Impurity effects on the spin excitation spectrum in anisotropic superconductors.

Abstract

Effects of nonmagnetic and magnetic impurities on [ital d]-wave and extended-[ital s]-wave superconductors in two dimensions are studied. After the effects on the density of states are investigated, the effects on the dynamical spin susceptibility are studied. In the clean limit, the magnetic response has significant wave-vector dependence if the gap has a large anisotropy. When impurities are introduced this dependence is smeared. In the extended-[ital s]-wave superconductor with a fully open gap, the response at low frequencies is strongly suppressed at low temperatures in the clean limit. Magnetic impurities can give rise to low-lying excitations in the gap and enhance the low-frequency magnetic response at low temperatures. It is found, however, that the intensity is still quite weak with a moderate concentration of the impurities. It is discussed how these results can help in the determination of the symmetry of the order parameter of the high-[ital T][sub [ital c]] superconductors.