MICROWAVE CONDUCTIVITY OF THE ORGANIC CHARGE-DENSITY-WAVE CONDUCTOR (FLUORANTHENE)2X

We have investigated the temperature dependence of the microwave condu ctivity of the organic CDW-conductor (FA)2X at frequencies of 4.2, 10. 2 and 23.5 GHz and compared our data with the d.c. conductivity of the se crystals [1]. Down to temperatures of about 80 to 60 K an excellent agreement has been found. In the quasi-metallic high temperature rang e no frequency dependence of the conductivity is observed. Upon coolin g a sharp phase transition already known from d.c. measurements occurs at temperatures T(p) of 182 to 175 K depending on the counterion X- = PF6-, SbF6-. At temperatures far below this Peierls transition, howev er, deviations from d.c. results are observed; while the d.c. conducti vity displays thermally activated behaviour the microwave conductivity leads into a plateau and remains almost constant down to 4 K. The ons et of this plateau lies at temperatures from 80 to 60 K. At 20 K the m icrowave conductivity at 10.2 GHz is about 10 orders of magnitude high er than the d.c. values. We interpret these results as a clear signatu re of the Frohlich mode resulting from the CDW ground state in (FA)2X. The agreement of microwave and d.c. data even at temperatures below t he Peierls transition can be ascribed to screening of the CDW by therm ally excited charge carriers.