CaFeAs2: A staggered intercalation of quantum spin Hall and high-temperature superconductivity

Abstract

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.

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

“…Second harmonic generation experiments confirmed the space group P 2 1 for La doped compounds [15]. More recently, theoretical studies focusing on the spacer layers determined that the As p x and p y orbitals are responsible for the Dirac cones, and the spin orbit coupling can open a gap and induce topological phases on these layers, suggesting the 112 compounds as prime candidates for proximity induced topological superconductivity [16,17].…”

Observation of the anisotropic Dirac cone in the band dispersion of 112-structured iron-based superconductor Ca0.9La0.1FeAs2

“…Thus, the S 4 symmetry for As 4p orbitals should be broken. The X and Y points would no longer be equivalent in BZ, and thus the Dirac-cone like feature could only be observed around one point 22,23 . Actually, our DFT calculation [ Fig.…”

“…Recently, high-temperature (high-T c ) superconductors such as Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) have been also applied as the substrate materials to synthesize artificial topological insulator/superconductor heterostructures, based on the expectation that a larger superconducting gap could be resolved since more robust cooper pairs might be introduced into the normal side through superconducting proximity effect [19][20][21] . However, whether this proximity system can really host Majorana fermions is still lack of further support, and whether high-T c superconductors could work in realizing Majorana fermions is a seductive but still open question 22,23 .…”

“…Consequently, below T c the proximity effect would drive the zigzag As chain of the CaAs layer to become a one-dimensional topological superconductor. In this proposal, the key argument is that the p x and p y orbitals of the As atoms in the unique arsenic zigzag bond layers of CaFeAs 2 would result in an anisotropic Dirac cone near the Fermi level (E F ), which is gapped by the spin-orbit coupling (SOC) and topological nontrivial 22,23 . Once this Dirac-cone like band dispersion could be confirmed experimentally, the staggered intercalation of quantum spin Hall insulator and superconductor in this 112-type iron-based superconductor could provide us a unique opportunity to realize Majorana modes in solids.…”

“…Very recently, there is one fascinating proposal which suggests the newly discovered 112-type iron-based hightemperature superconductor parent compound CaFeAs 2 be naturally a promising candidate to host Majorana fermions 23 .…”