Order Parameter "Holes" and Theory of Microwave Conductivity in YBCO

We propose that the low temperature discrepancy between simple d-wave models of the microwave conductivity and existing experiments on single crystals of YBCO can be resolved by including the scattering of quasiparticles from "holes" of the order parameter at impurity sites. Within a framework proposed previously, we find in particular excellent agreement with data of Hosseini et al. on slightly overdoped YBCO samples over the entire temperature range down to about 2-3K, and for a wide range of frequencies. Remaining discrepancies in the "universal" regime at very low temperatures are discussed.