Low-frequency dielectric response of charge-density wave pinned by commensurability in (2,5(OCH3)2DCNQI)2Li

We have used complex conductivity measurements between 20 Hz and 1 MHz to probe the dielectric response in the low-temperature phase of the $\!\rm (2,5(OCH_3)_2\rm DCNQI)_2Li$ organic conductor. The observed data reveal the existence of the relaxation process which is well described by a Debye-like expression for an overdamped response of a system with a single degree of freedom. Both the static dielectric constant ($\varepsilon_0 =3 \times 10^7$) and the characteristic relaxation time ($\tau_0$) are much larger than that expected for single-particle excitations. $\tau_0$ is thermally activated in a manner similar to the dc conductivity. We attribute the observed relaxation to N=4 charge density wave pinned by commensurability to the underlying lattice.