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

窄带氧化物中的轨道顺序和键长波动

• 批准号: 0132282
• 负责人: John Goodenough
• 金额: $ 39.2万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-02-01 至 2005-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0132282&HistoricalAwards=false
• 关键词: Orbital; Order; Bond; Length; Fluctuations

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.

该项目旨在研究过渡金属氧化物在从局域电子行为转变为巡回电子行为以及轨道有序-无序转变时遇到的新物理性质。当主要电子能量从原子内变为原子间时，发生局域巡回电子跃迁。 这种转变是一阶的，并且在原子扩散温度太低的情况下会发生相分离，小长度尺度上的动态旋节线偏析是通过局部协同键长波动来实现的。 在降低温度时，波动的排序可能会产生可移动或固定的电荷密度波。 轨道有序-无序转变发生在具有轨道简并性的局部构型处，通过降低局部位点对称性来消除轨道简并性。 这些 Jahn-Teller 畸变可能是长程协同、静态或短程动态，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域电子行为到巡回电子行为的交叉处。氧化锰钙钛矿中的巨大磁阻与局部巡回电子跃迁和长程协同轨道排序上轨道涨落磁场的稳定有关，因为轨道涨落产生各向同性铁磁相互作用，而静态长程协同畸变则产生反铁磁有序。 此外，在单晶上测量作为压力和温度函数的输运和磁性的重要性使得可以在不改变化学性质的情况下微调原子内与原子间相互作用。 将研究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有助于珀耳帖冷却。该项目具有非常强大的教育成分，为研究生和博士后学生提供了学习如何合成和表征化学和结构多晶和单晶样品的机会。这类过渡金属氧化物表现出磁性、输运、热电势、比热和导热性能，这表明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巨大潜力。在这些领域接受过培训的学生将能够很好地竞争可用的工业和学术职位。