NMR study on the spin-density wave phase of (TMTSF)2X

We have measured the spin-lattice relaxation rate T1−1 of 1H NMR in the spin-density wave (SDW) phase of (TMTSF)2X (X = PF 6, AsF6 and ClO4). Besides the sharp peak observed at the transition temperature TSDW, we found another divergent peak at the T∗ much lower than TSDW in the temperature dependent curve of T1−1 in three salts commonly. This temperature T∗ is scaled to TSDW as TSDW/T∗ ⋍ 3.5. The relaxation rate shows the thermally activated type behavior below T∗ and is roughly proportional to the inverse of NMR frequency. These behaviors strongly suggest that there is the phase transition with the magnetic fluctuation, which divides the SDW phase into two sub-phases.