GAPLESS SUPERCONDUCTIVITY IN ZN-DOPED YBA2CU3O7 STUDIED BY CU NMR AND NQR - POSSIBILITY OF D-WAVE SUPERCONDUCTIVITY IN HIGH-TC OXIDES

The superconducting properties of Zn-doped YBa2Cu3O7 have been investigated by the Cu-63-NQR and -NMR technique. Both the Knight shift K and the nuclear-spin-lattice relaxation time T1 were measured by using the oriented powder. The residual spin Knight shift of Cu-63 at 4.2 K increases appreciably with increasing Zn-content from x = 0.01 to 0.02. Correspondingly, the T1T = constant law holds at low temperatures far below T(c) = 79 K and 68 K for x = 0.01 and 0.02 respectively, providing a clear sign of gapless superconductivity with finite density of states at the Fermi level, whereas the behavior of rapid decrease of 1/T1 just below T(c) is almost unchanged even though Zn impurities are doped into the CuO2 plane. The NMR results in the Zn-doped and undoped YBa2Cu3O7 can be well interpreted by the 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-impurities and by some imperfection of the crystal existing inevitably even in undoped YBa2Cu3O7.