Quantum Monte Carlo study of the $S_4$ symmetric microscopic model for   iron-based superconductors
  The $S_4$ symmetric microscopic model with two iso-spin components has been studied via constrained-path quantum Monte Carlo simulation. Our results demonstrate a stable $(\pi,0)$ or $(0,\pi)$ magnetic order which is significantly enhanced on increasing both the Coulomb repulsion $U$ and Hund's coupling strength $J$. Also, our simulation indicates that the magnetic order tends to be in an orthomagnetic one, in which the nearest-neighbour magnetic moment are orthogonal to each other, rather than in a collinear antiferromagnetic state. Interestingly, when the system is doped away from half filling, the magnetic order is obviously elevated in the low doping density, and then significantly suppressed when more electrons are introduced. Meanwhile, we find that an $A_{1g}$ $s_{\pm}$-wave pairing dominates all the singlet nearest-neighbour pairings, and is significantly enhanced via electron doping. 
Guang-Kun Liu, Zhong-Bing Huang, and Yong-Jun Wang
cond-mat.str-el