While writing assessment is of tremendous importance, effectively and equitably assessing student writing can be difficult, especially when teaching diverse student populations. In an effort improve the quality of writing assessment, this dissertation focused on developing Student-Centric Learning Contracts (SCLCs) as a more effective alternative to conventional forms of assessment. SCLCs are a hybrid of labor and learning contracts, that aim to provide students with increased agency in the assessment process. In SCLCs, students and instructors work collaboratively to outline what a student wants to learn, how they will learn it, and what kinds of language will help them reach their audience
Persisting correlations of a central spin coupled to large spin baths
The decohering environment of a quantum bit is often described by the
coupling to a large bath of spins. The quantum bit itself can be seen as a spin
which is commonly called the central spin. The resulting central spin
model describes an important mechanism of decoherence. We provide
mathematically rigorous bounds for a persisting magnetization of the central
spin in this model with and without magnetic field. In particular, we show that
there is a well defined limit of infinite number of bath spins. Only if the
fraction of very weakly coupled bath spins tends to 100\% does no magnetization
persist.Comment: 19 pages, 15 figures, rigorous bounds for the central spin mode
Nuclear magnetic resonance spectroscopy of nonequilibrium steady states in quantum dots
The optically induced polarization of localized electron spins in an ensemble
of quantum dots (QDs) dephases due to the interaction with the surrounding
nuclear spins. Despite this dephasing, the spins in the QDs can be controlled
to respond coherently by applying periodic laser pulses, leading to a revival
of the spin polarization before each pulse. This effect, known as spin mode
locking, strongly depends on an emerging selection of certain polarizations of
the nuclear spin bath which is driven to a steady state far from equilibrium.
We investigate the influence of the nuclear composition in InGaAs
QDs on this nonequilibrium behavior and demonstrate that nuclear magnetic
resonances (NMR) appear as very sharp dips in the revival signal. This suggests
a novel kind of NMR spectroscopy.Comment: 7 pages including 4 figures; 3 pages supplement including 3 figure
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