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 $p_z$ 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 ${\text{CaFeAs}}_2$, 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 $k_y=\pi$ plane are twofold degenerate but in $k_x=\pi$ plane are lifted. This degeneracy is protected by a hidden symmetry $\widehat{{\rm Y}}=\widehat{T}{\widehat{R}}_y$. 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 features may help us to identify the sign change and pairing symmetry in iron-based superconductors.

• Received 26 February 2014
• Revised 21 April 2014

DOI:https://doi.org/10.1103/PhysRevB.89.205102