Unusual Electronic State of Layered θ-(ET)2 X Studied by Raman Spectroscopy

Employing Raman spectroscopy, we have studied the electronic structure of the charge-ordering system, θ−(ET)2X, the dihedral angle (φ) of which ranges from 100 to 121°. The most stable metal, θ−(ET)2I3, (φ=100°) has a uniform charge distribution in a whole temperature range. The compounds with φ > 111° have an inhomogeneous charge distribution even above the CO transition, which implies that the charges have localized nature in a whole temperature range. These localized charges are long-range ordered in the CO phase, whereas they are fluctuating randomly or forming short-range ordered domains above the CO phase transition.