Magnetic excitation and superconductivity in overdoped TlSr2CaCu2O7−δ: A Cu63 NMR study

Abstract

We report extensive measurements of the Knight shift K, the nuclear spin-lattice relaxation rate 1/T1, and the Gaussian spin-echo decay rate 1/T2G of Cu63 in overdoped TlSr2CaCu2O7−δ (Tl1212) with Tc = 70 K, 52 K, and 10 K, in order to elucidate the origin of the reduction in Tc 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/T1 and 1/T2G, 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), ΓQ, transfer to a higher-energy region and concomitantly reduces the magnetic correlation length ξm 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/T1 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-Tc cuprates so far. Eventually, the reduction in Tc 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 Tc from 93 K in YBa2Cu3O7 with double CuO2 layers to 115–135 K in Tl2Ba2Ca2Cu3O10 and HgBa2Ca2Cu3O8+δ with triple CuO2 layers was shown to be due to the increase in ΓQ with ξm unchanged appreciably. This finding was compatible with the relationship of Tc∝ΓQξm2exp(-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 Tc irrespective of increasing ΓQ in Tl1212 is proposed to be due to the significant reduction in ξm which makes the pairing interaction weaken and λ in the above formula reduced. © 1996 The American Physical Society.

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