Magnetic Excitation And Superconductivity In Overdoped Tlsr2cacu2o7-Delta: A Cu-63 Nmr Study

We report extensive measurements of the Knight shift K, the nuclear spin-lattice relaxation rate 1/T-1, and the Gaussian spin-echo decay rate 1/T-2G of Cu-63 in overdoped TlSr2CaCu2O7-delta (Tl1212) with T-c = 70 K, 52 K, and 10 K, in order to elucidate the origin of the reduction in T-c with increasing holes and to identify the symmetry of the order parameter. In the normal state, it is shown that 1/T1T obeys the Curie-Weiss law, pointing to the presence of the antiferromagnetic (AF) spin correlation. From the analyses of 1/T-1 and 1/T-2G, it is found that the increase of the hole content in Tl1212 compounds makes the characteristic energy of the AF spin fluctuation around a zone boundary, SZ =(pi/a, pi/a), Gamma(Q), transfer to a higher-energy region and concomitantly reduces the magnetic correlation length xi(m) significantly. The AF spin correlation is concluded to become less distinct in going from the optimum-doped to the overdoped regime. In the superconducting state, the T dependences of K and 1/T-1 have revealed that the superconductivity is in the gapless regime with a finite density of states at the Fermi level. The NMR results are consistently interpreted in the d-wave model in which the impurity scattering is incorporated in terms of the unitarity Limit as demonstrated in most of the high-T-c cuprates so far. Eventually, the reduction in T-c from 70 K to 52 K in Tl1212 is concluded to be not due to the impurity effect associated with the oxygen content. In the previous works, the enhancement of T-c from 93 K in YBa2Cu3O7 with double CuO2 layers to 115-135 K in Tl2Ba2Ca2Cu2O10 and HgBa2Ca2Cu2O8+delta with triple CuO2 layers was shown to be due to the increase in Gamma(Q) with xi(m) unchanged appreciably. This finding was compatible with the relationship of T-c proportional to Gamma(Q) xi(m)(2)exp(-1/lambda) based on the spin-fluctuation-induced mechanism for the d-wave superconductivity. Within the same scheme, the origin of the marked decrease in T-c irrespective of increasing Gamma(Q) in Tl1212 is proposed to be due to the significant reduction in xi(m) which makes the pairing interaction weaken and lambda in the above formula reduced.