ABSTRACT
The motivation for the optical studies of materials containing Cu-0 planes has been to try to understand high temperature superconductivity. This chapter presents measurements of the electrical conductivity σ as a function of four 'variables': the charge density added by doping: n; the lattice structure; the temperature of the crystals: T; and the energy of the photons used as a probe: E or ω. It reframes some of the question currently being asked about high temperature superconductors. The conductivity added by doping in the insulating state is important because absorption occurs near the same energy in the metallic and superconducting states. The Mott criterion indicates that the metal to insulator transition occurs at a density where only the tails of the wave functions overlap. Drude model corresponds to elastic scattering by a fixed number of scatterers. This type of scattering may arise when defects are present which have no low frequency excitations.
