NQR AND NMR STUDIES OF SPIN DYNAMICS IN HIGH T sub c SUPERCONDUCTING CUPRATES.

Recent NQR and NMR studies of Cu spin dynamics in metallic YBa sub 2 Cu sub 3 O sub (6+x) are reviewed. The Cu nuclear relaxation rates are shown to be inconsistent with weak correlation (band) theory and the Korringa relation, but the Cu spins retain important characteristics of itinerancy. These include pairing of the Cu d-spins in the superconducting state and strong sensitivity of the Cu spin dynamics to changes in carrier concentration associated with variation of the oxygen content. Samples of reduced oxygen content (T sub c = 60 K) are found to contain a high concentration of Cu sup + Cu(1) sites associated with "full chain - empty chain" vacancy ordering. These samples also exhibit remarkable precursive suppression of planar Cu (2) nuclear relaxation above T sub c. This is the written version of an invited talk presented at the 10th International Symposium on Quadrupole Resonance Spectroscopy held in Takayama, Japan, August 1989.