Study of the charge ordering in organic conductors

有机导体中的电荷有序源于一个非常窄的带宽。我们用电荷敏感的分子内振动模式观察了电荷分布。在这个研究项目中，我们取得了以下成果。(1)拉曼有源电荷敏感模式的响应并不像第一阶段预期的那么简单。在理论计算的基础上，我们阐明了当系统处于带电有序状态时，电子-分子振动耦合常数a-mv的大小如何导致振动带分裂。我们发现在BEDT-TTF的电荷转移盐中，弱耦合的ν_2模式有助于估计歧化比，强耦合的ν_3模式有助于获得有序电荷模式的对称性。(2)除了在θ-(BEDT-TTF)_RbZn(SCN)_4中呈水平条纹外，在θ-(BDT-TTF)_2Cu(NCS)_2中发现了垂直条纹，在θ-(BEDT-TTF)_2TlZn(SCN)_4中发现了斜条纹。这些发现支持了理论预测，即实际材料位于这些条纹相之间的边界附近。(3)系统地研究了θ型BEDT-TTF盐的电子相图。θ型BEDT-TTF盐的低温绝缘相为电荷有序态。通过拉曼光谱分析，我们认为高温导电相不是相干金属态，而是载流子非相干跳变的非金属态。随着相邻BEDT-TTF分子间二面角的减小，跳变速率增大。θ-(BEDT-TTF)_2CsZn(SCN)_4位于绝缘相和金属相（或超导相）的分界处。我们发现该化合物的绝缘相不是电荷有序态，而是弱局域态。(4)我们证实了β " -(BEDT-TTF)_3X_2 （X=ReO_4， ClO_4和HSO_4）的相变伴随着电荷顺序。

The charge ordering in organic conductors originates from a very narrow bandwidth. We have observed the charge distribution using charge-sensitive intra-molecular vibrational modes. In this research project, we have achieved the following results. (1) The response of the Raman-active charge-sensitive mode is not so simple as it was expected in the first stage. Based on the theoretical calculation, we elucidated how the vibronic bands split depending upon the magnitude of the a-mv (electron-molecular vibration) coupling constant when the system undergoes a charged-ordered state. We found that in the charge-transfer salts of BEDT-TTF, the weakly coupled ν_2 mode was useful for estimating the disproportionation ratio and the strongly coupled ν_3 mode was useful for obtaining the symmetry of the ordered-charge pattern. (2) We found a vertical stripe in θ-(BDT-TTF)_2Cu(NCS)_2 and a diagonal stripe in θ-(BEDT-TTF)_2TlZn(SCN)_4 in addition to the horizontal stripe in θ-(BEDT-TTF)_RbZn(SCN)_4. These findings support the theoretical prediction that the actual materials are located near the boundary among these stripe phases. (3) The electronic phase diagram of θ-type BEDT-TTF salts was systematically studied. The low-temperature insulating phase of θ-type BEDT-TTF salts was identified as the charge-ordered state. We suggested through the Raman spectra that the high-temperature conducting phase is not a coherent metallic state but a non-metallic state in which the charge carriers are hopping incoherently. The hopping rate increases as the dihedral angle between adjacent BEDT-TTF molecules decreases. θ-(BEDT-TTF)_2CsZn(SCN)_4 is located at the boundary which separates the insulating and metallic (or superconducting) phase. We found that the insulating phase of this compound is not a charge-ordered state but a weakly localized state. (4) We have verified that the phase transitions of β''-(BEDT-TTF)_3X_2 (X=ReO_4, ClO_4, and HSO_4) are accompanied by charge ordering.