Order-parameter holes and theory of microwave conductivity in YBa2CU3O7-δ

We propose that the low-temperature discrepancy between simple d-wave models of the microwave conductivity and existing experiments on single crystals of YBa2Cu3O7-delta 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 YBa2Cu3O7-delta samples over the entire temperature range down to about 2-3 K, and for a wide range of frequencies. Remaining discrepancies in the "universal'' regime at very low temperatures are discussed.