@phdthesis{Schmitt2022,
  author    = {Schmitt, Matthias},
  title     = {High Energy Spin- and Momentum-Resolved Photoelectron Spectroscopy of Complex Oxides},
  doi       = {10.25972/OPUS-26475},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-264757},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Spin- and \(k\)-resolved hard X-ray photoelectron spectroscopy (HAXPES) is a powerful tool to probe bulk electronic properties of complex metal oxides. Due to the low efficiency of common spin detectors of about \(10^{-4}\), such experiments have been rarely performed within the hard X-ray regime since the notoriously low photoionization cross sections further lower the performance tremendously. This thesis is about a new type of spin detector, which employs an imaging spin-filter with multichannel electron recording. This increases the efficiency by a factor of \(10^4\) and makes spin- and \(k\)-resolved photoemission at high excitation energies possible. Two different technical approaches were pursued in this thesis: One using a hemispherical deflection analyzer (HDA) and a separate external spin detector chamber, the other one resorting to a momentum- or \(k\)-space microscope with time-of-flight (TOF) energy recording and an integrated spin-filter crystal. The latter exhibits significantly higher count rates and - since it was designed for this purpose from scratch - the integrated spin-filter option found out to be more viable than the subsequent upgrade of an existing setup with an HDA. This instrumental development is followed by the investigation of the complex metal oxides (CMOs) KTaO\(_3\) by angle-resolved HAXPES (HARPES) and Fe\(_3\)O\(_4\) by spin-resolved HAXPES (spin-HAXPES), respectively. KTaO\(_3\) (KTO) is a band insulator with a valence-electron configuration of Ta 5\(d^0\). By angle- and spin-integrated HAXPES it is shown that at the buried interface of LaAlO\(_3\)/KTO - by the generation of oxygen vacancies and hence effective electron doping - a conducting electron system forms in KTO. Further investigations using the momentum-resolution of the \(k\)-space TOF microscope show that these states are confined to the surface in KTO and intensity is only obtained from the center or the Gamma-point of each Brillouin zone (BZ). These BZs are furthermore square-like arranged reflecting the three-dimensional cubic crystal structure of KTO. However, from a comparison to calculations it is found that the band structure deviates from that of electron-doped bulk KTaO\(_3\) due to the confinement to the interface. There is broad consensus that Fe\(_3\)O\(_4\) is a promising material for spintronics applications due to its high degree of spin polarization at the Fermi level. However, previous attempts to measure the spin polarization by spin-resolved photoemission spectroscopy have been hampered by the use of low photon energies resulting in high surface sensitivity. The surfaces of magnetite, though, tend to reconstruct due to their polar nature, and thus their magnetic and electronic properties may strongly deviate from each other and from the bulk, dependent on their orientation and specific preparation. In this work, the intrinsic bulk spin polarization of magnetite at the Fermi level (\(E_F\)) by spin-resolved photoelectron spectroscopy, is determined by spin-HAXPES on (111)-oriented thin films, epitaxially grown on ZnO(0001) to be \(P(E_F) = -80^{+10}_{-20}\) \%.},
  subject      = {Elektronenkorrelation},
  language  = {en}
}
@phdthesis{Henn2014,
  author    = {Henn, Tobias},
  title     = {Hot spin carriers in cold semiconductors : Time and spatially resolved magneto-optical Kerr effect spectroscopy of optically induced electron spin dynamics in semiconductor heterostructures},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-110265},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {The present thesis "Hot spin carriers in cold semiconductors" investigates hot carrier effects in low-temperature photoinduced magneto-optical Kerr effect (MOKE) microscopy of electron spins in semiconductor heterostructures. Our studies reveal that the influence of hot photocarriers in magneto-optical pump-probe experiments is twofold. First, it is commonly assumed that a measurement of the local Kerr rotation using an arbitrary probe wavelength maps the local electron spin polarization. This is the fundamental assumption that underlies the widely used two-color MOKE microscopy technique. Our continuous-wave (cw) spectroscopy experiments demonstrate that this assumption is not correct. At low lattice temperatures the nonresonant spin excitation by the focused pump laser inevitably leads to a strong heating of the electron system. This heating, in turn, locally modifies the magneto-optical coefficient which links the experimentally observed Kerr rotation to the electron spin polarization. As a consequence, the spin-induced local Kerr rotation is augmented by spin-unrelated changes in the magneto-optical coefficient. A spatially resolved measurement of the Kerr rotation then does not correctly map the electron spin polarization profile. We demonstrate different ways to overcome this limitation and to correctly measure the electron spin profile. For cw spectroscopy we show how the true local electron spin polarization can be obtained from a quantitative analysis of the full excitonic Kerr rotation spectrum. Alternatively, picosecond MOKE microscopy using a spectrally broad probe laser pulse mitigates hot-carrier effects on the magneto-optical spin detection and allows to directly observe the time-resolved expansion of optically excited electron spin packets in real-space. Second, we show that hot photocarriers strongly modify the spin diffusion process. Owing to their high kinetic energy, hot carriers greatly enhance the electron spin diffusion coefficient with respect to the intrinsic value of the undisturbed system. Therefore, for steady-state excitation the spin diffusivity is strongly enhanced close to the pump spot center where hot electrons are present. Similarly, for short delays following pulsed excitation the high initial temperature of the electrons leads to a very fast initial expansion of the spin packet which gradually slows as the electrons cool down to the lattice temperature. While few previous publications have recognized the possible influence of hot carriers on the electron spin transport properties, the present work is the first to directly observe and quantify such hot carrier contributions. We develop models which for steady-state and pulsed excitation quantitatively describe the experimentally observed electron spin diffusion. These models are capable of separating the intrinsic spin diffusivity from the hot electron contribution, and allow to obtain spin transport parameters of the undisturbed system. We perform extensive cw and time-resolved spectroscopy studies of the lattice temperature dependence of the electron spin diffusion in bulk GaAs. Using our models we obtain a consistent set of parameters for the intrinsic temperature dependence of the electron spin diffusion coefficient and spin relaxation time and the hot carrier contributions which quantitatively describes all experimental observations. Our analysis unequivocally demonstrates that we have, as we believe for the first time, arrived at a coherent understanding of photoinduced low-temperature electron spin diffusion in bulk semiconductors.},
  subject      = {Galliumarsenid},
  language  = {en}
}
@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{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}
}