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 .