Local Enhancement of Spin Susceptibility around a Nonmagnetic Impurity in the Normal State of High-T_c Cuprate Superconductors

We investigate local characters of spin susceptibility around a nonmagnetic impurity in the normal state of high-T_c cuprate superconductors. In a model two-dimensional system with strong antiferromagnetic (AF) spin fluctuations and with an extended impurity potential which is finite not only at the impurity site but also around it, we find that the AF susceptibility and the uniform one are strongly enhanced around the impurity. In particular, the uniform susceptibility which is almost temperature-independent in the clean system remarkably increases with decreasing temperature. We also show that the local enhancement of the susceptibility is due to (1) an increase of the local density of states at the Fermi level near the impurity site, and (2) a mode-mode coupling effect which is caused by the impurity potential. Our results can explain the recent Knight shift measurements in YBa₂(Cu_1-xZn_x)₃O_6+y and YBa₂(Cu_1-xLi_x)₃O_6+y showing a Curie-Weiss temperature dependence of the uniform susceptibility at (around) the impurities, and also agree with the recent experiments on the nuclear spin-lattice relaxation rate in Zn-doped YBa₂Cu₃O_6+y and YBa₂Cu₄O₈ which observed anomalous enhancement of the low-energy AF spin fluctuations.