Optical properties of Na_0.68CoO₂ thin film

The optical reflectance of single-crystalline Na_0.68CoO ₂ (thickness around 3000 Å) thin film has been measured over a wide frequency (40-55000 cm^-1) and at temperatures between 20 and 300 K. The roomtemperature optical conductivity spectrum displays a metallic character. In addition, one E_1u phonon mode at about 518 cm ^-1 is identified, while the other four phonon peaks are due to the sapphire substrate. As the temperature is lowered, the Drude plasma frequency exhibits little temperature dependence, whereas the scattering rate monotonically decreases with decreasing temperature. Notably, the Drude scattering rate at 20 K is found to approach zero, consistent with the small residual resistivity, indicating high quality of Na_0.68CoO ₂ thin film.