Non-stripe charge order in the θ-phase organic conductors

Phase diagrams of charge order patterns of the theta-phase organic conductors are investigated on the basis of the static-limit extended Hubbard model including the second nearest Coulomb repulsion V. Since the theta-phase is close to the triangular lattice rather than the square lattice, a 3-fold non-stripe charge order is stable in a considerably large region between the stripe phases and the uniform phase. At finite temperatures, such unevenly ordered patterns as the 3-fold phase D+D21/4+ become more stable than the evenly ordered stripes, (D+D0), and it is unlikely that the stripe phase directly transforms to the uniform phase. These results suggest the existence of an unevenly ordered pattern in the metallic state of theta-(BEDT-TTF)(2)RbM(SCN)(4) [BEDT-TTF: bis(ethyleneditho)tetrathiafulvalene, M = Zn and Co] above T-MI = 190 K, accounting for the observation of modulated structures in the X-ray measurement, broadening of the NMR line, flat temperature dependence of the resistivity, and the first-order character of the 190 K transition.