Charge ordering in θ-(BEDT-TTF)2RbZn(SCN)4:: Cooperative effects of electron correlations and lattice distortions

Combined effects of electron correlations and lattice distortions on the charge ordering in theta-(BEDT-TTF)(2)RbZn(SCN)(4) are investigated theoretically in a two-dimensional 3/4-filled extended Hubbard model with electron-lattice couplings. It is known that this material undergoes a phase transition from a high-symmetry metallic state to a low-symmetry insulating state with a horizontal-stripe charge order (CO) by lowering temperature. By means of the exact-diagonalization method, we show that electron-phonon interactions are crucial to stabilize the horizontal-stripe CO and to realize the low-symmetry crystal structure.