Nonmagnetic impurity effects on antiferromagnetic spin fluctuations in high-T_c superconductivity

We investigate nonmagnetic impurity effects on antiferromagnetic (AF) spin fluctuations in d_x2-y2-wave superconductivity. We show that, far below the transition temperature (T_c), the low-energy AF spin fluctuations are enhanced by impurity scatterings in the unitarity limit owing to the formation of low-energy bound states around the impurities. As a result, the nuclear-spin lattice relaxation rate on Cu (1/(T₁T)_Cu) increases at low temperatures even below T_c. On the other hand, the enhancement of the AF spin fluctuations does not occur in the case of the Born scattering because of the absent of the bound states. The result in the unitarity limit can explain the anomalous increase of 1/(T₁T)_Cu which was recently observed in Zn-doped YBa₂Cu₄O₈.