Zn-impurity effect and interplay of $s_±$- and $s_++$-pairings in Fe-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-base superconductors. We study a model incorporating both $s_++$ and $s_±$ pairing couplings by using Bogoliubov de-Gennes equation, and show that the Zn-impurity strongly suppresses $s_±$ pairing and may induce a transition from $s_±$ to $s_++$-wave. Our theory is consistent with various experiments on the impurity effect. We present new experimental data on the Zn-doping SmFe$_1-x$Zn$_x$AsO$_0.9$F$_0.1$ of T$_c=$ 50K, in further support of our proposal.