The optical properties of the linear chain compound bis-(tetramethyltetraselenafulvalene)hexafluoro-phosphate, (TMTSF${)}_2$P${\mathrm{F}}_6$, were examined in an extremely wide frequency range in the metallic state, above the spin density wave transition ( $T>\mathrm{}12\mathrm{}\mathrm{K}$). 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.

• Received 15 March 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.398