Coexistence of SDW and purely-electronic CDW in quarter-filled organic conductors

Recently an X-ray experiment reported a coexisting phase of purely-electronic 2kF charge density wave (CDW) and the conventional 2kF spin density wave (SDW) in (TMTSF)2PF6. In order to understand such coexistence, we have performed a mean-field calculation for a Hubbard-type model of a quarter-filled one-dimensional chain. It is found that the coexistence of 2kF SDW and purely-electronic 2kF CDW is realized due to the effect of next-nearest-neighbor Coulomb repulsion. Phase diagrams are obtained as a function of nearest-and next-nearest-neighbor Coulomb interaction energies. Competition between the two coexistent states with 2kF CDW and with 4kF CDW is revealed, which gives a first order transition. It turns out that the internal degrees of freedom inside a dimer (consisting of two TMTSF molecules) play an important role. Our result supports the X-ray result of a coexisting phase in (TMTSF)2PF6.