NMR and NQR studies of spin dynamics and superconductivity in High-T _c oxides

The magnetic and superconducting properties in the high-T _c cuprates have been investigated over a wide hole doping range by ⁶³Cu, ¹⁷O and ²⁰⁵Tl NMR and NQR in the lightly-doped La _2-xSr _xCuO ₄ (LSCO), the heavily-doped Tl ₂Ba ₂CuO _6+y (TBCO) and the Zn-doped YBa ₂Cu ₃O ₇ (YBCO ₇). In low doping region, the large antiferromagnetic (AF) spin correlation around the zone boundary (q=Q) causes the Curie-Weiss behavior of ⁶³(1/T ₁T) associated with that of the staggered susceptibility χ _O(T) in LSCO. In the vicinity of the hole content where T _c has a peak, the AF spin correlation still survives, although the magnetic coherence length ξ _M is considerably short being presumably (ξ _M/a) ∼ 1. The further doping destroys progressively the AF spin correlation, which is no longer present is non-superconducting TBCO compounds. These NMR evidences signify that there is an intimate relation between the presence of the AF spin correlation and the onset of the superconductivity. The local collapse of AF spin correlation is a primary cause for the unexpected strong reduction of T _c in case of the substitution of Zn impurities into the CuO ₂ plane. The superconducting properties clarified by NMR experiments 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 the NMR results, if the finite density of states at the Fermi level is taken into consideration and is associated with the pair breaking effect. There are increasing evidences that the magnetic mechanism for the superconductivity is promising in high-T _c cuprates.