Charge ordering in θ-(BEDT-TTF)2X materials

We theoretically investigate charge ordered states on the anisotropic triangular lattice characteristic of the theta-(BEDT-TTF)(2)X materials. Using exact diagonalization studies, we establish that the charge order (CO) pattern corresponds to a "horizontal" stripe structure, with...1100...CO along the two directions with larger electron hopping (p-directions), and...1010...CO along the third direction (c-direction). The CO is accompanied by co-operative bond dimerizations along all three directions in the highest spin state. In the lowest spin state bonds along the p-directions are tetramerized. Our theory explains the occurrence of a charge-induced high temperature transition as well as a spin gap transition at lower temperature.