Recently, Professor Ren Junfeng and his team published a paper entitled "Quadratic band crossing induced quantum anomalous Hall effect in monolayer" in the Nature Index journal Physical Review B MoTe2F2 "research paper. Shandong Normal University is the sole signatory, master student Chen Fanzheng is the first author of the paper, and Professor Ren Junfeng is the corresponding author.

Quantum anomalous Hall effect (QAHE) has great potential applications in low-power devices and has received great attention in recent years. However, QAHE still faces the limitations of a relatively small global band gap and a low number of cycles. To obtain large global bandgap and high number of QAHE has always been the focus of QAHE research. Based on the tight binding model, it is found that ferromagnetic materials with secondary band crossing can achieve large global band gaps. In addition, considering the secondary band crossing under spin-orbit coupling effect, the high term topological state of C=−2 can be achieved.

Based on the results of the tightly bound model, the author constructs a single-layer MoTe2F2. Through first-principles calculation and low-energy k.p model analysis, it is found that the single-layer MoTe2F2 has a quadratic band crossing near the Fermi plane and a linear band crossing below the Fermi plane. In addition, the paper also found that the secondary band crossing can produce a high number of cycles. The hybridization and the different contributions of the orbitals form the secondary band crossing and at the same time lead to a large global band gap. The secondary band crossing can be further regulated by applying biaxial strain and vertical electric field to obtain anomalous Valley Hall effect. This work not only provides a way to discover two-dimensional materials with large global bandgaps and high aging numbers, but also combines topological electronics and valley electronics, which is expected to promote the development of low-power devices.

The above research was supported by the National Natural Science Foundation of China and the Natural Science Foundation of Shandong Province.