Single Impurity Effects in Multiband Superconductors with Different Sign Order Parameters

概要

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A single impurity problem is investigated for multiband $s$-wave superconductors with different sign order parameters ($s_{\pm}$-wave superconductors) suggested in Fe-pnictide superconductors. Not only intraband but also interband scattering is considered at the impurity. The latter gives rise to impurity-induced local boundstates close to the impurity. We present an exact form of the energy of the local boundstates as a function of strength of the two types of impurity scattering. The essential role of the impurity is unchanged in finite number of impurities. The main conclusions for a single impurity problem help us understand effects of dense impurities in the $s_{\pm}$-wave superconductors. Local density of states around the single impurity is also investigated. We suggest impurity site nuclear magnetic resonance as a suitable experiment to probe the local boundstates that is peculiar to the $s_{\pm}$-wave state. We find that the $s_{\pm}$-wave model is mapped to a chiral $d_{x^{2}-y^{2}}\pm\mathrm{i} d_{xy}$-wave, reflecting the unconventional nature of the sign reversing order parameter. For a quantum magnetic impurity, interband scattering destabilizes the Kondo singlet.
2009-08-15