Study of the charge ordering in organic conductors

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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.