Local and nonlocal effects on the surface resistance of d-wave superconductors

Evidence for d-wave superconductivity from measurements of the local conduc tivity is rather indirect and not very compelling. Very recently, experimen tal data on the surface resistance taken at five different frequencies on v ery high quality single crystals of YBa2Cu3O7 have become available. Having the frequency as additional variable provides a much more stringent test f or the predictive powers of the d-wave model. Because of the high quality o f the samples we expect that at low temperatures the nonlocal limit applies to the surface resistance. A nonlocal relation between current and field w ould allow a direct investigation of the order parameter anisotropy by vary ing the wave vector q of the radiation relative to the crystal axes.