New two-fluid superconduction model applied to penetration depth and microwave surface resistance

A two-fluid theory of superconductivity derived from an analysis of covalent interactions in the polaronic high-T/sub c/ cuprates is used to predict the penetration depth of YBa/sub 2/Cu/sub 3/O/sub 7/ as a function of temperature. In a related study, the model is applied to the temperature dependence of microwave surface resistance measurements on both YBa/sub 2/Cu/sub 3/O/sub 7/ and Nb films. The results suggest that polaronic cuprates can be superconducting with a much smaller fraction of available carriers than metals. It therefore follows that superconduction would survive to higher temperatures than in metals. If supercarrier populations are governed by Boltzmann tails as proposed in this model, however, the lower carrier densities in high-T/sub c/ materials would become even smaller as temperatures approach the critical limit.<>