17O NMR study of local hole density and spin dynamics in YBa 2Cu 4O 8

Abstract

17O NMR has been carried out in YBa 2Cu 4O 8 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/ T 1 T 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 CuO 2 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/ T 1 between 17O and 63Cu, which is also seen in YBa 2Cu 3O 7 ( T c = 92 K) and YBa 2Cu 3O 6.63 ( T c = 60 K), seems to be essential for high- T c superconductivity.