14 Externe wissenschaftliche Einrichtungen

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/15

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    Density functional and linear response studies of sp materials
    (2008) Kunstmann, Jens; Andersen, Ole Krogh (Prof. Dr.)
    In this thesis density functional theory and density functional perturbation theory are employed to study structural, electronic, and vibrational properties of sp materials, in particular boron, lithium, and aluminum. We develop a theory that describes the properties of the recently discovered boron nanotubes. Our theory is based on a structure model of a broad boron sheet, being a single quasiplanar layer of boron. Based on the properties of that boron sheet, we propose a new route to achieve control over the atomic structure of nanotubes during their synthesis. Our results show that structure control can be accomplished by nanotubes which are rolled up from sheets with anisotropic in-plane mechanical properties. We further study the high-pressure phase diagram of various bulk structures of boron. In particular, we investigate layered boron materials, which are a new family of hypothetical bulk phases which we regard as stacked arrangement of different broad boron sheets. These metallic materials are likely to exist at elevated pressures, or even at ambient conditions, and there are strong indications that they are conventional superconductors. Therefore, layered bulk phases of boron have the potential to explain the experimentally observed high-pressure superconductivity. Furthermore, we present the first realization of the generalized pseudoatom concept introduced by Ball, which we call enatom. This enatom is calculated using numerical linear response methods, and the enatom quantities are analyzed for both fcc Li and Al at pressures of 0, 35, and 50 GPa. These simple metals show different physical behaviors under pressure, which reflects the increasing covalency in Li and its absence in Al. Our results establish a method to construct the enatom, whose potential is to obtain a real-space understanding of solids, their vibrational properties, and electron-phonon interactions.
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    Electron phonon interaction in strongly correlated materials
    (2005) Rösch, Oliver; Andersen, Ole Krogh (Prof. Dr.)
    We study electron-phonon interaction in strongly correlated systems where the interplay with electron-electron interactions needs to be taken into account explicitly. We specifically consider cuprate high-temperature superconductors but also investigate in more general terms the influence of strong correlations. We first develop a model framework by deriving an effective low-energy model from a three-band model of the copper oxide planes in the cuprates allowing for the modulation of its parameters by lattice distortions. The electron-phonon interaction in the resulting t-J model with phonons is dominated by the on-site coupling due to phonon-induced changes in the large energy gain of Zhang-Rice singlets. Using exact diagonalization of finite clusters, we find that this model successfully describes the anomalous softening upon doping of the oxygen half-breathing mode in these compounds. Both the dependence on doping and the phonon wavevector are in good agreement with experiment. A comparison with results from a Hartree-Fock mean-field approximation of the three-band model shows the importance of treating strong correlations directly. After deriving the additional electron-phonon interaction that arises in undoped cuprates from the modulation of the electrostatic potential, we conclude that the coupling is strong enough to lead to the polaronic behavior seen in photoemission from these systems. Based on an adiabatic approximation, we explain the dispersion of the phonon side band according to predictions from purely electronic models and develop an efficient method for calculating spectra. Applying it to our model yields results that agree well with experiment. Finally, we use sum rules for the phonon and the electron self-energy to show generally that due to strong correlations the effect of electron-phonon interaction can be at variance from what is expected for non-interacting electrons. Electronic and phononic properties are affected differently leading to a mismatch of the respective apparent electron-phonon couplings. We also discuss the importance of vertex corrections to the electron-phonon interaction in strongly correlated materials.
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    Phenomenological approach to spin fluctuations in itinerant magnets and superconductors from ab initio calculations
    (2013) Ortenzi, Luciano; Andersen, Ole Krogh (Prof. Dr.)
    In this thesis I study the interplay between magnetism and superconductivity in itinerant magnets and superconductors. I do this by applying a semiphenomenological method to four representative compounds. In particular I use the discrepancies (whenever present) between density functional theory (DFT) calculations and the experiments in order to construct phenomenological models which explain the magnetic, superconducting and optical properties of four representative systems. I focus my attention on the superconducting and normal state properties of the recently discovered APt3P superconductors, on superconducting hole-doped CuBiSO, on the optical properties of LaFePO and finally on the ferromagnetic-paramagnetic transition of Ni3Al under pressure. At the end I present a new method which aims to describe the effect of spin fluctuations in itinerant magnets and superconductors that can be used to monitor the evolution of the electronic structure from non magnetic to magnetic in systems close to a quantum critical point.