Modified two-fluid model for superconductor surface impedance calculation

The traditional two-fluid model of superconducting conductivity was modified to reflect the BCS coherence effects in the conductivity of a superconductor, which are incorporated through an effective ratio of normal-to-superconducting electrons. The ratio is a simple analytical expression which depends on frequency, temperature, and material parameters. This modified two-fluid model allows accurate and rapid calculation of the microwave surface impedance of a superconductor in the clean and dirty limits and in the weak- and strong-coupled regimes. The model compares well with surface resistance data for Nb. Numerical calculations of surface impedance with the modified two-fluid model are one to five orders of magnitude faster than equivalent BCS calculations.>