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

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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.