Damping of the spin-density-wave phase mode by defect scattering

We have investigated the pinning of the spin-density wave and the low-frequency damping of the phase excitations in the solid solutions (TMTSF${)}_2$(${\mathrm{AsF}}_6$ ${)}_{1\mathrm{-}\mathit{x}}$(${\mathrm{SbF}}_6$ ${)}_{\mathit{x}}$ (0≤x≤1). The pinning is strongly enhanced by the weak nonmagnetic disorder introduced by the alloying. The damping is characterized by the nonlinear dc conductivity and by the dielectric relaxation. In a broad but limited temperature range we describe a temperature-independent damping with a strong dependence on defect concentration, suggesting elastic scattering of phasons by defects.