Spin density wave in quasi-one-dimensional organic conductors

The incommensurate spin density wave (SDW) phases in (TMTSF)₂PF₆ and (TMTTF)₂Br were investigated by transport measurements under pressure and in magnetic field parallel to the c*-axis. For the SDW phase of (TMTSF)₂PF₆ slightly suppressed by the pressure, the SDW transition temperature T_SDW increases with the applied magnetic field. The field dependence of T_SDW is described by a quadratic behavior and the coefficient of the quadratic term increases with increasing pressure. For the incommensurate SDW phase of (TMTTF)₂Br stabilized under the pressure above 0.5 GPa, the SDW transition shows similar behavior as in the case of (TMTSF)₂PF₆ 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)₂PF₆ and (TMTTF)₂Br are understood in a common pressure axis with taking the reduction of the coupling constant by the pressure into account.