Non-linear transport in the incommensurate SDW phase of (TMTTF)2Br under pressure

The spin-density wave (SDW) phase with an incommensurate wave vector is induced as a ground state in the organic compound (TMTTF)2Br under the pressure above 0.5 GPa. We investigated the collective sliding motion of the SDW condensate with measuring the non-linear conductivity in this SDW phase. The measurement was done with dc and pulse methods. Below the SDW transition temperature TsDW, the sharp increase of conductivity from the ohmic one was observed with the clearly defined threshold electric field ET. This indicates the depinning of the SDW and the excess conductivity above ET is associated with the sliding motion of the SDW. The absolute value of ET is fairly larger than that observed in (TMTSF)2PF6, whose ground state is the incommensurate SDW at ambient pressure. With decreasing the temperature from TSDW, ET increases gradually and shows a relatively sharp drop after a peak around 03TSDW. This temperature is supposed to be connected with the sub-phase transition proposed from the temperature dependence of NMR relaxation rate. In addition, the excess conductivity normalized to the ohmic one, which corresponds to the current carried by the SDW condensate, shows a dip at 0.3TSDW with decreasing the temperature. In the dc I-V characteristics, the hysteretic behavior near ET appears only around 0.3TSDW. These behaviors suggest that the sliding mechanism of the SDW condensate varies across 0.3TSDW. We discuss the dynamics of the SDW in (TMTTF)2Br in comparison with that in (TMTSF)2X, considering the sub-phase transition.