Optical conductivity in high-Tcsuperconductors

The optical conductivity of a superconductor with arbitrary mean free path is calculated with use of an imaginary-frequency formulation. We obtain good agreement with the results of a recent real-axis formulation and, in most cases, with another imaginary-frequency formulation. Extending the work to a simple version of a recently proposed phenomenological charge-transfer resonance model for the CuO, it is found that in the clean limit the absorption will start at four times the gap (${\mathrm{\Delta }}_0$) while introducing impurities produces absorption starting at twice ${\mathrm{\Delta }}_0$. In agreement with experiment, strong coupling effects at higher energies need not be large in this model.