Intralayer Non-Collinear Antiferromagnetism Induced Flat Bands in Two-Dimensional CoBi₂Te₄

The interplay of the topology of electronic wavefunctions with spin configurations in intrinsic magnetic topological insulators (TIs) gives rise to various exotic topological states. Here we predict existence of stable, two-dimensional CoBi₂Te₄ with an intralayer non-collinear antiferromagnetic state. Such an exotic magnetism is explained by the competing Heisenberg exchange interaction based on Co triangular lattice, where the weak exchange coupling between next-nearest neighbour Co plays a vital role in determining the non-collinear order. CoBi₂Te₄ presents trivial insulating state in one septuple-layer structure with a large band gap of 309 meV. In CoBi₂Te₄ two septuple layers, the band gap reduces to 52 meV, and the topological nature is determined by an obvious band inversion. Most interestingly, a flat band is observed in the edge state of nanoribbon with non-collinear antiferromagnetic termination. The appearance of the flat band can be attributed to the magnetic inhomogeneities caused by the intralayer non-collinear antiferromagnetic coupling.