Direct Observation of the Spin-Density-Wave Gap in(TMTSF)2PF6

We have measured the optical properties of the linear chain compound bis-(tetramethyltetraselenafulvalene)hexafluorophosphate, $(\mathrm{TMTSF}{)}_2{\mathrm{PF}}_6$, in the spin-density-wave state which develops below $T_{\mathrm{SDW}}=12\mathrm{}$ K. In the direction perpendicular to the chains, we observe clear signatures of the spin-density-wave gap and model these features with a formalism similar to the one worked out for the superconducting state but with case I coherence factors.