Far-infrared study of bis(tetramethyltetraselenafulvalene) hexafluoroantimonate [${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$${\mathrm{SbF}}_6$]: Coexistence of metallic and semiconducting states

Detailed reflectance measurements of ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$ ${\mathrm{SbF}}_6$ in the far-infrared region, above and below the metal-insulator transition temperature, are presented. The compound undergoes a phase transition at 12-14 K, and below this temperature a series of sharp lines appear in the $a$-axis polarization. We interpret these lines as "phase phonons" of the theory of Rice for the electron-phonon interaction of a one-dimensional conductor with an energy gap. Below the transition temperature a background absorption due to phonons generated by the Holstein mechanism can also be seen, suggesting that the semiconducting state coexists with a metallic state. We propose that the semiconducting state is related to the formation of a spin-density wave with an energy gap of 180 ${\mathrm{cm}}^{-1\mathrm{}}$ (22 meV).