We present measurements of the optical spectra on ${\mathrm{N}\mathrm{a}}_{0.7}\mathrm{C}\mathrm{o}{\mathrm{O}}_2$ single crystals. The optical conductivity shows two broad interband transition peaks at 1.6 eV and 3.1 eV, and a weak midinfrared peak at 0.4 eV. The intraband response of conducting carriers is different from that of a simple Drude metal. A peak at low but finite frequency is observed, which shifts to higher frequencies with increasing temperature, even though the dc resistivity is metallic. The origin of the interband transitions and the low-frequency charge dynamics have been discussed and compared with other experiments.

• Received 24 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.93.237007