Far-infrared optical properties of semiconducting tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ), including the pinned charge-density wave

Abstract

The far-infrared bolometric response of a thick single crystal of TTF-TCNQ at 12 K has been used to give directly $1\ensuremath{-}R$, where $R$ is the power reflectivity, for radiation polarized with $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$, and wave numbers from 10 to 830 ${\mathrm{cm}}^{\ensuremath{-}1}$. Accurate and high-resolution values of $R$ are obtained. These have been combined with measurements at higher energies and a Kramers-Kronig analysis performed to yield the optical properties. The conductivity shows a strong resonance at 35 ${\mathrm{cm}}^{\ensuremath{-}1}$, which has been attributed to the phase mode of the pinned charge-density wave, in agreement with Tanner, Cummings, and Jacobsen [Phys. Rev. Lett. 47, 597 (1981)]. We find, however, no large resonance at higher wave numbers but instead a strong broad midgap conductivity possibly attributable to solitons. The optical properties are confirmed by comparison with powder transmission measurements. The effect of small particles on the powder and thin-film spectra is demonstrated. Finally, temperature-dependent powder measurements have been used to show the activated nature of the anomalous $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$ vibrational spectrum.