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

We have performed a mean field calculation in order to determine the ground state of a modified Hubbard model of a quarter-filled one-dimensional chain. It is found that the coexistence of 2k(F) spin density wave (SDW) and purely-electronic 2k(F) charge density wave (CDW) is realized due to the effect of next-nearest-neighbor Coulomb repulsion. Phase diagrams are obtained as the values of nearest-and next-nearest-neighbor Coulomb interaction energies are varied. Competition between the two coexistent states with 2k(F) CDW and 4k(F) CDW is revealed, which gives a first order transition. Our result supports the X-ray result confirming the presence of a coexisting phase of purely-electronic 2k(F) CDW and 2k(F) SDW in (TMTSF)(2)PF6, a quarter-filled one-dimensional chain in terms of holes.