Uwe Stephan Dissertation This work is concerned with methods and results for the calculation of electronic properties of amorphous carbon models (a-C). These investigations are based upon a very efficient non-selfconsistent ab-initio procedure for the evaluation of electronic states of extended systems using modified self- consistent DFT-LDA states and potentials of neutral atoms. Starting from the LCAO matrices constructed in this method, the electronic densities of states (DOS) of model systems are calculated by diagonalization or with use of the recursion method. Both techniques and, in particular, several versions of the recursion method will be investigated and compared with respect to their numerical efficiency and practical applicability. For DOS calculations in carbon systems a modification of the atomic SCF routine will be proposed and tested in application to the crystalline carbon allotropes diamond and graphite. In this work, the investigation of a-C structures is based on various structural models which have been generated in the author's research group by means of molecular-dynamics simulations using the empirical Tersoff potential as well as the just mentioned DFT-LDA approach. The total energy in this latter procedure is calculated as the sum of the band-structure energy and an empirical repulsive pair potential; contrary to the purely empirical approach, this scheme therefore includes pi-bonding effects and gives rise to a superior description of defect states in these models. As suggested by an analysis of the localization properties of the eigenstates, the defect structure in a-C models depends primarily on the ability of pi- and weak-sigma-bonded undercoordinated atoms to cluster. To investigate these clustering effects, a pi-bonding analysis will be proposed which enables the quantification and classification of the defect states and the estimation of gaps between pi bands. This procedure, which will be justified by local DOS calculations, provides essential structure-property correlations in dependence on the mass densities of the models. Within predominantly fourfold-coordinated models, the occurrence of a certain fraction of threefold-coordinated atoms turns out to stabilize the network by achieving optimum stress and defect minimization due to the preferred formation of pi-bonded atom pairs. Such models exhibit mass densities and pi gaps of about 3.0 g/cm^3 and 2.4 eV, respectively, in close agreement with recent experimental results.

The thesis is devoted to the study of in-chain impurities in spin 1/2 antiferromagnetic Heisenberg chains (S=1/2 aHC's)---a model which accompanies the research on magnetism since the early days of quantum theory and which is one of the few integrable spin systems. With respect to impurities it is special insofar as an impurity perturbs the system strongly due to its topology: there is no way around the defect. To what extend the one-dimensional picture stays a good basis for the description of real materials even if the chains are disturbed by in-chain impurities is an interesting question which is addressed in this work. For this purpose, Cu Nuclear Magnetic Resonance (NMR) measurements on the cuprate spin chain compounds SrCuO2 and Sr2CuO3 intentionally doped with nickel (Ni), zinc (Zn) and palladium (Pd) are presented. These materials are well known to be among the best realizations of the S=1/2 aHC model and their large exchange coupling constants allow the investigation of the low-energy dynamics within experimentally easily feasible temperatures. NMR provides the unique ability to study the static and dynamic magnetic properties of the spin chains locally which is important since randomly placed impurities break the translational invariance. Because copper is the magnetically active ion in those materials and the copper nuclear spin is most directly coupled to its electron spin, the NMR measurements have been performed on the copper site. The measurements show in all cases that there are changes in the results of these measurements as compared to the pure compounds which indicate the opening of gaps in the excitation spectra of the spin chains and the emergence of oscillations of the local susceptibility close to the impurities. These experimental observations are compared to theoretical predictions to clarify if and to what extend the already proposed model for these doped systems---the finite spin chain---is suitable to predict the behavior of real materials. Thereby, each impurity shows peculiarities. While Zn and Pd are know to be spin 0 impurities, it is not clear if Ni carries spin 1. To shed some light on this issue is another scope of this work. For Zn impurities, there are indications that they avoid to occupy copper sites, other than in the layered cuprate compounds. Also this matter is considered.

Background: Ablative surgery of oropharyngeal tumors frequently leads to defects in the speech organs, resulting in impairment of speech up to the point of unintelligibility. The aim of the present study was the assessment of selected parameters of speech with and without resection prostheses. Patients and Methods: The speech sounds of 22 patients suffering from maxillary and mandibular defects were recorded using a digital audio tape (DAT) recorder with and without resection prostheses. Evaluation of the resonance and the production of the sounds /s/, /sch/, and /ch/ was performed by 2 experienced speech therapists. Additionally, the patients completed a non-standardized questionnaire containing a linguistic self-assessment. Results: After prosthesis supply, the number of patients with rhinophonia aperta decreased from 7 to 2 while the number of patients with intelligible speech increased from 2 to 20. Correct production of the sounds /s/, /sch/, and /ch/ increased from 2 to 13 patients. A significant improvement of the evaluated parameters could be observed only in patients with maxillary defects. The linguistic self-assessment showed a higher satisfaction in patients with maxillary defects. Conclusion: In patients with maxillary defects due to ablative tumor surgery, an increase in speech performance and intelligibility is possible by supplying resection prostheses.

Die Rekombinationsaktivität von Versetzungen und Korngrenzen in multikristallinem Silicium wird von Kupfer und anderen metallischen Verunreinigungen wie Eisen mitbestimmt. Das Hauptziel der Arbeit war es, die Verteilung von Kupfer und dessen Wirkung auf die Rekombinationsaktivität von Versetzungen und Korngrenzen genauer zu untersuchen. Dazu wurden optische und elektrische Untersuchungen an gezielt mit Metallen verunreinigten Modellmaterialien durchgeführt. Nicht alle Versetzungen sind rekombinationsaktiv. Es konnte gezeigt werden, dass der Anteil rekombinationsaktiver Versetzungen am Gesamtinventar und die Hintergrunddiffusionslänge von der Verunreinigung mit Metallen abhängig sind. Ergebnisse von Untersuchungen an Proben, die Diffusionsexperimenten unterzogen wurden, deuten auf unterschiedliches Ausscheidungsverhalten von Kupfer und Eisen hin sowie auf Wechselwirkungen mit Versetzungen und Korngrenzen, die mit der Diffusionstemperatur und den Abkühlbedingungen in Zusammenhang stehen.

This work investigates the defect energies, band alignments, and charge carrier recombination in polycrystalline Cu(In1-xGax)(Se1-ySy)2 chalcopyrite thin films and the interrelationship with the alloy composition. Photoluminescence spectroscopy of investigated Cu-poor Cu(In,Ga)(Se,S)2 layers generally shows broad emission lines with the corresponding maxima shifting towards higher energies under decreasing temperature or under increasing excitation power. Admittance spectroscopy of Cu-poor ZnO/CdS/Cu(In,Ga)(Se,S)2 chalcopyrite devices shows that the activation energies of the dominant defect distributions involving donors at the CdS/absorber interface and deep acceptors in the chalcopyrite bulk, increase upon alloying CuInSe2 with S. The band alignments within the Cu(In1-xGax)(Se1-ySy)2 system are determined using the energy position of the bulk acceptor state as a reference. The band gap enlargement under Ga alloying is accommodated almost exclusively in the rise of the conduction band edge, whereas the increase of band gap upon alloying with S is shared between comparable valence and conduction band offsets. The extrapolated band discontinuities [delta]EV(CuInSe2/CuInS2) = -0.23 eV, [delta]EC(CuInSe2/CuInS2) = 0.21 eV, [delta]EV(CuInSe2/CuGaSe2) = 0.036 eV, and [delta]EC(CuInSe2/CuGaSe2) = 0.7 eV are in good agreement with theoretical predictions. Current-voltage analysis of Cu-poor ZnO/CdS/Cu(In,Ga)(Se,S)2 devices reveals recombination barriers which follow the band gap energy of the absorber irrespective of alloy composition, as expected for dominant recombination in the chalcopyrite bulk. In turn, the recombination at the active junction interface prevails in Cu-rich devices which display substantially smaller barriers when compared to the band gap energy of the absorber. The result indicates that the Cu-stoichiometry is the driving compositional parameter for the charge carrier recombination in the chalcopyrite heterojunctions under investigations.

III-valent dopants have shown to be most effective in stabilizing the ferroelectric, crystalline phase in atomic layer deposited, polycrystalline HfO₂ thin films. On the other hand, such dopants are commonly used for tetragonal and cubic phase stabilization in ceramic HfO₂. This difference in the impact has not been elucidated so far. The prospect is a suitable doping to produce ferroelectric HfO₂ ceramics with a technological impact. In this paper, we investigate the impact of Al, Y, and La doping, which have experimentally proven to stabilize the ferroelectric Pca21 phase in HfO₂, in a comprehensive first-principles study. Density functional theory calculations reveal the structure, formation energy, and total energy of various defects in HfO₂. Most relevant are substitutional electronically compensated defects without oxygen vacancy, substitutional mixed compensated defects paired with a vacancy, and ionically compensated defect complexes containing two substitutional dopants paired with a vacancy. The ferroelectric phase is strongly favored with La and Y in the substitutional defect. The mixed compensated defect favors the ferroelectric phase as well, but the strongly favored cubic phase limits the concentration range for ferroelectricity. We conclude that a reduction of oxygen vacancies should significantly enhance this range in Y doped HfO₂ thin films. With Al, the substitutional defect hardly favors the ferroelectric phase before the tetragonal phase becomes strongly favored with the increasing concentration. This could explain the observed field induced ferroelectricity in Al-doped HfO₂. Further Al defects are investigated, but do not favor the f-phase such that the current explanation remains incomplete for Al doping. According to the simulation, doping alone shows clear trends, but is insufficient to replace the monoclinic phase as the ground state. To explain this fact, some other mechanism is needed.

Verbindungshalbleiter mit einer großen Bandlücke wie Galliumnitrid (GaN) sind aufgrund ihrer hervorragenden elektronischen Eigenschaften für die Halbleiterindustrie von großem Interesse. Die Hochtemperatur-Gasphasenepitaxie, die auf dem physikalischen Gasphasentransport von Gallium basiert, ist eine alternative Methode der Gasphasenepitaxie von GaN. Im Mittelpunkt der vorliegenden Arbeit standen die Weiterentwicklung der Methode hinsichtlich der Verringerung der Kontamination und die Reduzierung der Versetzungsdichte in den GaN-Schichten. Dazu wurde eine neue Verdampfungszelle entwickelt und die komplexen, mehrstufigen Nukleations- und Wachstumsprozesse systematisch untersucht. Insgesamt wurden zu kommerziell verfügbaren GaN-Schichten vergleichbare Defektdichten erreicht. Zusätzlich wurde die Methode zur Abscheidung auf Saphir-Substraten mit einem Durchmesser von bis zu 2 Zoll aufskaliert.

Die konforme Feldtheorie findet in verschiedenen Bereichen Anwendungen, von statistischen Systemen in der Nähe kritischer Punkte bis hin zur Quantengravitation durch die AdS/CFT-Korrespondenz. Diese Theorien unterliegen starken Einschränkungen, die eine systematische nicht-perturbative Analyse ermöglichen. Konforme Defekte bieten eine kontrollierte Möglichkeit, die Symmetrie zu brechen und neue physikalische Phänomene einzuführen, während wichtige Vorteile der zugrunde liegenden konformen Symmetrie erhalten bleiben. Diese Dissertation untersucht konforme Liniendefekte sowohl im schwachen als auch im starken Kopplungsregimes. Es werden zwei verschiedene Klassen von Modellen untersucht. Wir konzentrieren uns zuerst auf die supersymmetrische Wilson-Linie in N = 4 Super Yang-Mills, die als ideales Testfeld für die Entwicklung innovativer Techniken wie dem analytischen konformen Bootstrap dient. Die zweite Klasse besteht aus magnetische Linien in Yukawa-Modellen, die faszinierende Anwendungen in 3d kondensierten Materiesystemen haben. Diese Systeme haben das Potenzial, Phänomene des Standardmodells in einem Niedrigenergieszenario nachzubilden. / Conformal field theory finds applications across diverse fields, from statistical systems at criticality to quantum gravity through the AdS/CFT correspondence. These theories are subject to strong constraints, enabling a systematic non-perturbative analysis. Conformal defects provide a controlled means of breaking the symmetry, introducing new physical phenomena while preserving crucial benefits of the underlying conformal symmetry. This thesis investigates conformal line defects in both the weak- and strong-coupling regimes. Two distinct classes of models are studied. First, we focus on the supersymmetric Wilson line in N = 4 Super Yang–Mills, which serves as an ideal testing ground for the development of innovative techniques such as the analytic conformal bootstrap. The second class consists of magnetic lines in Yukawa models, which have fascinating applications in 3d condensed-matter systems. These systems have the potential to emulate phenomena observed in the Standard Model in a low-energy setting.

Ziel der Arbeit ist die theoretische Analyse von Kohlenstoff-Clustern der Größe 100 - 500 Atome. Die experimentellen Beobachtungen sind bei dieser geringen Anzahl der Atome schwierig. Anderseits sind Kenntnisse über solche Cluster sehr wichtig, z.B. für die Keimbildung von Diamant auf Substraten, oder für die Kohlenstoff-Nanotechnologie (Fullerene, Nanotubes), oder für strukturelle Defekte in Kohlenstoff-Systemen. Es wurden gekrümmte Grenzflächen im Diamant simuliert. Zuerst mit einem empirischen Potential. Es wurde danach eine Methode entwickelt, bei der die schwach gestörten Gebiete einem empirischen Potential gehorchen, und die stark gestörten Gebiete, wo eine genaue Berechnung erforderlich ist, durch eine quantenmechanische Näherung beschrieben wurden. Somit kann man mit guter Genauigkeit große Systeme, bestehend aus einigen 10 (hoch)4 Atomen, simulieren, bei denen nur lokal quantenmechanische Methoden erforderlich sind. Mit diesem Hybrid-Code wurden weiterhin Diamantkeime auf Silizium gerechnet. Es wurden Aussagen bezüglich der Stabilität des Diamants auf dem Siliziumsubstrat, der kritischen Keimgröße, der Änderungen, die der Keim erfährt, gemacht. Ein anderes Gebiet ist die molekulardynamische Simulation bezüglich der Stabilität und des Transformationsverhaltens von Kohlenstoff-Nanoteilchen. Es wurden als »Rohstoffe» sowohl Diamant- und Graphitkristalle sphärischer, ellipsoidischer oder quadratischer Form benutzt, als auch amorpher Kohlenstoff. Es wurde demonstriert, daß sich Diamant unter höherer Temperatur und Bestrahlung in Kohlenstoffzwiebeln transformiert. Es wurde der innere Kern, bestehend aus zwei Schalen, der Kohlenstoffzwiebel simuliert. Es wurde, nach meinem Wissen, zum ersten Mal gezeigt, daß zwischen den Schalen der Kohlenstoffzwiebel Quer-Verbindungen (cross-links) existieren. Diese waren von den Experimentatoren vermutet worden. Sie bilden die Initiatoren der Diamantkeime der Kohlenstoffzwiebel bei ihrer ohne äußeren Druck möglichen Transformation in Diamant. Die Zentren der Kohlenstoffzwiebeln befinden sich bereits in der Entstehung der Zwiebel unter einem Selbstdruck. Bei den größeren Kohlenstoffzwiebeln beträgt der experimentell bestimmte Abstand zwischen den Schalen von außen nach innen von 3.34 Å bis 2.2 Å. Anlagen: nano1.mpg (91,8 MB); nano2.mpg (131 MB) Nutzung: Referat Informationsvermittlung der SLUB / The scope of this work is the analysis of carbon clusters of about 100 - 500 atoms. The experimental studies are at such small clusters heavy. Knowledges about thus clusters are very important, for example in the field of the nucleation of diamond on substrates, or for the carbon nano-technology (fullerene, nanotubes), or for local defects in carbon systems. There were simulated curved interfaces in diamond. Firstly with an empirical potential. Than I developed a method, in wich the defects and the structure around them are treated by a quantum mechanical algorithm and the rest with a near to ideal structure with an empirical potential. So, it is possible an accurate calculation of great systems of about 10 (high)4 atoms on wich only locally quantum mechanical methods are necessary. With this hybrid-code diamond nuclei on silicon substrate were simulated. The stability of the diamond nuclei on the silicon substrate, the critical radius of the nuclei and the changes of the nuclei during his transformation was investigated. Another field of investigations is the molecular dynamics simulation of free carbon clusters. The initial structures had spherical, ellipsoidical or square form and consists of diamond and graphite or a free form in the case of amorphous carbon. It was demonstrated that diamond transforms at higher temperatures and under irradiation in carbon onions. The genesis of the nucleus of a carbon onion with two shells was here for the first time simulated. The existence of the cross-links between the shells of a carbon onion was demonstrated. These existence was expected from the experimentators. The cross-links are the initiators of the transformation of carbon onions to diamond. The center of carbon onions is under self-pressure, because the distance between the outer shells is about 3.34 Å and between the inner shells about 2.2 Å. Appendix: nano1.mpg (91,8 MB); nano2.mpg (131 MB) Usage: Referat Informationsvermittlung/ SLUB

Owing to their versatile orbital character with both local and highly dispersive degrees of freedom, transition metal oxides span the range of ionic, covalent and metallic bonding. They exhibit a vast diversity of electronic phenomena such as high dielectric, piezoelectric, pyroelectric, ferroelectric, magnetic, multiferroic, catalytic, redox, and superconductive properties. The nature of these properties arises from sensitive details in the electronic structure, e.g. orbital mixing and orbital hybridization, due to non-stoichiometry, atomic displacements, broken symmetries etc., and their coupling with external perturbations. In the work presented here, these variations of the electronic structure of crystals due to structural and electronic defects have been investigated, exemplarily for the quasi-binary system SrO - SrTiO3 - TiO2. A number of binary and ternary structures have been studied, both experimentally as well as by means of electronic modeling. The applied methods comprise Resonant X-ray Scattering techniques like Diffraction Anomalous Fine Structure, Anisotropy of Anomalous Scattering and X-ray Absorption Fine Structure, and simultaneously extensive electronic calculations by means of Density Functional Theory and Finite Difference Method Near-Edge Structure to gain a thorough physical understanding of the underlying processes, interactions and dynamics. It is analyzed in detail how compositional variations, e.g. manifesting as oxygen vacancies or ordered stacking faults, alter the short-range order and affect the electronic structure, and how the severe changes in mechanical, optical, electrical as well as electrochemical properties evolve. Various symmetry-property relations have been concluded and interpreted on the basis of these modifications in electronic structure for the orbital structure in rutile TiO2, for distorted TiO6 octahedra and related switching mechanisms of the Ti valence, for elasticity and resistivity in strontium titanate, and for surface relaxations in Ruddlesden-Popper phases. Highlights of the thesis include in particular the methodical development regarding Resonant X-Ray Diffraction, such as the first use of partially forbidden reflections to get the complete phase information not only of the tensorial structure factor but of each individual atomic scattering tensor for a whole spectrum of energies, as well as the determination of orbital degrees of freedom and details of the partial local density of states from these tensors. On the material side, the most prominent results are the identification of the migration-induced field-stabilized polar phase and the exergonic redox behavior in SrTiO3 caused by defect migration and defect separation.