RESTORING FORCE AND DISPLACEMENT OF THE PINNED SPIN-DENSITY-WAVE CONDENSATE IN (TMTSF)2PF6

We report NMR spin echo measurements of the average spin density,wave condensate displacement at 4.2 K in (TMTSF)2PF6 When it is driven by an electric field below the depinning threshold over time scales of 10-25 mus. The displacement, which corresponds to internal deformations, varies linearly as a function of the electric field and has a restoring force constant per electronic charge k = ( 1.3 +/- 0.3) x 10(-9) N/m. At the depinning threshold the average condensate displacement is 0.46 +/- 0.06 angstrom. The corresponding low frequency dielectric constant is (3.4 +/- 0.7) x 10(9), which is close to that obtained from electrical transport measurements.