PROTON NMR-STUDY OF SPIN-DENSITY-WAVE THERMAL FLUCTUATIONS IN (TMTSF)2PF6, WHERE TMTSF IS TETRAMETHYLTETRASELENAFULVALENE

Measurements of the proton spin-lattice relaxation rate (1/T1) at a pr essure of 1 bar in a single crystal of (TMTSF)2PF6 (TMTSF is tetrameth yltetraselenafulvalene) are reported for the magnetic field in the ran ge 0.25-1.48 T aligned along the b'- and c- axes over the temperature range 2-30 K. The methyl group rotation contribution to 1/T1 is subtr acted to obtain the spin density wave (SDW) contribution. Through meas urements of 1/T1 below and above the spin flop transition it is determ ined that phason fluctuations of the SDW are responsible for most of t he relaxation. Depinning the SDW's by an electric field leaves 1/T1 ne arly unchanged, which indicates that the SDW and its fluctuation rate near 10(8) rad/s persist when the SDW is sliding. Analysis of the peak in 1/T1 near 3 K on the applied magnetic field suggests that it repre sents a slowing of thermally activated fluctuations with an activation energy DELTA/k(B) = 18.3 +/- 4.0 K, which is close to the single part icle gap for this material. The contribution of critical fluctuations to 1/T1 is consistent with the transition being weakly first order. Un like the relaxation in the ordered phase, the contribution of the crit ical fluctuations is isotropic and has little, if any dependence on ma gnetic field.