Optical anisotropy of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$

The optical anisotropy of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ in the 0.08–5.6 eV region is investigated by polarized reflectance measurements on single crystals, and by transmittance, reflectance, and ellipsometric measurements on oriented thin films. In the visible–near-uv spectral region the anisotropy is relatively mild, but a huge anisotropy is observed in the infrared. Here, the reflectance increases with decreasing frequency for both polraizations, consistent with metallic conductance both parallel and perpendicular to the ab plane. The c-axis plasma frequency is strikingly low, but there is strong damping which could originate from a continuum of low-energy interband transitions.