Magnetic field dependence of incommensurate SDW transition in (TMTTF)₂Br

Resistivity measurements were performed on (TMTTF)₂Br under pressure. Magnetic field was applied along c^*-axis up to 16 T. Above 0.5 GPa, (TMTTF)₂Br exhibits incommensurate spin density wave (ISDW) transition at low temperature. With increasing pressure, spin density wave (SDW) transition temperature T_SDW decreases from 19.5 K at 0.5 GPa to 12.5 K at 2.1 GPa. In the incommensurate SDW phase, with increasing magnetic field, T_SDW increases quadratically and the coefficient of quadratic term increases with increasing pressure. These results are qualitatively consistent with the prediction of the mean-field theory based on the imperfection of the nesting of Fermi surface. The relation between the coefficient of the quadratic term and T_SDW at zero magnetic field can be explained by the mean-field theory with taking into account the reduction of the coupling constant N(0)I by pressure.