Flat Bands Induced by Non-Collinear Antiferromagnetism in CoBi2Te4.

The interplay of the topology of electronic wavefunctions with spin configurations in intrinsically magnetic topological materials causes various exotic electronic states, attracting much attention in condensed matter physics. Non-collinear antiferromagnetic (ncAFM) state, characterized by spins lacking a specific orientation, remains enigmatic. Through first-principles calculations and Wannier simulations, we predicted that the AB-stacked CoBi2Te4 two-septuple layer (2SL) hosts an intrinsically intralayer ncAFM state while preserving a band inversion between Bi-p and Te-p orbitals in its bulk band structure. Intriguingly, we identified a group of flat bands near the Fermi level in the edge state of (210) nanoribbon terminated by ncAFM coupling. These flat bands persist in the edge state of a CoBi2Te4-Bi2Te3 heterostructure, where intralayer ncAFM coupling is maintained. In contrast, they vanish in systems lacking either band inversion or ncAFM coupling. This suggests that the formation of flat bands arises from the interplay between spin-orbit coupling-induced band inversion and the spatially varying local exchange field generated by intralayer ncAFM ordering. Our findings not only uncover the topological properties and electronic states of an intrinsically ncAFM configuration for the first time but also provide a novel strategy for realizing flat bands, which could have implications for strongly correlated electron systems.