NMR investigation of spin density wave repinning in (TMTSF)2PF6

We report proton spin echo measurements of the spin density wave (SDW) repinning rate of sliding SDW's in (TMTSF)2PF6. This method, which complements conventional transport measurements, provides a local measurement of the square of the magnitude of the SDW velocity averaged over the sample. For these measurements, the current in the sample is raised above the depinning threshold and then lowered below it. At the end of the current pulse, a slow, complex decay of the SDW current is observed which we attribute to the corresponding adjustment of the SDW phase to the pinning centers and relaxation of the strain of the sliding SDW. At temperatures above about 4 K, a thermally activated behavior for the characteristic decay time is observed whose activation energy of 22±3 K is close to the single particle excitation gap. This observation indicates that the repinning rate is determined by thermally excited normal electrons in this temperature range. Measurements at lower temperatures indicate a crossover to a much weaker temperature dependence which suggests that tunneling may become the dominant repinning mechanism at lower temperatures.