IMPURITY SCATTERING AND ORDER-PARAMETER SYMMETRY IN HIGH-T-C SUPERCONDUCTORS

I review the theory of impurities in a d-wave superconductor, with a v iew towards devising tests which can identify the order parameter symm etry in the cuprates particularly those involving electromagnetic resp onse. Existing microwave data can be semiquantitatively fit by ''dirty d-wave'' theory, extended to include inelastic scattering by antiferr omagnetic spin fluctuations although some discrepancies remain. The mo del also yields alternative explanations for some well-known features of the optical conductivity in single crystals. The theory is compared with the predictions for various anisotropic s-wave states which have been proposed recently.