NMR and NQR Studies of ¹⁷O and ⁶³Cu in CuO₂ Plane of High-T_c YBa₂Cu₃O_6.65 with T_c = 61 K

The microscopic property of each oxygen and copper site in the CuO₂ plane of YBa₂Cu₃O_6.65 with T_c=61 K has been investigated by ¹⁷O NMR and ⁶³Cu NQR techniques. It has been found that the nuclear spin-lattice relaxation rate 1/T₁ for Cu sites is depressed by three orders of magnitude as compared with that for YBa₂Cu₃O₇ with T_c=92 K, whereas 1/T₁ for oxygen sites exhibits a rather small change, decreasing approximately to one-fourth (1/4). From this difference of the relaxation behavior between oxygen and Cu sites, it is derived that the progressive reduction of 1/T₁ for oxygen sites is responsible for the decrease of the p-hole carriers, while the extraordinary decrease of 1/T₁ for Cu sites is due to the dramatic suppression of the low-frequency components of an antiferromagnetic spin fluctuation of nearly localized d electrons. The spin fluctuation at the Cu sites is found to strongly correlate with the p-hole content.