2-FLUID INTERPRETATION OF THE CONDUCTIVITY OF CLEAN BCS SUPERCONDUCTORS

The temperature and frequency dependence of the microwave and optical conductivity are studied for a BCS superconductor with weak elastic sc attering [1/tau < 2DELTA(0), where tau is the scattering time and DELT A(T) is the BCS energy gap]. The results are interpreted in terms of a two-fluid model in which the combined oscillator strength of the delt a function at the origin and the thermally excited free-carrier conduc tivity is found to be only weakly temperature dependent. The low-frequ ency conductivity is shown to be a function of omegatau, in the clean limit, with a characteristic logarithmic divergence at omega=0. The fr equency dependence of the microwave surface resistance is discussed an d comparison is made to the recent experimental results of Bonn, Hardy , and co-workers on YBa2Cu3O7-delta single crystals.