GROUND-STATE SYMMETRY AND IMPURITY EFFECTS IN SUPERCONDUCTORS

The effect of non-magnetic impurity scatterers on superconducting ground state and quasiparticle spectrum in s- and d-wave superconductors is considered. The calculated t-matrix has no poles within the s-wave gap, but reveals low lying resonances in the d-wave spectrum. The superconducting order parameter is found to be always suppressed at the impurity site. The quasiparticle spectrum for s-wave is just insensitive to impurities, while (relatively broad) resonances in the density of states (DOS) for d-wave are well pronounced already at very low impurity concentrations (similar to 10(-3)). Comparison of these results with experimental data on metal-oxide superconductors better testifies in favour of d-wave symmetry, in particular, permitting to explain the peculiar ''small gap'' behaviour in the tunnelling characteristics.