CU NMR AND NQR STUDY OF IMPURITY-DOPED YBA2CU3O7

The superconducting property in impurity-doped YBa2(Cu1-xMx)3O7 (M = Zn and Ni) has been investigated by measuring the nuclear spin-lattice relaxation time T1 and the Knight shift of Cu-63 in the CuO2 plane. We have found that Zn doping into the CuO2 plane in YBa2Cu3O7 causes gapless superconductivity at low T, giving rise to a finite density of states at the Fermi surface, while magnetic Ni doping into the CuO2 plane reduces T(c) moderately, although Ni spins behave as local moments. The gapless feature in Zn-doped YBCO7 can be consistently interpreted in the framework of the d-wave model with gap zeros of lines at the Fermi surface, as argued for heavy-fermion superconductors.