Complex low-frequency dielectric relaxation of the charge-density wave state in the (2.5(OCH ₃ ) ₂ DCNQI) ₂ Li

Abstract:

We report a detailed characterization of an unique 2- and 4-fold commensurate charge density wave (CDW) state in single crystals of the organic conductor (2.5(OCH₃)₂DCNQI)₂Li by means of low-frequency (0.1 Hz-1 MHz) dielectric spectroscopy. We show that in addition to the narrow Debye-like relaxation mode of strength Δɛ≈ 10 ⁷ with Arrhenius-like resistive decay, associated with long wavelength phason excitations, a new mode of weaker strength Δɛ≈ 10 ⁶ appears below 40 K and broadens with decrease in temperature. Below 25 K the mean relaxation time of the new mode saturates, whereas the Debye mode continues gradual slowing down with weaker temperature dependence, again similar to the DC conductivity. We attribute the new process to short wavelength CDW excitations that become dominant once the free carrier density is substantially reduced at low temperatures.