Spin-density-wave phase transitions in quasi-one-dimensional dimerized quarter-filled organic conductors

We have studied spin density wave (SDW) phase transitions in dimerized quarter-filled Hubbard chains weakly coupled via interchain one-particle hopping, t(bO). It is shown that there exists a critical value of t(bO), t(b)*, between the incoherent metal regime (t(bO) < t(b)*) and the Fermi liquid regime (t(bO) > t(b)*) in the metallic phase above the SDW transition temperature. By using the 2-loop perturbative renormalization-group (PRG) approach together with the random-phase-approximation, we propose a SDW phase diagram covering both of the regimes. The SDW phase transition from the incoherent metal phase for t(bO) < t(b)* is caused by growth of the intrachain electron-electron umklapp scattering toward low temperatures, which is regarded as preformation of the Mott gap. We discuss relevance of the present result to the SDW phase transitions in the quasi-one-dimensional dimerized quarter-filled organic conductors, (TMTTF)(2)X and (TMTSF)(2)X.