Magnetic excitation and superconductivity in overdoped TlSr2CaCu2O7- delta : A 63Cu NMR study.

Abstract

We report extensive measurements of the Knight shift K, the nuclear spin-lattice relaxation rate 1/${\mathit{T}}_{1}$, and the Gaussian spin-echo decay rate 1/${\mathit{T}}_{2\mathit{G}}$ of $^{63}\mathrm{Cu}$ in overdoped ${\mathrm{TlSr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (Tl1212) with ${\mathit{T}}_{\mathit{c}}$ = 70 K, 52 K, and 10 K, in order to elucidate the origin of the reduction in ${\mathit{T}}_{\mathit{c}}$ with increasing holes and to identify the symmetry of the order parameter. In the normal state, it is shown that 1/${\mathit{T}}_{1}$T obeys the Curie-Weiss law, pointing to the presence of the antiferromagnetic (AF) spin correlation. From the analyses of 1/${\mathit{T}}_{1}$ and 1/${\mathit{T}}_{2\mathit{G}}$, 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=(\ensuremath{\pi}/a,\ensuremath{\pi}/a), ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{Q}}$, transfer to a higher-energy region and concomitantly reduces the magnetic correlation length ${\ensuremath{\xi}}_{\mathit{m}}$ 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/${\mathit{T}}_{1}$ 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-${\mathit{T}}_{\mathit{c}}$ cuprates so far. Eventually, the reduction in ${\mathit{T}}_{\mathit{c}}$ 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 ${\mathit{T}}_{\mathit{c}}$ from 93 K in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ with double ${\mathrm{CuO}}_{2}$ layers to 115--135 K in ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ and ${\mathrm{HgBa}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ with triple ${\mathrm{CuO}}_{2}$ layers was shown to be due to the increase in ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{Q}}$ with ${\ensuremath{\xi}}_{\mathit{m}}$ unchanged appreciably. This finding was compatible with the relationship of ${\mathit{T}}_{\mathit{c}}$\ensuremath{\propto}${\mathrm{\ensuremath{\Gamma}}}_{\mathit{Q}}$${\ensuremath{\xi}}_{\mathit{m}}^{2}$exp(-1/\ensuremath{\lambda}) based on the spin-fluctuation-induced mechanism for the d-wave superconductivity. Within the same scheme, the origin of the marked decrease in ${\mathit{T}}_{\mathit{c}}$ irrespective of increasing ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{Q}}$ in Tl1212 is proposed to be due to the significant reduction in ${\ensuremath{\xi}}_{\mathit{m}}$ which makes the pairing interaction weaken and \ensuremath{\lambda} in the above formula reduced. \textcopyright{} 1996 The American Physical Society.