Role of electron-lattice couplings on charge order in quasi-two-dimensional organic conductors

Charge ordering with structural distortion in quasi-two-dimensional organic conductors θ-(ET)₂RbZn(SCN)₄ (ET=BEDT-TTF) and α-(ET)₂I₃ is investigated for an extended Hubbard model with Peierls-type electron-lattice interactions within the Hartree-Fock approximation. It is found that the lattice effects stabilize the experimentally observed charge order substantially in θ-(ET)₂RbZn(SCN) ₄, whereas the energy gain by a lattice distortion is small and its band structure and Coulomb interactions play an important role to realize the charge order in α-(ET)₂I₃. The results are consistent with experimental observations and show contrastive roles of lattice degrees of freedom on charge orders in these compounds.