The effect of As-Chain layers in CaFeAs$_2$
  The new discovered iron-based superconductors have chain-like As layers. These layers generate an additional 3-dimensional hole pocket and cone-like electron pockets. The former is attributed to the Ca $d$ and As1 $p_z$ orbitals and the latter are attributed to the anisotropic Dirac cone, contributed by As1 $p_x$ and $p_y$ orbitals. We find that large gaps on these pockets open in the collinear antiferromagnetic ground state of CaFeAs$_2$, suggesting that the chain-like As layers are strongly coupled to FeAs layers. Moreover due to the low symmetry crystal induced by the As layers, the bands attributed to FeAs layers in $k_y=\pi$ plane are two-fold degenerate but in $k_x=\pi$ plane are lifted. This degeneracy is protected by a hidden symmetry $\hat{\Upsilon}=\hat{T}\hat{R}_y$. Ignoring the electron cones, the materials can be well described by a six-band model, including five Fe $d$ and As1 $p_z$ orbitals. We suggest that these new features may help us to identify the sign change and pairing symmetry in iron based superconductors. 
Xianxin Wu, Congcong Le, Yi Liang, Shengshan Qin, Heng Fan, and   Jiangping Hu
cond-mat.supr-con