Janus monolayers Fe₂SSeX₂ (X = Ga, In, and Tl): Robust nontrivial topology with high Chern number

High-performance quantum anomalous Hall (QAH) systems are crucial materials for exploring emerging quantum physics and magnetic topological phenomena. Inspired by layered FeSe materials with excellent superconducting properties, the Janus monolayers Fe₂SSeX₂ (X = Ga, In and Tl) are built by the decoration of Ga, In and Tl atoms in monolayer Fe₂SSe. In first-principles calculations, Fe₂SSeX₂ have stable structures and prefer ferromagnetic (FM) ordering, and can be considered as Weyl semimetals without spin–orbit coupling. For out-of-plane (OOP) magnetic anisotropy, large nontrivial gaps are opened and the Fe₂SSeX₂ are predicted to be large-gap QAH insulators with a high Chern number C = 2, proved by two chiral edge states and Berry curvature. When the magnetization is flipped, the two chiral edge states can be simultaneously changed and C = −2 can be obtained, revealing the fascinating behavior of chiral spin–edge state locking. It is found that the QAH properties of Fe₂SSeX₂ are robust against strain. In particular, nontrivial topological quantum states can spontaneously appear for Fe₂SSeGa₂ and Fe₂SSeIn₂ because the orientations of the easy magnetic axis are adjusted from in-plane to OOP by the biaxial strain. Our studies provide excellent candidate systems to realize QAH properties with a high Chern number, and suggest more experimental explorations combining superconductivity and topology.