@phdthesis{Seiberlich2008,
  author    = {Seiberlich, Nicole},
  title     = {Advances in Non-Cartesian Parallel Magnetic Resonance Imaging using the GRAPPA Operator},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28321},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {Magnetic Resonance Imaging (MRI) is an imaging modality which provides anatomical or functional images of the human body with variable contrasts in an arbitrarily positioned slice without the need for ionizing radiation. In MRI, data are not acquired directly, but in the reciprocal image space (otherwise known as k-space) through the application of spatially variable magnetic field gradients. The k-space is made up of a grid of data points which are generally acquired in a line-by-line fashion (Cartesian imaging). After the acquisition, the k-space data are transformed into the image domain using the Fast Fourier Transformation (FFT). However, the acquisition of data is not limited to the rectilinear Cartesian sampling scheme described above. Non-Cartesian acquisitions, where the data are collected along exotic trajectories, such as radial and spiral, have been shown to be beneficial in a number of applications. However, despite their additional properties and potential advantages, working with non-Cartesian data can be complicated. The primary difficulty is that non-Cartesian trajectories are made up of points which do not fall on a Cartesian grid, and a simple and fast FFT algorithm cannot be employed to reconstruct images from non-Cartesian data. In order to create an image, the non-Cartesian data are generally resampled on a Cartesian grid, an operation known as gridding, before the FFT is performed. Another challenge for non-Cartesian imaging is the combination of unusual trajectories with parallel imaging. This thesis has presented several new non-Cartesian parallel imaging methods which simplify both gridding and the reconstruction of images from undersampled data. In Chapter 4, a novel approach which uses the concepts of parallel imaging to grid data sampled along a non-Cartesian trajectory called GRAPPA Operator Gridding (GROG) is described. GROG shifts any acquired k-space data point to its nearest Cartesian location, thereby converting non-Cartesian to Cartesian data. The only requirements for GROG are a multi-channel acquisition and a calibration dataset for the determination of the GROG weights. Chapter 5 discusses an extension of GRAPPA Operator Gridding, namely Self-Calibrating GRAPPA Operator Gridding (SC-GROG). SC-GROG is a method by which non-Cartesian data can be gridded using spatial information from a multi-channel coil array without the need for an additional calibration dataset, as required in standard GROG. Although GROG can be used to grid undersampled datasets, it is important to note that this method uses parallel imaging only for gridding, and not to reconstruct artifact-free images from undersampled data. Chapter 6 introduces a simple, novel method for performing modified Cartesian GRAPPA reconstructions on undersampled non-Cartesian k-space data gridded using GROG to arrive at a non-aliased image. Because the undersampled non-Cartesian data cannot be reconstructed using a single GRAPPA kernel, several Cartesian patterns are selected for the reconstruction. Finally, Chapter 7 discusses a novel method of using GROG to mimic the bunched phase encoding acquisition (BPE) scheme. In MRI, it is generally assumed that an artifact-free image can be reconstructed only from sampled points which fulfill the Nyquist criterion. However, the BPE reconstruction is based on the Generalized Sampling Theorem of Papoulis, which states that a continuous signal can be reconstructed from sampled points as long as the points are on average sampled at the Nyquist frequency. A novel method of generating the "bunched" data using GRAPPA Operator Gridding (GROG), which shifts datapoints by small distances in k-space using the GRAPPA Operator instead of employing zig-zag shaped gradients, is presented in this chapter. With the conjugate gradient reconstruction method, these additional "bunched" points can then be used to reconstruct an artifact-free image from undersampled data. This method is referred to as GROG-facilitated Bunched Phase Encoding, or GROG-BPE.},
  subject      = {NMR-Tomographie},
  language  = {en}
}
@phdthesis{Galmbacher2007,
  author    = {Galmbacher, Matthias},
  title     = {Lernen mit dynamisch-ikonischen Repr{\"a}sentationen aufgezeigt an Inhalten zur Mechanik},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-29271},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2007},
  abstract  = {Im Physikunterricht wurde lange Zeit die Bedeutung quantitativer Zusammenh{\"a}nge f{\"u}r das Physiklernen {\"u}berbewertet, qualitative Zusammenh{\"a}nge spielten dagegen eine eher untergeordnete Rolle. Dies f{\"u}hrte dazu, dass das Wissen der Sch{\"u}ler zumeist ober­fl{\"a}ch­lich blieb und nicht auf neue Situationen angewendet werden konnte. TIMSS und Pisa offenbarten diese Schwierigkeiten. In den Abschlussberichten wurde kritisiert, dass die Sch{\"u}ler kaum in der Lage seien, Lernstoff zu transferieren oder pro­blem­l{\"o}send zu denken. Um physikalische Abl{\"a}ufe deuten und entsprechende Probleme l{\"o}sen zu k{\"o}nnen, ist qua­litativ-konzeptuelles Wissen n{\"o}tig. Dieses kann, wie Forschungs­ergebnisse belegen, am besten durch die konstruktivistisch motivierte Gestaltung von Lern­situationen sowie durch die Inte­gration externer Repr{\"a}sentationen von Versuchs­aussagen in den Schul­unter­richt er­reicht werden. Eine konkrete Umsetzung dieser Bedingungen stellt der Ein­satz rechner­gest{\"u}tzter Experimente dar, der heutzutage ohne allzu großen technischen Aufwand rea­lisiert werden kann. Diese Experimente erleichtern es dem Lernenden, durch den direk­ten Umgang mit realen Abl{\"a}ufen, physikalische Konzepte zu erschließen und somit qua­litative Zusammenh{\"a}nge zu verstehen. W{\"a}hrend man lange Zeit von einer grunds{\"a}tzlichen Lernwirksamkeit animierter Lern­um­gebungen ausging, zeigen dagegen neuere Untersuchungen eher Gegenteiliges auf. Sch{\"u}ler m{\"u}ssen offensichtlich erst lernen, wie mit multicodierten Re­pr{\"a}­sentationen zu arbeiten ist. Die vorliegende Arbeit will einen Beitrag dazu leisten, he­raus­zufinden, wie lernwirksam sogenannte dynamisch-ikonische Repr{\"a}sentationen (DIR) sind, die physikalische Gr{\"o}ßen vor dem Hintergrund konkreter Versuchsabl{\"a}ufe visuali­sieren. Dazu bearbeiteten im Rahmen einer DFG-Studie insgesamt 110 Sch{\"u}ler jeweils 16 Projekte, in denen mechanische Konzepte (Ort, Geschwindigkeit, Beschleu­nigung und Kraft) aufgegriffen wurden. Es zeigte sich, dass die Probanden mit den ein­ge­setzten DIR nicht erfolgreicher lernen konnten als ver­gleich­bare Sch{\"u}ler, die die gleichen Lerninhalte ohne die Unter­st{\"u}tzung der DIR erarbeiteten. Im Gegen­teil: Sch{\"u}ler mit einem geringen visuellen Vorstellungsverm{\"o}gen schnitten aufgrund der Darbietung einer zus{\"a}tzlichen Codierung schlechter ab als ihre Mit­sch{\"u}ler. Andererseits belegen Untersuchungen von Blaschke, dass solche Repr{\"a}sen­ta­tionen in der Erarbeitungsphase einer neu entwickelten Unter­richts­kon­zep­tion auch und gerade von schw{\"a}cheren Sch{\"u}lern konstruktiv zum Wissens­erwerb genutzt werden konnten. Es scheint also, dass die Lerner zun{\"a}chst Hilfe beim Umgang mit neuartigen Re­pr{\"a}­sen­ta­tions­formen ben{\"o}tigen, bevor sie diese f{\"u}r den weiteren Aufbau ad{\"a}qua­ter physi­ka­lischer Modelle nutzen k{\"o}nnen. Eine experimentelle Unter­suchung mit Sch{\"u}­lern der 10. Jahrgangsstufe best{\"a}tigte diese Vermutung. Hier lernten 24 Probanden in zwei Gruppen die mechanischen Konzepte zu Ort, Geschwin­dig­keit und Beschleunigung kennen, bevor sie im Unter­richt behandelt wurden. W{\"a}hrend die Teil­nehmer der ersten Gruppe nur die Simulationen von Bewegungsabl{\"a}ufen und die zuge­h{\"o}rigen Liniendiagramme sahen, wurden f{\"u}r die zweite Gruppe unterst{\"u}tzend DIR eingesetzt, die den Zusammenhang von Bewe­gungs­ablauf und Linien­diagramm veranschaulichen sollten. In beiden Gruppen war es den Probanden m{\"o}glich, Fragen zu stellen und Hilfe von einem Tutor zu erhalten. Die Ergebnis­se zeigten auf, dass es den Sch{\"u}lern durch diese Maßnahme erm{\"o}glicht wurde, die DIR erfolgreich zum Wissens­er­werb einzusetzen und sig­nifikant besser abzuschneiden als die Teilnehmer in der Kon­troll­­gruppe. In einer weiteren Untersuchung wurde abschließend der Frage nachgegangen, ob DIR unter Anleitung eines Tutors eventuell bereits in der Unterstufe sinnvoll eingesetzt werden k{\"o}nnen. Ausgangspunkt dieser {\"U}berlegung war die Tatsache, dass mit der Einf{\"u}hrung des neuen bayerischen G8-Lehrplans wesentliche Inhalte, die Bestand­teil der vorherigen Untersuchungen waren, aus dem Physik­unterricht der 11. Jgst. in die 7. Jahrgangsstufe verlegt wurden. So bot es sich an, mit den Inhalten auch die DIR in der Unterstufe ein­zusetzen. Die Un­tersuchungen einer quasiexperimentellen Feldstudie in zwei siebten Klassen belegten, dass die betrachte­ten Repr{\"a}sentationen beim Aufbau entsprechender Kon­zepte keinesfalls hinderlich, sondern sogar f{\"o}rder­lich sein d{\"u}rften. Denn die Sch{\"u}ler­gruppe, die mit Hilfe der DIR lernte, schnitt im direkten hypothesenpr{\"u}fenden Vergleich mit der Kontrollklasse deutlich besser ab. Ein Kurztest, der die Nachhaltigkeit des Gelernten nach etwa einem Jahr {\"u}berpr{\"u}fen sollte, zeigte zudem auf, dass die Sch{\"u}ler der DIR-Gruppe die Konzepte, die unter Zuhilfenahme der DIR erarbeitet wurden, im Vergleich zu Sch{\"u}lern der Kontrollklasse und zu Sch{\"u}lern aus 11. Klassen insgesamt {\"u}berraschend gut verstanden und behalten hatten.},
  subject      = {Multimedia},
  language  = {de}
}
@phdthesis{Fechner2008,
  author    = {Fechner, Susanne},
  title     = {Quantenkontrolle im Zeit-Frequenz-Phasenraum},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28569},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {Die in der vorliegenden Arbeit eingef{\"u}hrte von Neumann-Darstellung beschreibt jeden Laserpuls auf eineindeutige Weise als Summe von an verschiedenen Punkten des Zeit-Frequenz-Phasenraumes zentrierten, bandbreitebegrenzten Gaußimpulsen. Diese Laserpulse bilden sozusagen die „elementaren" Bausteine, aus denen jeder beliebige Lichtimpuls konstruiert werden kann. Die von Neumann-Darstellung vereint eine Reihe von Eigenschaften, die sie f{\"u}r eine Anwendung auf dem Gebiet der Quantenkontrolle besonders geeignet erscheinen l{\"a}sst. So ist sie eine bijektive Abbildung zwischen den Freiheitsgraden des verwendeten Impulsformers und der Phasenraumdarstellung der resultierenden, geformten Laserpulse. Jeder denkbaren Wahl von Impulsformerparametern entspricht genau eine von Neumann-Darstellung und umgekehrt. Trotzdem erm{\"o}glicht sie, ebenso wie die Husimi- oder die Wigner-Darstellung, eine intuitive Interpretation der dargestellten Lichtimpulse, da deren zeitliche und spektrale Struktur sofort zu erkennen ist.},
  subject      = {Femtosekundenlaser},
  language  = {de}
}
@phdthesis{Behr2008,
  author    = {Behr, Volker Christian},
  title     = {Entwicklung und Optimierung von Resonatoren und Detektionsverfahren in der magnetischen Kernspinresonanz},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28635},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {No abstract available},
  subject      = {Kernspintomograph},
  language  = {de}
}
@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{Wolpert2008,
  author    = {Wolpert, Daniel},
  title     = {Quantum Control of Photoinduced Chemical Reactions},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-27171},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {The control of quantum mechanical processes, especially the selective manipulation of photochemical reactions by shaped fs laser pulses was successfully demonstrated in many experiments in the fields of physics, chemistry and biology. In this work, attention is directed to the control of two systems that mark a bridge to real synthetic chemistry. In a liquid phase environment the outcome of the photo-induced Wolff rearrangement of an industrially relevant diazonaphthoquinone compound, normally used in photoresists (e.g. Novolak) was optimized using shaped fs laser pulses. In the second series of experiments chemical reactions on a catalyst metal surface which comprise laser induced molecular bond formation channels were selectively manipulated for the first time. The control of liquid phase reactions necessitates adequate spectroscopic signals that are characteristic for the formed product species. Therefore, a pump-probe setup for transient absorption spectroscopy in the mid-infrared for the purpose of investigating ultrafast structural changes of molecules during photoreactions was constructed. This versatile setup enables to monitor structural changes of molecules in the liquid phase and to find appropriate feedback signals for the control of these processes. Prior to quantum control experiments, the photoinduced Wolff-rearrangement reaction of 2-diazo-1-naphthoquinone (DNQ) dissolved in water and methanol was thoroughly investigated. Steady state absorption measurements in the mid-infrared in combination with quantum chemical density functional theory (DFT) calculations revealed the characteristic vibrational bands of DNQ and of possible products. A mid-infrared transient absorption study was performed, to illuminate the structural dynamics of the ultrafast rearrangement reaction of DNQ. The experimental observations indicate, that the Wolff rearrangement reaction of DNQ proceeds within 300 fs. A model for the relaxation dynamics of the ketene photoproduct and DNQ after photoexcitation can be deduced that fits the measured data very well. The object of the quantum control experiments on DNQ was the improvement of the ketene yield. It was shown that the ketene formation after Wolff rearrangement of DNQ is very sensitive to the shape of the applied excitation laser pulses. The variation of single parameters, like the linear chirp as well as the pulse separation of colored double pulses lead to the conclusion that the well known intrapulse dumping mechanism is responsible for the impact of the frequency ordering within the excitation pulse on the photoproduct yield. Adaptive optimizations using a closed learning loop basically lead to the same result. Adaptive fs quantum control was also applied to surface reactions on a catalyst metal surface for the first time. Therefore, the laser-induced catalytic reactions of carbon monoxide (CO) and hydrogen (H2) on a Pd(100) single crystal surface were studied. This photochemical reaction initiated with fs laser pulses has not been observed before. Several product molecules could be synthesized, among them also species (e.g. CH^3+) for whose formation three particles are involved. The systematic variation of different parameters showed that the reactions are sensitive to the catalyst surface, the composition of the adsorbate and to the laser properties. A pump-probe study revealed that they occur on an ultrafast time scale. These catalytic surface reactions were then investigated and improved with phaseshaped fs laser pulses. By applying a feedback optimal control scheme, the reaction outcome could be successfully manipulated and the ratio of different reaction channels could be selectively controlled. Evidence has been found that the underlying control mechanism is nontrivial and sensitive to the specific conditions on the surface. The experiments shown here represent the first successful experiment on adaptive fs quantum control of a chemical reaction between adsorbate molecules on a surface. In contrast to previous quantum control experiments, reaction channels comprising the formation of new molecular bonds rather than the cleavage of already existing bonds are controlled. This work successfully showed that quantum control can be extended to systems closer to situations encountered in synthetic chemistry as was demonstrated in the two examples of the optimization of a complicated rearrangement reaction and the selective formation of chemical bonds with shaped fs laser pulses.},
  subject      = {Nichtlineare Spektroskopie},
  language  = {en}
}
@phdthesis{Heidemann2008,
  author    = {Heidemann, Robin},
  title     = {Non-Cartesian Parallel Magnetic Resonance Imaging},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26893},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {Besides image contrast, imaging speed is probably the most important consideration in clinical magnetic resonance imaging (MRI). MR scanners currently operate at the limits of potential imaging speed, due to technical and physiological problems associated with rapidly switched gradient systems. Parallel imaging (parallel MRI or pMRI) is a method which allows one to significantly shorten the acquisition time of MR images without changing the contrast behavior of the underlying MR sequence. The accelerated image acquisition in pMRI is accomplished without relying on more powerful technical equipment or exceeding physiological boundaries. Because of these properties, pMRI is currently employed in many clinical routines, and the number of applications where pMRI can be used to accelerate imaging is increasing. However, there is also growing criticism of parallel imaging in certain applications. The primary reason for this is the intrinsic loss in the SNR due to the accelerated acquisition. In addition, other effects can also lead to a reduced image quality. Due to unavoidable inaccuracies in the pMRI reconstruction process, local and global errors may appear in the final reconstructed image. The local errors are visible as noise enhancement, while the global errors result in the so-called fold-over artifacts. The appearance and strength of these negative effects, and thus the image quality, depend upon different factors, such as the parallel imaging method chosen, specific parameters in the method, the sequence chosen, as well as specific sequence parameters. In general, it is not possible to optimize all of these parameters simultaneously for all applications. The application of parallel imaging in can lead to very pronounced image artifacts, i.e. parallel imaging can amplify errors. On the other hand, there are applications such as abdominal MR or MR angiography, in which parallel imaging does not reconstruct images robustly. Thus, the application of parallel imaging leads to errors. In general, the original euphoria surrounding parallel imaging in the clinic has been dampened by these problems. The reliability of the pMRI methods currently implemented is the main criticism. Furthermore, it has not been possible to significantly increase the maximum achievable acceleration with parallel imaging despite major technical advances. An acceleration factor of two is still standard in clinical routine, although the number of independent receiver channels available on most MR systems (which are a basic requirement for the application of pMRI) has increased by a factor of 3-6 in recent years. In this work, a novel and elegant method to address this problem has been demonstrated. The idea behind the work is to combine two methods in a synergistic way, namely non-Cartesian acquisition schemes and parallel imaging. The so-called non-Cartesian acquisition schemes have several advantages over standard Cartesian acquisitions, in that they are often faster and less sensitive to physiological noise. In addition, such acquisition schemes are very robust against fold-over artifacts even in the case of vast undersampling of k-space. Despite the advantages described above, non-Cartesian acquisition schemes are not commonly employed in clinical routines. A reason for that is the complicated reconstruction techniques which are required to convert the non-Cartesian data to a Cartesian grid before the fast Fourier transformation can be employed to arrive at the final MR image. Another reason is that Cartesian acquisitions are routinely accelerated with parallel imaging, which is not applicable for non-Cartesian MR acquisitions due to the long reconstruction times. This negates the speed advantage of non-Cartesian acquisition methods. Through the development of the methods presented in this thesis, reconstruction times for accelerated non-Cartesian acquisitions using parallel imaging now approach those of Cartesian images. In this work, the reliability of such methods has been demonstrated. In addition, it has been shown that higher acceleration factors can be achieved with such techniques than possible with Cartesian imaging. These properties of the techniques presented here lead the way for an implementation of such methods on MR scanners, and thus also offer the possibility for their use in clinical routine. This will lead to shorter examination times for patients as well as more reliable diagnoses.},
  subject      = {NMR-Bildgebung},
  language  = {en}
}
@phdthesis{Scheibner2007,
  author    = {Scheibner, Ralf},
  title     = {Thermoelectric Properties of Few-Electron Quantum Dots},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26699},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2007},
  abstract  = {This thesis presents an experimental study of the thermoelectrical properties of semiconductor quantum dots (QD). The measurements give information about the interplay between first order tunneling and macroscopic quantum tunneling transport effects in the presence of thermal gradients by the direct comparison of the thermoelectric response and the energy spectrum of the QD. The aim of the thesis is to contribute to the understanding of the charge and spin transport in few-electron quantum dots with respect to potential applications in future quantum computing devices. It also gives new insight into the field of low temperature thermoelectricity. The investigated QDs were defined electrostatically in a two dimensional electron gas (2DEG) formed with a GaAs/(Al,Ga)As heterostructure by means of metallic gate electrodes on top of the heterostructure. Negative voltages with respect to the potential of the 2DEG applied to the gate electrodes were used to deplete the electron gas below them and to form an isolated island of electron gas in the 2DEG which contains a few ten electrons. This QD was electrically connected to the 2DEG via two tunneling barriers. A special electron heating technique was used to create a temperature difference between the two connecting reservoirs across the QD. The resulting thermoelectric voltage was used to study the charge and spin transport processes with respect to the discrete energy spectrum and the magnetic properties of the QD. Such a two dimensional island usually exhibits a discrete energy spectrum, which is comparable to that of atoms. At temperatures below a few degrees Kelvin, the electrostatic charging energy of the QDs exceeds the thermal activation energy of the electrons in the leads, and the transport of electrons through the QD is dominated by electron-electron interaction effects. The measurements clarify the overall line shape of thermopower oscillations and the observed fine structure as well as additional spin effects in the thermoelectrical transport. The observations demonstrate that it is possible to control and optimize the strength and direction of the electronic heat flow on the scale of a single impurity and create spin-correlated thermoelectric transport in nanostructures, where the experimenter has a close control of the exact transport conditions. The results support the assumption that the performance of thermoelectric devices can be enhanced by the adjustment of the QD energy levels and by exploiting the properties of the spin-correlated charge transport via localized, spin-degenerate impurity states. Within this context, spin entropy has been identified as a driving force for the thermoelectric transport in the spin-correlated transport regime in addition to the kinetic contributions. Fundamental considerations, which are based on simple model assumptions, suggest that spin entropy plays an important role in the presence of charge valence fluctuations in the QD. The presented model gives an adequate starting point for future quantitative analysis of the thermoelectricity in the spin-correlated transport regime. These future studies might cover the physics in the limit of single electron QDs or the physics of more complex structures such as QD molecules as well as QD chains. In particular, it should be noted that the experimental investigations of the thermopower of few-electron QDs address questions concerning the entropy transport and entropy production with respect to single-bit information processing operations. These questions are of fundamental physical interest due to their close connection to the problem of minimal energy requirements in communication, and thus ultimately to the so called "Maxwell's demon" with respect to the second law of thermodynamics.},
  subject      = {Quantenpunkt},
  language  = {en}
}
@phdthesis{Pracht2007,
  author    = {Pracht, Eberhard},
  title     = {Entwicklung und Optimierung von Bildgebungssequenzen f{\"u}r die 1H-Magnetresonanztomographie der Lunge},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26398},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2007},
  abstract  = {No abstract available},
  subject      = {NMR-Tomographie},
  language  = {de}
}
@phdthesis{Arnold2008,
  author    = {Arnold, Johannes F. T.},
  title     = {Funktionelle Bildgebung der Lunge und des Bronchialkarzinoms mittels Magnetresonanztomographie},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26388},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {Ziel dieser Arbeit war es, die Magnetresonanztomographie (MRT) an der Lunge als Alternative zur traditionellen Lungenbildgebung voranzutreiben. So sollten MRT-Verfahren zur regionalen und quantitativen Lungenfunktionspr{\"u}fung f{\"u}r die klinische Routine entwickelt werden. Im Hinblick auf die Strahlentherapie von Patienten mit Bronchialkarzinom sollen funktionelle Lungenareale erkannt werden, um diese w{\"a}hrend der Bestrahlung optimal schonen zu k{\"o}nnen. An den zahlreichen Luft-Gewebe-Grenzfl{\"a}chen in der Lunge entstehen Magnetfeldinhomogenit{\"a}ten. Daraus resultiert ein schneller Zerfall des MRT-Signals in der Lunge. Es wurde in dieser Arbeit ein Ansatz aufgezeigt, um die Ursache f{\"u}r den raschen Signalzerfall, n{\"a}mlich die unterschiedlichen magnetischen Suszeptibilit{\"a}ten von Luftr{\"a}umen und Lungengewebe, zu beseitigen. Durch die intravaskul{\"a}re Injektion von paramagnetischen Kontrastmitteln kann die Suszeptibilit{\"a}t des Blutes an die Suszeptibilit{\"a}t der Luftr{\"a}ume angeglichen werden. Durch die Entwicklung einer MR-kompatiblen aktiven Atemkontrolle (MR-ABC) wurde in dieser Arbeit ein weiteres fundamentales Problem der Lungen-MRT adressiert: Die Bewegung w{\"a}hrend der Datenakquisition. Die MR-ABC detektiert Herzschlag und Atemposition und ist in der Lage die Atembewegung in jeder beliebigen Atemphase reproduzierbar f{\"u}r eine definierte Zeit auszusetzen. Dies wird durch einen Verschluss der Atemluftzufuhr realisiert. Traditionelle Verfahren k{\"o}nnen zwar ebenfalls die Atemphase detektieren, gestatten jedoch nicht deren Konservierung. Es wurde demonstriert, dass mit der MR-ABC hochaufl{\"o}sende Bilder der Lunge in hoher Bildqualit{\"a}t und durch die Verwendung langer Akquisitionsfenster in relativ kurzer Messzeit erreicht werden k{\"o}nnen. Eine regionale Lungenfunktionspr{\"u}fung ist f{\"u}r die Diagnose und Evaluierung vieler Krankheitsbilder vorteilhaft. In diesem Sinne wird seit einigen Jahren das Potential der Sauerstoff-verst{\"a}rkten Lungen-MRT erforscht, die auf den paramagnetischen Eigenschaften des molekularen Sauerstoffs basiert. Im Blut gel{\"o}ster Sauerstoff f{\"u}hrt zu einer Verk{\"u}rzung der T1-Relaxationszeit. Statt diese T1-Verk{\"u}rzung quantitativ zu bestimmen wird aus praktischen Gr{\"u}nden meist ein T1-gewichteter Ansatz gew{\"a}hlt. In dieser Arbeit wurde jedoch gezeigt, dass nicht-quantitative Verfahren ein erhebliches Risiko zur Falschinterpretation beinhalten. Um Fehldiagnosen zu vermeiden, sollten deshalb prinzipiell quantitative Methoden zur Messung der durch die Sauerstoff-Verst{\"a}rkung bedingten T1-Verk{\"u}rzung in der Lunge verwendet werden. Herk{\"o}mmliche Techniken zur quantitativen T1-Messung ben{\"o}tigen allerdings l{\"a}ngere Messzeiten. Deshalb war zur Vermeidung von Bewegungsartefakten bisher die Datenaufnahme im Atemanhaltezustand notwendig. Wiederholtes Atemanhalten von mehreren Sekunden Dauer ist allerdings f{\"u}r einige Patienten sehr belastend. Aus diesem Grund wurden in dieser Arbeit zwei Methoden entwickelt, die eine quantitative Lungenfunktionspr{\"u}fung mittels MRT bei freier Atmung der Patienten erm{\"o}glichen. Eine gute Sauerstoffversorgung des Tumors wirkt sich positiv auf den Erfolg der Bestrahlung aus. Ein Ansatz zur Verbesserung der Strahlentherapie des Bronchialkarzinoms k{\"o}nnte daher in der Beatmung der Patienten mit hyperoxischen hypercapnischen Atemgasen w{\"a}hrend der Bestrahlung bestehen. In diesem Zusammenhang k{\"o}nnte die quantitative Messung der T1-Ver{\"a}nderung im Tumor nach Carbogenatmung ein Selektionskriterium darstellen, um diejenigen Patienten zu identifizieren, die von einer Carbogenbeatmung w{\"a}hrend der Bestrahlung profitieren k{\"o}nnen. Die Differenzierung zwischen vitalem Tumorgewebe, Nekrosen und atelektatischem Lungengewebe ist von großer Bedeutung bei der Bestrahlungsplanung des Bronchialkarzinoms. Einen neuen Ansatz bildet die in dieser Arbeit vorgestellte Magnetiserungstransfer-MRT. Um einen Magnetisierungstransfer zu erzeugen, wurde ein speziell auf die Bildgebung an der Lunge optimiertes Pr{\"a}parationsmodul entworfen. In Verbindung mit einer schnellen Bildakquisitionstechnik konnte die Magnetisierungstransfer-Lungenbildgebung in einem kurzen Atemstopp durchgef{\"u}hrt werden. Diese Technik wurde an mehreren Patienten mit Bronchialkarzinom evaluiert und die Ergebnisse mit denen der Fluor-Deoxyglykose-Positronen-Emissions-Tomographie (FDG-PET) verglichen. Es wurde festgestellt, dass mit diesem MRT-Verfahren {\"a}hnliche diagnostische Erkenntnisse erzielt werden k{\"o}nnen. Allerdings besitzt die MRT Vorteile im Hinblick auf r{\"a}umliche Aufl{\"o}sung, Messzeit, Bildqualit{\"a}t, Kosten und Strahlenbelastung. Das erhebliche Potential f{\"u}r die Bestrahlungsplanung des Bronchialkarzinoms durch eine Magnetisierungstransfer-Bildgebung wurde damit nachgewiesen.},
  subject      = {Magnetische Resonanz},
  language  = {de}
}