Local Hole Distribution in the CuO₂ Plane of High-T_c Cu-Oxides Studied by Cu and Oxygen NQR/NMR

The hole numbers at Cu and oxygen sites in the CuO₂ plane in a wide variety of high-T_c materials have been extracted from the measured nuclear quadrupole frequency, v_Q. The hole number in the [formula omitted] orbit, [formula omitted] is found to be more than 0.85 in La_2-xSr_xCuO₄ (x=0.075-0.24), while decreases to 0.74 in Tl₂Ba₂Ca₂Cu₃O₁₀. Thus estimated [formula omitted] is supported by the result that the orbital Knight shift along the c-axis, [formula omitted] increases in proportion to [formula omitted] In contrast to the decrease of [formula omitted] the hole number in the O-2p_σ orbit, [formula omitted] increases in going from the La- to the Tl-systems. It has been found that T_c is determined by two parameters about holes, i.e., [formula omitted] and [formula omitted] The former tells the microscopic “doping level” of the CuO₂ plane, while a smaller [formula omitted] results in a weaker antifer-romagnetic spin fluctuations of low energy. It has been found that not only a proper [formula omitted] but also a sufficiently small [formula omitted] are needed to obtain a high value of T_c.