@phdthesis{Vaeth2016,
  author    = {V{\"a}th, Stefan Kilian},
  title     = {On the Role of Spin States in Organic Semiconductor Devices},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-141894},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {The present work addressed the influence of spins on fundamental processes in organic semiconductors. In most cases, the role of spins in the conversion of sun light into electricity was of particular interest. However, also the reversed process, an electric current creating luminescence, was investigated by means of spin sensitive measurements. In this work, many material systems were probed with a variety of innovative detection techniques based on electron paramagnetic resonance spectroscopy. More precisely, the observable could be customized which resulted in the experimental techniques photoluminescence detected magnetic resonance (PLDMR), electrically detected magnetic resonance (EDMR), and electroluminescence detected magnetic resonance (ELDMR). Besides the commonly used continuous wave EPR spectroscopy, this selection of measurement methods yielded an access to almost all intermediate steps occurring in organic semiconductors during the conversion of light into electricity and vice versa. Special attention was paid to the fact that all results were applicable to realistic working conditions of the investigated devices, i.e. room temperature application and realistic illumination conditions.},
  subject      = {Organischer Halbleiter},
  language  = {en}
}
@phdthesis{Koenig2007,
  author    = {K{\"o}nig, Markus},
  title     = {Spin-related transport phenomena in HgTe-based quantum well structures},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-27301},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2007},
  abstract  = {Within the scope of this thesis, spin related transport phenomena have been investigated in HgTe/HgCdTe quantum well structures. This material exhibits peculiar band structure properties, which result in a strong spin-orbit interaction of the Rashba type. An inverted band structure, i.e., a reversed ordering of the energy states in comparison to common semiconductors, is obtained for quantum well layers above a critical thickness. Furthermore, the band structure properties can be controlled in the experiments by moderate gate voltages. Most prominently, the type of carriers in HgTe quantum wells can be changed from n to p due to the narrow energy gap. Along with the inverted band structure, this unique transition is the basis for the demonstration of the Quantum Spin Hall state, which is characterized by the existence of two one-dimensional spin-polarized edge states propagating in opposite directions, while the Fermi level in the bulk is in the energy gap. Since elastic scattering is suppressed by time reversal symmetry, a quantized conductance for charge and spin transport is predicted. Our experiments provide the first experimental demonstration of the QSH state. For samples with characteristic dimensions below the inelastic mean free path, charge conductance close to the expected value of 2e^2/h has been observed. Strong indication for the edge state transport was found in the experiments as well. For large samples, potential fluctuations lead to the appearance of local n-conducting regions which are considered to be the dominant source of backscattering. When time reversal symmetry is broken in a magnetic field, elastic scattering becomes possible and conductance is significantly suppressed. The suppression relies on a dominant orbital effect in a perpendicular field and a smaller Zeeman-like effect present for any field direction. For large perpendicular fields, a re-entrant quantum Hall state appears. This unique property is directly related to the non-trivial QSH insulator state. While clear evidence for the properties of charge transport was provided, the spin properties could not be addressed. This might be the goal of future experiments. In another set of experiments, the intrinsic spin Hall effect was studied. Its investigation was motivated by the possibility to create and to detect pure spin currents and spin accumulation. A non-local charging attributed to the SHE has been observed in a p-type H-shaped structure with large SO interaction, providing the first purely electrical demonstration of the SHE in a semiconductor system. A possibly more direct way to study the spin Hall effects opens up when the spin properties of the QSH edge states are taken into account. Then, the QSH edge states can be used either as an injector or a detector of spin polarization, depending on the actual configuration of the device. The experimental results indicate the existence of both intrinsic SHE and the inverse SHE independently of each other. If a spin-polarized current is injected from the QSH states into a region with Rashba SO interaction, the precession of the spin can been observed via the SHE. Both the spin injection and precession might be used for the realization of a spin-FET similar to the one proposed by Datta and Das. Another approach for the realization of a spin-based FET relies on a spin-interference device, in which the transmission is controlled via the Aharonov-Casher phase and the Berry phase, both due to the SO interaction. In the presented experiments, ring structures with tuneable SO coupling were studied. A complex interference pattern is observed as a function of external magnetic field and gate voltage. The dependence on the Rashba splitting is attributed to the Aharonov-Casher phase, whereas effects due to the Berry phase remain unresolved. This interpretation is confirmed by theoretical calculations, where multi-channel transport through the device has been assumed in agreement with the experimental results. Thus, our experiments provide the first direct observation of the AC effect in semiconductor structures. In conclusion, HgTe quantum well structures have proven to be an excellent template for studying spin-related transport phenomena: The QSHE relies on the peculiar band structure of the material and the existence of both the SHE and the AC effect is a consequence of the substantial spin-orbit interaction. While convincing results have been obtained for the various effects, several questions can not be fully answered yet. Some of them may be addressed by more extensive studies on devices already available. Other issues, however, ask, e.g., for further advances in sample fabrication or new approaches by different measurements techniques. Thus, future experiments may provide new, compelling insights for both the effects discussed in this thesis and, more generally, other spin-orbit related transport properties.},
  subject      = {Spin-Bahn-Wechselwirkung},
  language  = {en}
}
@phdthesis{Fuchs2016,
  author    = {Fuchs, Moritz Jakob},
  title     = {Spin dynamics in the central spin model: Application to graphene quantum dots},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-136079},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {Due to their potential application for quantum computation, quantum dots have attracted a lot of interest in recent years. In these devices single electrons can be captured, whose spin can be used to define a quantum bit (qubit). However, the information stored in these quantum bits is fragile due to the interaction of the electron spin with its environment. While many of the resulting problems have already been solved, even on the experimental side, the hyperfine interaction between the nuclear spins of the host material and the electron spin in their center remains as one of the major obstacles. As a consequence, the reduction of the number of nuclear spins is a promising way to minimize this effect. However, most quantum dots have a fixed number of nuclear spins due to the presence of group III and V elements of the periodic table in the host material. In contrast, group IV elements such as carbon allow for a variable size of the nuclear spin environment through isotopic purification. Motivated by this possibility, we theoretically investigate the physics of the central spin model in carbon based quantum dots. In particular, we focus on the consequences of a variable number of nuclear spins on the decoherence of the electron spin in graphene quantum dots. Since our models are, in many aspects, based upon actual experimental setups, we provide an overview of the most important achievements of spin qubits in quantum dots in the first part of this Thesis. To this end, we discuss the spin interactions in semiconductors on a rather general ground. Subsequently, we elaborate on their effect in GaAs and graphene, which can be considered as prototype materials. Moreover, we also explain how the central spin model can be described in terms of open and closed quantum systems and which theoretical tools are suited to analyze such models. Based on these prerequisites, we then investigate the physics of the electron spin using analytical and numerical methods. We find an intriguing thermal flip of the electron spin using standard statistical physics. Subsequently, we analyze the dynamics of the electron spin under influence of a variable number of nuclear spins. The limit of a large nuclear spin environment is investigated using the Nakajima-Zwanzig quantum master equation, which reveals a decoherence of the electron spin with a power-law decay on short timescales. Interestingly, we find a dependence of the details of this decay on the orientation of an external magnetic field with respect to the graphene plane. By restricting to a small number of nuclear spins, we are able to analyze the dynamics of the electron spin by exact diagonalization, which provides us with more insight into the microscopic details of the decoherence. In particular, we find a fast initial decay of the electron spin, which asymptotically reaches a regime governed by small fluctuations around a finite long-time average value. Finally, we analytically predict upper bounds on the size of these fluctuations in the framework of quantum thermodynamics.},
  subject      = {Elektronenspin},
  language  = {en}
}
@phdthesis{Frey2011,
  author    = {Frey, Alexander},
  title     = {Spin-Dependent Tunneling and Heterovalent Heterointerface Effects in Diluted Magnetic II-VI Semiconductor Heterostructures},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-78133},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {The contribution of the present thesis consists of three parts. They are centered around investigating certain semiconductor heterointerfaces relevant to spin injection, exploring novel, diluted magnetic single barrier tunneling structures, and further developing diluted magnetic II-VI resonant tunneling diodes.},
  subject      = {Zwei-Sechs-Halbleiter},
  language  = {en}
}
@phdthesis{Edelhaeuser2011,
  author    = {Edelh{\"a}user, Lisa},
  title     = {Model Independent Spin Determination at Hadron Colliders},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-71030},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {Mit dem Ende des Jahres 2011 haben die beiden LHC-Experimente ATLAS und CMS jeweils ungef\"ahr 5 inverse Femtobarn an Daten bei einer Energie von 7 TeV aufgenommen. Die bisher analysierten Daten geben nur sehr vage Hinweise auf neue Physik an der TeV-Skala. Trotzdem erwartet man, dass sich an dieser Skala neue Physik zeigt, die bekannte Probleme des Standardmodells behebt. In den letzten Jahrzehnten wurden viele Erweiterungen des Standardmodells der Teilchenphysik und ihre Ph\"anomenologie dazu ausgearbeitet. Sobald sich neue Physik zeigt, stellt sich die Aufgabe, ihre Beschaffenheit und das zugrunde liegende Modell zu finden. Erste Hinweise k\"onnen nat\"urlich schon das Massenspektrum und die Quantenzahlen wie z.B. die elektrische und die Farbladung der neuen Teilchen liefern. \\ In zwei sehr bekannten und gut untersuchten Modellklassen, Supersymmetrie und Extradimensionen, haben neue Teilchen allerdings sehr \"ahnliche Eigenschaften an der erreichbaren Energieskala. Beide Modelle f\"uhren Partnerteilchen zu den bekannten Standardmodell-Teilchen ein, die, abgesehen von der Masse, sehr \"ahnliche Eigenschaften besitzen. Aus diesem Grund ist es n\"otig, weitere Kriterien zu ihrer Unterscheidung einzusetzen.\\ Ein hilfreicher Unterschied ergibt sich aus der Konstruktion beider Modelle: W\"ahrend in Modellen mit Extradimensionen die Partnerteilchen gleichen Spin wie die Standardmodell-Teichen haben, ist der Spin der Partnerteilchen in supersymmetrischen Modellen um 1/2 verschieden. Dieser Unterschied hat nun interessante Auswirkungen auf die jeweilige Ph\"anomenologie der Modelle.\\ Zum Beispiel kann man ausnutzen, dass die unterschiedlichen Spins die absoluten Wirkungsquerschnitte beeinflussen. Diese Methode setzt allerdings voraus, dass man die Massen und Kopplungsst\"arken sehr genau kennt. Eine weitere Herangehensweise nutzt aus, dass Winkelverteilungen vom Spin der involvierten Teilchen abh\"angen k\"onnen. Eine wichtige darauf basierende Methode stellt einen Zusammenhang zwischen der invariante-Masse-Verteilung \$d\Gamma/d\sff\$ zweier Zerfallsprodukte und dem Spin des intermedi\"aren Teilchens, \"uber welches der Zerfall abl\"auft, her.\\ In dieser Arbeit untersuchen wir als erstes den Einfluss von Operatoren h\"oherer Ordnung auf die Spinbestimmung in Zerfallsketten. Wir klassifizieren als erstes die relevanten Operatoren der Dimension 5 und 6. Wir berechnen die neuen Beitr\"age und diskutieren ihre Auswirkungen auf die Bestimmung von Kopplungen und Spin in diesen Zerf\"allen.\\ Im weiteren betrachten wir zwei Szenarien, die nicht die \"ublichen Zerfallsketten beinhalten:\\ In Dreik\"orperzerf\"allen kann die oben erw\"ahnte Methode nicht angewendet werden, da das intermedi\"are Teilchen nicht auf die Massenschale gehen kann. Solche off-shell'' Zerf\"alle k\"onnen in Szenarien wie split-Supersymmetrie oder split-Universal Extra Dimensions'' wichtig sein. Man kann hier die sogenannte Narrow width approximation'' nicht anwenden, welche eine notwendige Voraussetzung f\"ur einen einfachen Zusammenhang zwischen Spin und der invariante-Masse-Verteilung ist. Wir arbeiten eine Strategie f\"ur diese Dreik\"orperzerf\"alle aus, mittels derer man zwischen den unterschiedlichen Spinszenarien unterscheiden kann. Diese Strategie beruht darauf, dass man hier die differentielle Zerfallsbreite als globalen Phasenraumfaktor mal einem Polynom in der invarianten Masse \$\sff\$ schreiben kann. Die hierbei auftretenden Koeffizienten sind nur Funktionen der involvierten Massen und Kopplungen, und wir zeigen, wie beispielsweise ihre Wertebereiche und Vorzeichen dazu benutzt werden k\"onnen, um den zugrunde liegenden Zerfall zu bestimmen. Am Ende testen wir diese Strategie in einer Reihe von Monte Carlo-Simulationen, und diskutieren auch den Einfluss des off-shell'' Teilchens. Im letzten Teil betrachten wir eine Topologie mit sehr kurzen Zefallsketten, in der man den oben genannten Zusammenhang zwischen Spin und invarianter Masse ebenfalls nicht anwenden kann. Wir untersuchen eine bestimmte Variable, die zur Unterscheidung von Supersymmetrie und Universal Extra Dimensions'' eingef\"uhrt wurde. Dabei nutzt man aus, dass sich das Problem im Hochenergielimes auf die zugrunde liegenden Produktionsprozesse reduziert. Wir diskutieren, wie man diese Variable auch in Szenarien anwenden kann, in denen dieser Limes keine gute N\"aherung darstellt. Dazu betrachten wir die m\"oglichen Spinszenarien mit renormierbaren Kopplungen und untersuchen im Detail, wie gut diese Variable zwischen verschiedenen Spin-, Massen- und Kopplungsszenarien unterscheiden kann. Wir finden beispielsweise, dass das Spinszenario, welches den supersymmetrischen Fall beinhaltet, von den meisten anderen Spinszenarien gut unterscheidbar ist.},
  subject      = {Elementarteilchenphysik},
  language  = {en}
}