Condensed Matter - Theory

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Allen 9725037 This is a renewal award to conduct theoretical research on metallic, or nearly metallic, oxides and on glasses. The normal modes of thermal vibration in glassy solids have been characterized in computer studies of up to 4096 atoms. These models yield realistic vibrations for amorphous silicon. Four new projects are planned: (1) Clarify the concept of diffusivity of normal modes; (2) Find connections between low energy "resonant"modes and low energy structural rearrangements which couple strongly to the resonant modes; (3) Develop a theory of the viscosity of the thermal vibrations and apply this to the study of internal friction in glasses; (4) Develop a new theory of hopping conduction of heat when localized states are concentrated. Although most important oxides are insulating, some are very good metals and the most interesting ones are on the borderline. The transition from metallic to insulating has usually two equally strong and very different driving forces, Coulomb correlation and lattice distortion. The former causes great theoretical difficulty even in the simplest fully interacting model, whereas the latter, solvable in principle, has multiple manifestations depending on the chemical species and crystal structure. A study of BaBiO3, where Coulomb effects are minimal, is currently being completed. To extract the common physics of many oxides, a simplified model due to Rice and Sneddon is used, where the only lattice degrees of freedom is the displacement of oxygen atoms parallel to the metal-oxygen bond. This not only captures the breathing distortion that drives the Bi(3+)/Bi(5+) disproportionation (and causes insulating behavior), but also captures the principle Jahn- Teller displacement which couples to the metal-insulator behavior of Eg metals and T2g metals and many isoelectronic materials. %%% This theoretical research will continue studies of metal oxide materials and their unusual properties. These materials are intr iguing due to their widely varied properties. Some conduct electricity, like normal metals, while others are insulating. Some are on the borderline between metallic properties and insulating properties. The research supported by this grant will investigate this difficult, but technologically important class of materials. ***