Gapless superconductivity in Zn-doped YBa 2Cu 3O 7 studied by Cu NMR and NQR : Possibility of d-wave superconductivity in high- Tc oxides

Abstract

The superconducting properties of Zn-doped YBa 2Cu 3O 7 have been investigated by the 63Cu-NQR and -NMR technique. Both the Knight shift K and the nuclear-spin-lattice relaxation time T 1 were measured by using the oriented powder. The residual spin Knight shift of 63Cu at 4.2 K increases appreciably with increasing Zn-content from x=0.01 to 0.02. Correspondingly, the T 1 T=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/ T 1 just below T c is almost unchanged even though Zn impurities are doped into the CuO 2 plane. The NMR results in the Zn-doped and undoped YBa 2Cu 3O 7 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 YBa 2Cu 3O 7.