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

本项目旨在研究过渡金属氧化物在从局部电子行为到流动电子行为的转变和轨道有序-无序转变中遇到的新物理性质。局域巡回电子跃迁发生在主导电子能从原子内到原子间变化的地方。这种转变是一级的，当相分离发生在原子扩散的温度过低时，小长度尺度上的动态旋量偏析是通过局部协同键长波动来完成的。在降低温度时，波动的顺序可能会产生可移动或固定的电荷密度波。轨道有序-无序跃迁发生在具有轨道简并的局域构型上，这种简并通过降低局域位对称性而消除。这些扬-泰勒扭曲可能是长期合作的和静态的，也可能是短期动态的，从而给出了键长波动的另一种机制。实验表明，铜氧化物中的高温超导性发生在从局部电子行为到流动电子行为的交叉处。锰氧化物钙钛矿中的巨磁电阻与局域到巡回的电子跃迁和轨道波动在远程协同轨道有序中的磁场稳定有关，因为轨道波动产生各向同性铁磁相互作用，而静态、远程协同扭曲则产生反铁磁有序。此外，在单晶上测量输运和磁性作为压力和温度的函数的重要性允许在不改变化学性质的情况下微调原子内与原子间的相互作用。通过键长波动抑制声子对热导率的贡献，并着眼于这种现象如何可能对珀尔帖冷却有用，将进行研究。该项目具有很强的教育成分，为研究生和博士后提供了一个学习如何合成和表征多晶和单晶样品的化学和结构的机会。这类过渡金属氧化物具有磁性、输运性、热电功率、比热和导热性能，在高性能磁性和电子设备以及固态制冷等高影响应用中具有重要潜力。在这些领域受过训练的学生将在工业和学术职位上竞争得很好。

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.