Far-infrared reflectivity of bis-tetramethyltetraselenafulvalene hexafluoroarsenate [(TMTSF)2AsF6] through the spin-density-wave phase transition

Measurements are presented of the ratio of the far-infrared reflectivity of a crystal mosaic of bis-tetramethyltetraselenafulvalene hexafluoroarsenate [(TMTSF${)}_2$ ${\mathrm{AsF}}_6$] above and below the spin-density-wave phase transition at 12 K. Using the already published 6-K reflectivity spectra measured by a composite-bolometer technique, the higher-temperature reflectivity spectra are obtained. For radiation polarized perpendicular to the conducting axis, the reflectivity shows a sharp increase if the temperature is raised through 12 K. The resulting conductivity shows a disappearance of the semiconducting gap and its replacement by a normal metallic conductivity. For radiation polarized parallel to the conducting stacks, on the other hand, a small decrease in reflectivity is observed. The resulting conductivity shows a persistent gap in the metallic state with a reduced peak near 70 ${\mathrm{cm}}^{\mathrm{-}1}$. These data are further evidence for a collective mode at zero frequency at temperatures well above the phase transition.