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

Abstract

We investigate nonmagnetic impurity effects on antiferromagnetic (AF) spin fluctuations in d x 2− y 2 -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/( T 1 T) 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/( T 1 T) Cu which was recently observed in Zn-doped YBa 2Cu 4O 8.