Theory for superconductivity in (Tl,K)Fe$_x$Se$_2$ as a doped Mott insulator

TL;DR

This paper develops a theoretical framework for understanding superconductivity in (Tl,K)Fe$_x$Se$_2$ as a doped Mott insulator, highlighting the role of electron doping and Hund's coupling in pairing symmetry.

Contribution

It introduces a two-orbital Hubbard model analysis to explain the emergence of superconductivity and pairing symmetry dependence on Hund's coupling in (Tl,K)Fe$_x$Se$_2$.

Findings

Electron doping arises from Fe vacancy filling.

s-wave pairing favored at low Hund's coupling.

d$_{x^2-y^2}$ wave pairing favored at high Hund's coupling.

Abstract

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.