Effect of As-chain layers in CaFeAs2

The recently discovered iron-based superconductors have chainlike As layers. These layers generate an additional three-dimensional hole pocket and conelike electron pockets. The former is attributed to the Ca d and As1 pz orbitals and the latter are attributed to the anisotropic Dirac cone, contributed by As1 px and py orbitals. We find that large gaps on these pockets open in the collinear antiferromagnetic ground state of CaFeAs2, suggesting that the chainlike 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 ky=π plane are twofold degenerate but in kx=π plane are lifted. This degeneracy is protected by a hidden symmetry Υ̂=T̂R̂y. Ignoring the electron cones, the materials can be well described by a six-band model, including five Fe d and As1 pz orbitals. We suggest that these features may help us to identify the sign change and pairing symmetry in iron-based superconductors.