Novel phase transition in quarter-filled SDW state with dimerization

We have examined spin density wave (SDW) state in a quasi-one-dimensional e lectron system with both commensurability of quarter-filled band and dimeri zation, which compete with each other. It is shown that there are two kinds of critical temperatures: T-c for the onset of the SDW state and T-c2(< T- c) for another phase at low temperatures, where both phase transitions are of the second order. The former is pure 2k(F)-SDW and the latter is followe d by 4k(F)-charge density wave with Fermi momentum, k(F).