Browsing by Author "Gulden, Tobias"

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    Epitaxial Growth of thin film strontium cobaltite: a feasibility study.
    (2012-06) Gulden, Tobias
    In this work we present a feasibility study of epitaxial growth of thin films of strontium cobaltite, SrCoO3-\delta. The properties of strontium doped lanthanum cobaltite, La1-xSrxCoO3, have been widely studied for dopant concentration x<0.5, but little work has been performed on the x=1 member of the series. The main issue is that this is not a thermodynamically preferable state and close to stochiometric SrCoO3 in polycrystalline samples can only be obtained under high pressures of oxygen (>10kbar) or by electrochemical oxidation. However, theoretical calculations predict a phase change with respect to strain in epitaxially grown samples, from ferromagnetic-metallic behaviour in the bulk state to insulating-ferroelectric-antiferromagnetic behaviour for strongly strained films. This provides strong motivation for epitaxial growth of SrCoO3-d films. In this work we will present a feasibility study by using the methods of high-pressure oxygen sputtering (typically 1.0-4.0mbar) on SrTiO3(001) and LaAlO3(001) substrates. As anticipated, the presence of oxygen vacancies is a severe problem, but also epitaxial stabilization of non-cubic phases, an unexpected issue, arises. These are found to grow in multiple orientations. Overall, the samples exhibit only weak or no ferromagnetism, even though bulk SrCoO3 is known to be a strong ferromagnet. Based on the results, we present an outline for suggested further research on this topic.
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    A Semiclassical Theory on Complex Manifolds with Applications in Statistical Physics and Quantum Mechanics
    (2016-08) Gulden, Tobias
    Increased interest in non-Hermitian quantum systems calls for the development of efficient methods to treat these. This interest was sparked by the introduction of PT-symmetry and the study of mathematical mappings which map conventional statistical or quantum mechanics onto non-Hermitian quantum operators. One of the most common methods in quantum mechanics is the semiclassial approximation which requires integration along trajectories that solve classical equations of motion. However in non-Hermitian systems these solutions are rarely attainable. We borrow concepts from algebraic topology to develop methods to avoid solving the equations of motion and avoid straightforward integration altogether. We apply these methods to solve the semiclassical problem for three largely dierent systems and demonstrate their usefulness for Hermitian and non-Hermitian systems alike.