Local Hole Distribution in the CuO2Plane of High-TcCu-Oxides Studied by Cu and Oxygen NQR/NMR

Abstract

The hole numbers at Cu and oxygen sites in the CuO ₂ plane in a wide variety of high- T _c materials have been extracted from the measured nuclear quadrupole frequency, ν _Q . The hole number in the Cu-3 d _{x ² - y ²} orbit, n _{x ² - y ²} , is found to be more than 0.85 in La _{2- x} Sr _x CuO ₄ ( x =0.075∼0.24), while decreases to 0.74 in Tl ₂ Ba ₂ Ca ₂ Cu ₃ O ₁₀ . Thus estimated n _{x ² - y ²} is supported by the result that the orbital Knight shift along the c -axis, K ^c _{o r b} , increases in proportion to n _{x ² - y ²} . In contrast to the decrease of n _{x ² - y ²} , the hole number in the O-2 p _σ orbit, n p _σ , 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., n _{x ² - y ²} +2 n p _σ and n _{x ² - y ²} /2 n p _σ . The former tells the microscopic \lq\lqdoping level” of the CuO ₂ plane, while a smaller n _{x ² - y ²} /2 n p _σ results in a weaker antiferromagnetic spin fluctuations of low energy. It has been found that not only a proper n _{x ² - y ²} +2 n p _σ but also a sufficiently small n _{x ² - y ²} /2 n p _σ (≃1) are needed to obtain a high value of T _c .