MICROWAVE SURFACE IMPEDANCE AS A PROBE OF UNCONVENTIONAL SUPERCONDUCTIVITY IN YBA2CU3O6.95

The surface resistance and penetration depth of single crystals of YBa2Cu3O6.95 exhibit a linear, rather than exponential, temperature dependence below 30 K that suggests the presence of nodes in the gap function. The surface resistance also exhibits non-monotonic temperature dependence at higher temperatures that can be attributed to a rapid collapse of the quasiparticle scattering rate below T(c). Both of these effects are potentially sensitive to the presence of defects, which may explain the rather different and variable results of measurements on thin films. We have explicitly tested the influence of impurities on the surface resistance by adding small amounts of Zn impurites to single crystals. The Zn-doped crystals have a monotonic surface resistance that is similar to that observed in state-of-the-art thin films, which suggests that some level of defects must be present in order to achieve the low microwave loss of these films.