NMR in highly correlated superconductors

Results of our systematic NMR study in high T_c cuprates are reviewed. The antiferromagnetic spin fluctuations (AFSF) decrease in the order of La_1.85Sr_0.15CuO₄. YBa₂Cu₃O₇ and Tl₂Ba₂CuO_6+y. 1/T₁ of⁶³Cu in the CuO₂ plane in the normal state follows essentially a Curie-Weiss law at high temperature and T₁T = const. law at low temperature. The temperature dependence of 1/T₁ and the Knight shift together with their impurity effect in the superconducting state strongly suggest d-wave pairing implying the AFSF to be responsible for the occurrence of superconductivity. From the NQR frequency measurement the density of Cu 3d and O 2p holes decreases and increases, respectively, in the order of La, Y and Tl compounds, which is consistent with the change of AFSF. The relation between T_c and v_Q, and their pressure dependence suggest that there exists and optimum value of the ratio of Cu 3d and O 2p hole density to give a maximum in T_c.