NMR and NQR studies of spin correlation and superconductivity in high-T_c oxides

The magnetic and superconducting properties in high-T_c cuprates have been investigated over a wide doping range by NMR and NQR of ⁶³C_u and ¹⁷O in the lightly-doped La_2⇔xSr_xCuO₄] (LSCO), YBa₂Cu₄O₈ (Y124) and the heavily-doped Bi₂Sr₂CaCu₂O₈ (BSCO), Tl₂Ba₂CuO_6+y (TBCO) and the Zn- and Ni-doped YBa₂Cu₃O₇ (YBCO₇). From extensive studies in the normal state, it is extracted that the uniform, χ_s(T) and the staggered, _χQ(T) susceptibilities originating from the antiferromagnetic (AF) spin correlation are suppressed with increasing hole content and furthermore, the T-dependence of _χs(T) and _χQ(T) is changed from the T-increasing behavior with temperature and the Curie-Weiss one into the T-independent behavior, respectively. The superconducting properties cannot be accounted for by the conventional BCS model or other isotropic s-wave models. A d-wave model is applicable in interpreting consistently most of NMR results, if the finite density of states at the Fermi level is taken into consideration associated with the pair breaking effect. There are increasing evidences that the magnetic mechanism for the superconductivity is promising in high-T_c cuprates.