Theory for superconductivity in (Tl,K)Fe _x Se ₂ as a doped Mott insulator

Abstract

Possible superconductivity in recently discovered (Tl,K)Fe$_x$Se$_2$\ncompounds is studied from the viewpoint of doped Mott insulator. The Mott\ninsulating phase is examined to be preferred in the parent compound at $x=1.5$\ndue to the presence of Fe vacancies. Partial filling of vacancies at the\nFe-sites introduces electron carriers and leads to electron doped\nsuperconductivity. By using a two-orbital Hubbard model in the strong coupling\nlimit, we find that the s-wave pairing is more favorable at small Hund's\ncoupling, and d$_{x^2-y^2}$ wave pairing is more favorable at large Hund's\ncoupling.\n