Observing topological phase transition in ferromagnetic transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDs) are considered a suitable platform to study topological properties such as the quantum anomalous Hall effect. However, this quantum transport property is usually found in systems with a small band gap. For large-band-gap TMDs, the nature of quantum transport is difficult to find. In this work, with the analysis of the k .p model, we investigate the topological phases of ferromagnetic TMDs in the cases of a large gap and small gap. By analyzing the orbital Berry curvature, we reveal that the orbital Hall effect in the system possesses a nontrivial topological invariant C-L=-1 when sgn (Delta(+1)Delta(-1))>0 , where Delta(tau) represents the band gap at the tau valley (K or K-' valley). Consequently, the large-gap ferromagnetic TMDs exhibit properties of an orbital Hall insulator. In the case of sgn (Delta(+1)Delta(-1))<0 , we demonstrate that the small-gap system transforms into a quantum anomalous Hall insulator with a Chern number C =1 . Both topological phases are valley polarized and robust in the presence of an out-of-plane magnetic moment. Subsequently, we illustrate this transition from an orbital Hall insulator to a quantum anomalous Hall insulator in FeCl2, a representative ferromagnetic TMD, by tuning the Coulomb correction U-eff to manipulate the valley band gaps.