Cu NMR Study in Single Crystal Bi₂Sr₂CaCu₂O₈ ---Observation of Gapless Superconductivity---

Abstract

The normal and superconducting properties in single crystal Bi₂Sr₂CaCu₂O₈ (Bi2212) with T_c=86, K have been investigated by systematic measurements of the ⁶³Cu Knight shift, ⁶³K(T) and the nuclear-spin-lattice relaxation time, T₁. In the normal state, the T-dependences of ⁶³K(T) and (T₁T)^-1 in Bi2212 share similar magnetic behaviors to the underdoped systems such as oxygen deficient YBa₂Cu₃O_6+x and YBa₂Cu₄O₈, namely, ⁶³K(T) decreases with lowering temperature and ⁶³(T₁T)^-1 has a peak above T_c. However, from the analyses of the anisotropy of 1/T₁ and the hyperfine field, Bi2212 is found to belong to the slightly over-doped region as in Tl₂Ba₂CuO_6+x and TlSr₂CaCu₂O_8-x. In the superconducting state, the finding of T₁T=const. law far below T_c has revealed that the superconductivity is in a gapless regime, accompanied by the finite density of states at the Fermi level.