Optical conductivity of the high-T_c's: Search for the energy gap

The infrared absorption of oxide superconductors will be discussed. In the normal state, σ₁(ω) consists of a strongly T-dependent part in the far infrared, above which is a weakly-T-dependent midinfrared absorption. In the superconducting state, there is absorption to very low frequencies, well below where the superconducting gap is expected to be. It is argued that this absorption is due to a second ("midinfrared") component which is only weakly affected by the onset of superconductivity. The free carrier component is in the clean limit and most of its oscillator strength goes into the superfluid condensate zero-frequency delta function response.