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Complex low-frequency dielectric relaxation of the charge-density wave state in the (2, 5(OCH3)2DCNQI)2Li

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(OCH_3)_2 DCNQI)_2 Li 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 \Delta\epsilon \approx 10^7 with Arrhenius-like resistive decay, associated with long wavelength phason excitations, a new mode of weaker strength \Delta\epsilon \approx 10^6 appears below 40K and broadens with decrease in temperature. Below 25K 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.

collective modes; low dimensional conductors; charge-density wave systems; dielectric loss and relaxation

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