DIELECTRIC-CONSTANT AND DIELECTRIC-RELAXATION OF THE PINNED SPIN-DENSITY-WAVE IN THE ALLOYS (TMTSF)2(ASF6)1-X(SBF6)X

We present the first measurements of the low frequency dielectric constant for a spin-density wave (SDW) system for a wide range of pinning energies. The pinning is created by a controlled introduction of disorder by alloying in the system (TMTSF)2(AsF6)1-x(SbF6)x, 0 less-than-or-equal-to x less-than-or-equal-to 1. The static dielectric constant is shown to be inversely proportional to the depinning threshold field. At high frequencies the dielectric constant falls off as omega(-alpha) with alpha = 0.7. This frequency dependence is attributed to a distribution of relaxation times of the pinned SDW. We show that for a wide range of pinning energies and temperatures the distribution of the relaxation times characterised by a does not change significantly. The mean dielectric relaxation time is a measure of the damping of the SDW excitations and it can therefore be compared to the dc nonlinear SDW conductivity. At temperatures below 2.5 K the SDW conductivity has a stronger decrease than the increase of damping calculated from the dielectric relaxation. We suggest that this is due to a transition to a commensurate phase in a small fraction of the sample volume.