Spin density wave in quasi-one-dimensional organic conductors

The incommensurate spin density wave (SDW) phases in (TMTSF)(2)PF6 and (TMTTF)(2)Br were investigated by transport measurements under pressure and in magnetic field parallel to the c*-axis. For the SDW phase of (TMTSF)(2)PF6 slightly suppressed by the pressure. the SDW transition temperature T-SDW increases with the applied magnetic field. The field dependence of T-SDW is de scribed by a quadratic behavior and the coefficient of the quadratic term increases with increasing pressure. For the incommensurate SDW phase of (TMTTF)(2)Br stabilized under the pressure above 0.5 GPa, the SDW transition shows similar behavior as in the case of (TMTSF)(2)PF6 with increasing magnetic field. These results are consistent with the prediction of the mean field theory based on the nesting of the Fermi surface. The incommensurate SDW phases in both (TMTSF)(2)PF6 and (TMTTF)(2)Br are understood in a common pressure axis with Laking the reduction of the coupling constant by the pressure into account.