Correlation-enhanced spin–orbit coupling in a quantum anomalous Hall insulator Fe2Br2 monolayer with a large band gap and robust ferromagnetism

Abstract

Nontrivial band topology combined with magnetic ordering can produce a quantum anomalous Hall insulator (QAHI), which may lead to advances in device concepts. Here, through first-principles calculations, stable monolayer Fe₂Br₂ is predicted as a room-temperature large-gap high-Chern-number QAHI by using a generalized gradient approximation plus U (GGA + U) approach. The large gap is due to the correlation-enhanced spin–orbit coupling (SOC) effect of Fe atoms, which equates with artificially increasing the strength of the SOC without electronic correlation. An out-of-plane magnetic anisotropy is very key to produce a quantum anomalous Hall (QAH) state because an in-plane magnetization will destroy nontrivial band topology. In the absence of SOC, Fe₂Br₂ is a half Dirac semimetal state protected by mirror symmetry, and the electronic correlation along with the SOC effect creates the QAH state with a sizable gap and two chiral edge modes. It is found that the QAH state is robust against biaxial strain (a/a₀: 0.96 to 1.04) in monolayer Fe₂Br₂ with stable ferromagnetic (FM) ordering and out-of-plane magnetic anisotropy. The calculated results show that the Curie temperature is sensitive to the correlation strength and strain. The reduced correlation and compressive strain are in favour of high Curie temperature. These analyses and results can be readily extended to other monolayer Fe₂XY (X/Y = Cl, Br and I), which possess the same Fe-dominated low-energy states as an Fe₂Br₂ monolayer. These findings open new opportunities to design new high-temperature topological quantum devices.