Optical conductivity inYBa2Cu3O7−δthin films

The real and imaginary parts of the optical conductivity ${\sigma }_1+i{\sigma }_2$ of partly untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ thin films were measured by submillimeter spectroscopy for frequencies from 100 GHz to 1 THz. The frequency dependence of the conductivity below $T_C$ can be described by a narrow Drude-like peak with strongly temperature-dependent relaxation rates. ${\sigma }_1\left(\nu \right)$ does not extrapolate to the universal d-wave value at 0 K. Impurity scattering significantly different to the unitary limit has to be assumed to achieve agreement with recent theoretical results. Including published infrared results we present ${\sigma }_1\left(\nu \right)$ in the spectral range from 0.1–100 THz.