Abstract
We report extensive measurements of the Knight shift K, the nuclear spin-lattice relaxation rate 1/, and the Gaussian spin-echo decay rate 1/ of in overdoped (Tl1212) with = 70 K, 52 K, and 10 K, in order to elucidate the origin of the reduction in with increasing holes and to identify the symmetry of the order parameter. In the normal state, it is shown that 1/T obeys the Curie-Weiss law, pointing to the presence of the antiferromagnetic (AF) spin correlation. From the analyses of 1/ and 1/, 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, Q=(π/a,π/a), , transfer to a higher-energy region and concomitantly reduces the magnetic correlation length 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/ 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- cuprates so far. Eventually, the reduction in 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 from 93 K in with double layers to 115–135 K in and with triple layers was shown to be due to the increase in with unchanged appreciably. This finding was compatible with the relationship of ∝exp(-1/λ) based on the spin-fluctuation-induced mechanism for the d-wave superconductivity. Within the same scheme, the origin of the marked decrease in irrespective of increasing in Tl1212 is proposed to be due to the significant reduction in which makes the pairing interaction weaken and λ in the above formula reduced. © 1996 The American Physical Society.
- Received 15 May 1996
DOI:https://doi.org/10.1103/PhysRevB.54.10131
©1996 American Physical Society