Deviations from Drude response in low-dimensional metals: Electrodynamics of the metallic state of (TMTSF)(2)PF6

The optical properties of the linear chain compound bis-(tetramethyltetraselenafulvalene)hexafluorophosphate, (TMTSF)(2)PF6, were examined in an extremely wide frequency range in the metallic state, above the spin density wave transition (T > 12 K). Parallel to the chains the conductivity has two distinct features,a zero frequency response and a finite energy absorption. Both features are temperature dependent with most of the spectral weight in the finite energy mode. Perpendicular to the chains, a Drude-like response is recovered. We discuss the implications of our findings.