NMR studies of spin excitations in superconducting Bi2Sr2CaCu2O8+δ single crystals

The oxygen NMR shift and the Cu nuclear spin-lattice relaxation rate (1T1) were measured in Bi2Sr2CaCu2O8+δ single crystals. While both the shift and 1T1T decrease sharply near Tc, 1T1T becomes nearly constant at low temperatures, indicating a gapless superconducting state with finite density of states at the Fermi level. From the oxygen shift data, the residual spin susceptibility at T=0 is estimated to be 10% of the value at room temperature. Our results are most consistent with a a-wave pairing model with strong (resonant) impurity scattering. © 1994 The American Physical Society.