Theory for superconductivity in (Tl,K)Fe$_x$Se$_2$ as a doped Mott   insulator
  Possible superconductivity in recently discovered (Tl,K)Fe$_x$Se$_2$ compounds is studied from the viewpoint of doped Mott insulator. The Mott insulating phase is examined to be preferred in the parent compound at $x=1.5$ due to the presence of Fe vacancies. Partial filling of vacancies at the Fe-sites introduces electron carriers and leads to electron doped superconductivity. By using a two-orbital Hubbard model in the strong coupling limit, we find that the s-wave pairing is more favorable at small Hund's coupling, and d$_{x^2-y^2}$ wave pairing is more favorable at large Hund's coupling. 
Yi Zhou, Dong-Hui Xu, Fu-Chun Zhang and Wei-Qiang Chen
cond-mat.supr-con cond-mat.str-el