Zn-impurity effect and interplay of s± and s++ pairings in iron-based superconductors

We report theoretical and experimental studies of the effect of Zn impurity in Fe-based superconductors. Zn impurity is expected to severely suppress sign reversed s(+/-)-wave pairing. The experimentally observed suppression of T-c under Zn doping strongly depends on the materials and the charge-carrier contents, which suggests competition of s(++) and s(+/-) pairings in Fe-based superconductors. We study a model incorporating both s(++)- and s(+/-)-pairing couplings by using the Bogoliubov de-Gennes equation, and we show that the Zn impurity strongly suppresses s(+/-) pairing and may induce a transition from the s(+/-) to s(++) wave. Our theory is consistent with various experiments on the impurity effect. We present new experimental data on the Zn-doping SmFe1-xZnxAsO0.9F0.1 of T-c = 50 K, in further support of our proposal.