Orbital Order and Bond-Length Fluctuations in Narrow-Band Oxides

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This project aims to study the novel physical properties encountered in transition-metal oxides at the transition from localized to itinerant electronic behavior and at orbital order-disorder transitions. The localized-itinerant electronic transition occurs where the dominant electronic energies change from intra-atomic to inter-atomic. This transition is first-order, and where phase separation would occur at too low a temperature for atomic diffusion, a dynamic spinodal segregation on a small length scale is accomplished by locally cooperative bond-length fluctuations. On lowering the temperature, ordering of the fluctuations may result in a charge-density wave that can be mobile or pinned. Orbital order-disorder transitions occur at localized configurations having an orbital degeneracy that is removed by lowering the local site symmetry. These Jahn-Teller distortions may be long-range cooperative and static or short-range dynamic to give another mechanism for bond-length fluctuations. Experiments have shown that the high-temperature superconductivity in the copper oxides occurs at a crossover from localized to itinerant electronic behavior. Colossal magnetoresistance in the manganese-oxide perovskites is associated with both a localized to itinerant electronic transition and the stabilization in a magnetic field of orbital fluctuations over long-range cooperative orbital ordering because orbital fluctuations give isotropic ferromagnetic interactions whereas static, long-range-cooperative distortions give antiferromagnetic order. Also, the importance of measuring on single crystals the transport and magnetic properties as a function of pressure as well as temperature allows fine tuning of the intraatomic versus interatomic interactions without changing the chemistry. The suppression of the phonon contribution to the thermal conductivity by bond-length fluctuations with an eye on how this phenomenon might be useful for peltier cooling will be studied. This project has a very strong educational component that provides graduate and post-doctoral students an opportunity to learn how to synthesize and characterize both chemically and structurally polycrystalline and single-crystal samples. This class of transition-metal oxides exhibits magnetic, transport, thermoelectric power, specific heat, and thermal conductivity behavior that suggests significant potential for high impact applications such as high performance magnetic and electronic devices and solid state refrigeration. Students trained in these areas will compete very well for available industrial and academic positions.