Optical properties of copper-oxygen planes in superconducting oxides and related materials

The optical spectra of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ and other Cu-O-based superconductors have several common features in the visible and near uv. Chemical changes that affect the conductivity of these materials also have a strong effect on some of these features. By comparing the spectra of many materials containing similar Cu-O structures, we have determined that some of the optical features are associated with specific local structures within the unit cell. Of particular interest is a sharp feature at 1.7 eV that appears for nonmetallic compositions of many of the materials and is removed by the introduction of carriers. Similar features in materials not yet showing superconductivity encourage further investigation of them.