NMR in highly correlated superconductors

Results of our systematic NMR study in highTc cuprates are reviewed. The antiferromagnetic spin fluctuations (AFSF) decrease in the order of La1.85Sr0.15CuO4. YBa2Cu3O7 and Tl2Ba2CuO6+y. 1/T1 of63Cu in the CuO2 plane in the normal state follows essentially a Curie-Weiss law at high temperature andT1T = 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 betweenTc andvQ, 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 inTc.