Far-infrared spectra of bis-(tetramethyltetraselenafulvalene) hexafluoroarsenate [(TMTSF${)}_2$${\mathrm{AsF}}_6$] and hexafluoroantimonate [(TMTSF${)}_2$${\mathrm{SbF}}_6$] in their spin-density-wave state

The far-infrared reflectivities of bis-(tetramethyltetraselenafulvalene) hexafluoroarsenate [(TMTSF${)}_2$ ${\mathrm{AsF}}_6$] and hexafluoroantimonate [(TMTSF${)}_2$ ${\mathrm{SbF}}_6$] at 6 K, when they are in their spin-density-wave semiconducting phase, have been measured by a composite-bolometer technique. Details and limitations of this novel technique have been given, along with a method of correcting for the most serious limitation, an unwanted ‘‘leakage’’ signal. The resulting conductivities for two polarizations, E∥a and E∥b’, are presented. The results are similar for both materials, and to those found previously for bis-(tetramethyltetraselenafulvalene) hexafluorophosphate [(TMTSF${)}_2$ ${\mathrm{PF}}_6$]. Both polarizations exhibit a conductivity with the form of the combined density of states of a one-dimensional semiconductor. The peak is close to the energy gap inferred from the dc conductivity activation energy. For the transverse polarization, the peak is wider and is shifted to higher energies and, in agreement with the sum rule, the magnitude is much lower. These results agree qualitatively with calculations of the conductivity using published energy bands and a simple charge-density-wave coupling theory. This theory predicts that the main difference between the two polarizations arises from the anisotropic effective mass.