Frequency and Temperature-Dependent Conductivity in YBa sub 2 Cu sub 3 O sub (6+x) Crystals.

The results of a systematic study of the optical properties of the YBa sub 2 Cu sub 3 O sub (6+x) insulators and superconductors are reported. Specifically, we present measurements and analysis of the optical reflectivity, R, from a series of YBa sub 2 Cu sub 3 O sub (6+x) crystals in the frequency range from 30 to 20,000cm sup (-1) (4meV to 2.5eV), and temperature range from 10 to 270K. From R we obtain the real part of the temperature and frequency-dependent optical conductivity, sigma(omega, T), by Kramers-Kronig analysis. In our discussion, we emphasize the development of structure, and spectral weight, in sigma as the compounds change from insulators to high-T sub c superconductors with varying O content or Al doping.