Browsing by Author "Grussendorff, Sharon Joy."
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Item First principles quantum mechanical studies of iridium : a focus on bulk and surface properties.(2003) Grussendorff, Sharon Joy.; Chetty, Nithaya.Recent high-pressure experiments on iridium show a transition to a 14 atomic layer superlattice structure. Since iridium has a high bulk modulus, it is used in many high-pressure applications, for instance as a gasket for high-temperature, high-pressure diamond anvil cell experiments. The effects of pressure on this material are hence of interest. Of the transition metals, the iridium surface has been one of the most extensively studied surfaces experimentally. The field ion microscope has made it possible to observe in detail the behaviour of adatoms on the surface, and has led to interesting discoveries of the nature of atomic adsorption on the lr(111) surface. A number of theoretical and semi-empirical studies have been made on this topic. However, none of these studies take atomic relaxations into account in a satisfactory manner, and therefore do not give a complete understanding of the process of incorporation of adatoms onto the surface. In the present work, first-principles total energy calculations based on the plane wave pseudopotential method within the framework of the density functional theory are employed in the study of the bulk properties of iridium, and the crystal phases and defect structures of iridium under pressure. The bond-orientation model is extended to include the effects of pressure, and used to compute all of the ~2n defect structures of iridium as a function of atomic volume. Allowance for full atomic relaxations is made in computing the ideal and relaxed surface formation energies of the three low-index surfaces of iridium, and in investigating the nature of adsorption of single adatoms on the lr(111) surface. The formation energy of a vacancy on the Ir(111) surface is also computed. This is the first time such a calculation has been made.Item A theoretical investigation of birefringences in cubic and uniaxial magnetic crystals.(1997) Grussendorff, Sharon Joy.; Raab, Roger Edouard.In this thesis a consistent multipole theory is used to describe light propagation in nonabsorbing magnetic cubic and uniaxial crystals to the order of electric octopoles and magnetic quadrupoles. The first chapter extends Maxwell's equations for a vacuum to their macroscopic form in matter by including bound-source contributions as multipole expansions. From these the corresponding forms for D and H are obtained, which ensure origin-independence of Maxwell's equations. A multipole eigenyalue equation describing light propagation in a source-free homogeneous medium is then derived, which is the basic equation applied in this thesis. In the second chapter it is shown how, from the multipolar form of the propagation equation for transverse waves, expressions can be derived for the various birefringences that may be exhibited in macroscopic platelets of the medium, as introduced by Jones in the formulation of his M-matrix. The following chapter presents the derivation, by means of first-order perturbation theory, of the quantum mechanical expressions for the polarizability tensors which enter the eigenvalue wave equation. The origin independence of the expressions for the various observable quantities is then established. In the fourth chapter the independent components of the polarizability tensors are calculated for two selected crystal point groups, namely 622 and 432, by way of illustration. In chapter five the components calculated in the manner illustrated in the previous chapter are presented in tabular form. The Jones method outlined in chapter two is then applied to the crystal point group 6m2, again as an illustration of the method used to determine the optical effects displayed by this point group. Chapter five concludes with a table containing a listing of the predicted optical effects calculated in this way for all of the magnetic uniaxial and cubic point groups. The thesis concludes with chapter six, in which a summary of the results of the work undertaken is given, together with a discussion of factors influencing measurements of the predicted optical effects.