¹⁷O NMR study of local hole density and spin dynamics in YBa₂Cu₄O₈

¹⁷O NMR has been carried out in YBa₂Cu₄O₈ with T_c = 74 K. The nuclear quadrupole tensors in the oxygen sites were determined and compared with those in other high-T_c cuprates. The hole densities in the oxygen and copper sites have been extracted from ν_z(O) and ν_Q(Cu) by assuming that the electric field gradient is dominated by the on-site holes. We propose that the hole distribution (the extent of covalency) between the planar oxygens and copper, which is changed by doping and pressure, is a hidden parameter determining the spin dynamics and the superconducting transition temperature T_c in high-T_c cuprates. It was found that 1/T₁T and the Knight shift of the planar oxygens decrease with lowering temperature in the normal state. The nuclear relaxation rates of ¹⁷O and ⁶³Cu in the CuO₂ plane exhibit different behaviors at temperatures higher than ∼ 120 K, reflecting the different q-window of spin fluctuations, while both of them share the same T-dependence below ∼ 120 K down to 55 K. This scaling of 1/T₁ between ¹⁷O and ⁶³Cu, which is also seen in YBa₂Cu₃O₇ (T_c = 92 K) and YBa₂Cu₃O_6.63 (T_c = 60 K), seems to be essential for high-T_c superconductivity.