Giant Conductivity Resonance in the Spin-Density-Wave State of an Organic Conductor

Frequency-dependent conductivity measurements between 4.5 and 35 GHz are reported in the spin-density-wave (SDW) state of the linear-chain organic conductor (TMTSF)₂PF₆, where TMTSF denotes tetramethyltetraselenafulvalenium. A giant conductivity resonance, implying long relaxation times, is observed at frequencies well below the single-particle gap. The frequency dependence is strongly depressed by small amounts of impurities. Either collective spin-density-wave conduction or excitations of the spin-density-wave mode are responsible for the observed frequency-dependent response.