POSSIBILITY OF D-WAVE SUPERCONDUCTIVITY IN HIGH-TC SUPERCONDUCTOR - NMR-STUDY IN YBA2(CU1-XMX)3O7 (M=ZN,NI)

The superconducting property of YBa2(Cu1-xMx) 3O7 (M = Zn and Ni) has been investigated by Cu-63 NQR and NMR techniques. In case of Zn(3d10) doped into CuO2, the residual spin Knight shift of Cu-63 increases appreciably with increasing concentration. Correspondingly, T1T = const. law holds far below T(c), providing a signature of the gapless superconductivity. The behavior of rapid decrease of 1/T1 just below T(c) has commonly been observed as in most of the high-T(c) materials. These results are well interpreted by a combined effect of the gap zeros of lines at the Fermi surface originating from a d-wave pairing and the residual density of states at the Fermi level induced by non-magnetic Zn-impurities1). In contrast, there is no clear sign of the gapless superconductivity caused by Ni(3d8) impurities, although we point out that the undoped system itself posseses more or less a gapless feature induced by some imperfection of the crystal.