Are the high Tc superconductors D-wave?

Although it has been widely accepted for several years that the normal state of high Tc superconductors is anomalous, only recently has there been growing evidence for the anomalous nature of the superconducting state. A number of recent experiments show clear evidence for the existence of low-lying excitations in the superconducting state. Moreover, the observation by Bonn, Hardy and co-workers of a linear temperature dependence in the microwave surface resistance and penetration depth of YBCO at low temperatures suggests d-wave pairing with line nodes. The evidence for and against unconventional superconductivity in the high Tc oxides is reviewed. Specific topics discussed include the different behaviours observed for the penetration depth in thin films and in single crystals, what is known about the absolute value of the penetration depth, the role of disorder and inelastic scattering, the relative merits of strong vs. weak correlation theories of d-wave superconductivity in the high Tc oxides, and most important, what have we actually learned about the penetration depth of high Tc superconductors from μSR? (orig.)