SUPERCONDUCTING OPTICAL CONDUCTIVITY FOR ARBITRARY TEMPERATURE AND MEAN FREE-PATH

Calculations of the optical conductivity of a superconductor valid for any temperature T and any mean free path l are presented. They are based on the method of Marsiglio, Schossmann, and Carbotte of computing the real-frequency-axis gap and renormalization function. The work is applied to a study of the signature of the opening of the gap edge DELTA(T) as the superconducting state develops. At finite temperature, the presence of a normal-fluid component leads to absorption down to zero frequency (omega). For sufficiently impure systems there remains, nevertheless, a sharp threshold for additional absorption from the condensate which starts at omega = 2-DELTA(T), so that it is possible to deduce from it an accurate value of DELTA(T). For the pure limit, the situation is more ambiguous.