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

Abstract

The normal and superconducting properties in single crystal Bi ₂ Sr ₂ CaCu ₂ O ₈ (Bi2212) with T _c =86 K have been investigated by systematic measurements of the ⁶³ Cu Knight shift, ⁶³ K( T ) and the nuclear-spin-lattice relaxation time, T ₁ . In the normal state, the T -dependences of ⁶³ K( T ) and ( T ₁ T ) ^-1 in Bi2212 share similar magnetic behaviors to the underdoped systems such as oxygen deficient YBa ₂ Cu ₃ O _{6+ x} and YBa ₂ Cu ₄ O ₈ , namely, ⁶³ K( T ) decreases with lowering temperature and ⁶³ ( T ₁ T ) ^-1 has a peak above T _c . However, from the analyses of the anisotropy of 1/ T ₁ and the hyperfine field, Bi2212 is found to belong to the slightly over-doped region as in Tl ₂ Ba ₂ CuO _{6+ x} and TlSr ₂ CaCu ₂ O _{8- x} . In the superconducting state, the finding of T ₁ T =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.