Spin Correlation in High-T_c Cuprate HgBa₂Ca₂Cu₃O_8+δ with T_c=133 K—An Origin of T_c-Enhancement Evidenced by ⁶³Cu-NMR Study

The Knight shift, the nuclear spin-lattice relaxation rate, 1/T₁, and the transverse relaxation rate, 1/T_2G, of ⁶³Cu have been separately measured for the square and the pyramidal CuO₂ planes in HgBa₂Ca₂Cu₃O_8+δ (Hg1223) with the highest transition temperature, T_c= 133 K to date. From the quantitative analysis of 1/T₁ and 1/T_2G, it is shown that the product of the staggered susceptibility and the characteristic energy of spin fluctuations around a zone boundary, χ_QΓ_Q, for the square CuO₂ plane site is larger than that for YBa₂Cu₃O₇ (YBCO₇), although that for the pyramidal CuO₂ plane site is almost the same. This enhancement of χ_QΓ_Q for the square site is considered to be responsible for the higher T_c in Hg1223 than in YBCO₇.