Cu NMR and NQR Studies of High-T_c Superconductor La_2-xSr_xCuO₄

抄録

Results of extensive Cu NMR and NQR studies in the superconducting La_2-xSr_xCuO₄ with x=0.075-- 0.24 have been presented. The rapid decrease of the nuclear spin-lattice relaxation rate, 1/T₁, and the spin Knight shift below T_c confirm microscopically the existence of the superconducting phase with lower T_c than 38, K for 0.10 ≤ x ≤ 0.24. For x=0.10, magnetic anomalies emerge at low-T in some small parts of the sample, e.g. T-independent T₁ and broadening of spectrum, having some relevance to the magnetic order in La_2-xBa_xCuO₄ with x=0.125. For x>0.15, the T₁T=const. behavior and the existence of the residual spin Knight shift far below T_c have revealed that the superconductivity is of a gapless type with a finite density of states at the Fermi level caused by the mixture of different structures, i.e. tetragonal and orthorhombic.