No abstract

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 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 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 ky = π plane are two-fold degenerate but in kx = π plane are lifted. This degeneracy is protected by a hidden symmetryΥ =TRy. 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 new features may help us to identify the sign change and pairing symmetry in iron based superconductors.

We predict that CaFeAs2, a newly discovered iron-based high temperature (Tc) superconductor, is a staggered intercalation compound that integrates topological quantum spin hall (QSH) and superconductivity (SC). CaFeAs2 has a structure with staggered CaAs and FeAs layers. While the FeAs layers are known to be responsible for high Tc superconductivity, we show that with spin orbital coupling each CaAs layer is a Z2 topologically nontrivial two-dimensional QSH insulator and the bulk is a 3-dimensional weak topological insulator. In the superconducting state, the edge states in the CaAs layer are natural 1D topological superconductors. The staggered intercalation of QSH and SC provides us an unique opportunity to realize and explore novel physics, such as Majorana modes and Majorana Fermions chains.

Anomalous surface states with Fermi arcs are commonly considered to be a fingerprint of Dirac semimetals (DSMs). In contrast to Weyl semimetals, however, Fermi arcs of DSMs are not topologically protected. Using first-principles calculations, we predict that β-cuprous iodide (β-CuI) is a peculiar DSM whose surface states form closed Fermi pockets instead of Fermi arcs. In such a fermiological Dirac semimetal, the deformation mechanism from Fermi arcs to Fermi pockets stems from a large cubic term preserving all crystal symmetries and from the small energy difference between the surface and bulk Dirac points. The cubic term in β-CuI, usually negligible in prototypical DSMs, becomes relevant because of the particular crystal structure. As such, we establish a concrete material example manifesting the lack of topological protection for surface Fermi arcs in DSMs.