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 TSDW 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, TSDW 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 TSDW 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.