Conductivity mode in blue bronze

We have extended far-infrared reflectance measurements of ${\mathrm{K}}_{0.3}$Mo${\mathrm{O}}_3$ to frequencies as low as 4 ${\mathrm{cm}}^{-1\mathrm{}}$ using a Martin-Puplett interferometer and a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-cooled bolometer. The new results, combined with other measurements, span an energy range from ∼ 0.5 meV to 6 eV and reveal two optically active modes arising from charge-density waves, the lower of which is near a frequency of 3 ${\mathrm{cm}}^{-1\mathrm{}}$ (corresponding to a thermal energy of 4 K, slightly above the sample temperature of 2 K in our experiment). We interpret the peaks as new evidence of pinned charge-density waves, in addition to the Raman-active mode previously seen at higher frequencies.