Cu NMR and NQR Studies of Impurities-Doped YBa₂(Cu_1-<i>x</i>M_x)₃O₇(M=Zn and Ni)

Abstract

The normal and superconducting properties in impurity-doped YBa 2 (Cu 1- x M x ) 3 O 7 (M=Zn and Ni) have been investigated by 63 Cu NMR and NQR techniques. The nonmagnetic Zn-doping into the CuO 2 plane in YBa 2 Cu 3 O 7 (YBCO 7 ) causes a local collapse of antiferromagnetic (AF) spin correlation near Zn impurities and induces a gapless superconductivity with a finite density of states at the Fermi surface. On the other hand, in the case of Ni doping, there is no appreciable modification except for a small decrease of T c , although the Ni spins behave actually as local moments. The gapless feature in Zn-doped YBCO 7 is shown to be consistently interpreted within the framework of the d -wave model with gap zeros of lines at the Fermi surface as argued in heavy fermion superconductors. It is pointed out that the AF spin fluctuation-induced superconductivity is applicable to high- T c cuprates.