Impurity Effect in High-TcSuperconductors –A Consistent Analysis byd-Wave Superconducting Model–

Abstract

The contrast between Zn- and Ni-impurity effects in YBa ₂ Cu ₃ O ₇ and the imperfection-induced gapless superconductivity in Bi ₂ Sr ₂ CaCu ₂ O ₈ reported previously are quantitatively analyzed based on the d -wave superconducting model. By taking account of the fact that the Zn impurity which caused the gapless superconductivity acts as a unitarity or strong scatterer, it is demonstrated that the Ni impurity does not give rise to the gapless superconductivity, although it is not so weak as to be treated in terms of Born scattering. The d -wave model can explain the characteristic feature such that even though T _c is moderately reduced by impurity scattering, the superconductivity changes into the gapless type with a finite density of states at the Fermi level.