Spin Density Wave Order and Dynamics in

Studies of spin density wave (SDW) order and dynamics of (TMTSF)_2X, where TMTSF is tetramethyltetraselenafulvalene and X^- = PF_6^ -, AsF_6^-, and ClO _4^-, with proton magnetic resonance are reported. The SDW transition is seen to be weakly first order for X^- = PF_6^- and AsF_6 ^-, and smeared in X^- = ClO_4^-. Proton spin-lattice relaxation 1/T_1 measurements reveal: (1) evidence that outside the critical regime the main contribution to 1/T_1 is from phason fluctuations, (2) evidence that the "unusual" behavior often seen near 3.5 K is due to a slowing of the fluctuations, and (3) evidence that depinning of the SDW has no significant effect on the order parameter. Spin echo measurements of the average SDW condensate displacement at 4.2 K in (TMTSF)_2PF_6 when it is driven by an electric field below the depinning threshold show a linear displacement as a function of the electric field with a restoring force constant per electronic charge k = 2.6 +/- 0.3 times 10^{-9} N/m. At the depinning threshold the average condensate displacement is 0.24 +/- 0.04 A. The corresponding low frequency dielectric constant is 1.7 +/- 0.4 times 10^9 , in agreement with electrical transport measurements. Measurement of the sliding velocity from observations of oscillations in the spin echo height are compared with narrow band noise measurements of the SDW velocity. It is concluded that the SDW pinning potential wavelength is half of the SDW wavelength. Measurements of the SDW in the repinning process following a pulse in an external current several times the depinning threshold are shown in (TMTSF)_2 PF_6 at 4.2 K. Analysis of the effect on the spin echo shows that the local SDW phase velocity persists for longer times than the drift velocity measured by electrical transport. This observation shows evidence of long-lived random motion of the SDW phase as successively lower minima in the complex free energy surface are reached during repinning. An activation energy of 22 K (which is close to the gap) is obtained for the repinning rate at temperatures larger than 3 K, suggesting that above 3 K the repinning is determined by the interaction of the SDW condensate with thermally excited normal electrons. This research has been partially supported by: NSF Grant No. DMR -9319304.