Browsing by Author "Kunstmann, Jens"

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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.