NMR and NQR studies of high T_c superconductors

^63,65Cu NMR and NQR studies of YBa₂Cu₃O_7-δ are described for compositions, δ, of 0.0, 0.3, and 0.9. Spin-lattice relaxation rates in YBa₂Cu₃O₇ reveal significantly different electron dynamics at the two Cu sites in both the normal and superconducting states. In YBa₂Cu₃O_6.7, the relaxation rates of the sites giving a 31 MHz NQR line are dramatically lower (by a factor ∼ 1 1500) than in YBa₂Cu₃O₇ indicating the essential vanishing of mobile carrier density on these sites. In contrast, relaxation at the 22 MHz sites is reduced only about 30%. Interpretation of these and other NMR and NQR data is discussed in relation to the question of assignment of the NQR lines to the appropriate crystalline sites.