Broadband infrared conductivity in an YBa2Cu3O6.7 thin film

Infrared reflectivity of an oxygen reduced YBa2Cu3O6.7 thin film Was measur ed in the frequency range 30 cm(-1) < v < 7000 cm(-1) and for temperatures 5 K < T < 300 K. At lower frequencies, 3 cm(-1) < v < 40 cm(-1), the comple x conductivity sigma*=sigma (1)+i sigma (2) was measured directly using a t ransmission technique. Both data sets were combined to obtain the reflectiv ity in a broad frequency range and thus to improve substantially the qualit y of the Kramers-Kronig analysis at low frequencies. The analysis of the co nductivity spectra reveals that the low-frequency spectral weight strongly depends on temperature directly above TC but saturates for T greater than o r similar to 150 K.