Origin of the 1 eV-reflectivity edges in high-Tc superconducting cuprates

The reflectivity edge commonly observed at around 1 eV in the optical spectrum is investigated for a number of high-Tc superconducting cuprates. We have found that the edge energy (ωedge) is almost independent of doping concentration in each material but varies widely among the materials dependent on the average CuO2-plane spacing dc. This is consistent with a view supposing that the observed reflectivity edge corresponds to the plasma edge associated with the renormalized two-dimensional band, which would be nearly half-filled and has been reconstructed on doping from the gap-separated states of the charge transfer insulator as a result of reduced renormalization. We could not find a universal correlation between Tc and ω'p2. (orig.)