Valley-polarized quantum anomalous Hall insulator in monolayer

Abstract

The coexistence of an intrinsic ferrovalley (FV) and nontrivial band topology attracts intensive interest, both for its fundamental physics and for its potential applications, namely a valley-polarized quantum anomalous Hall insulator (VQAHI). Here, based on first-principles calculations by using a generalized gradient approximation plus a U (GGA + U) approach, the VQAHI induced by electronic correlation or strain can occur in monolayer . For perpendicular magnetic anisotropy (PMA), the FV to half-valley-metal (HVM) to quantum anomalous Hall (QAH) to HVM to FV transitions can be driven by increasing the electron correlation U. However, there are no special QAH states and valley polarization for in-plane magnetic anisotropy. By calculating the actual magnetic anisotropy energy (MAE), the VQAHI can indeed exist between two HVM states due to PMA, a unit Chern number/a chiral edge state and spontaneous valley polarization. The increasing U can induce VQAHI, which can be explained by a sign-reversible Berry curvature or a band inversion between d xy / and orbitals. Even though the real U falls outside the range, the VQAHI can be achieved by strain. Taking U = 2.25 eV as a concrete case, the monolayer can change from a common ferromagentic (FM) semiconductor to VQAHI under about 0.985 compressive strain. It is noted that the edge states of VQAHI are chiral-spin-valley locking, which can achieve complete spin and valley polarizations for low-dissipation electronic devices. Both the energy band gap and the valley splitting of VQAHI in monolayer are higher than the thermal energy of room temperature (25 meV), which is key for device applications at room temperature . It is found that the electronic correlation or strain have important effects on the Curie temperature of monolayer . These results can be readily extended to other monolayer (M = Ru, Os; X/Y = Cl, Br I). Our work emphasizes the importance of electronic correlation and PMA to study FV materials, and provides a pathway to realize VQAHI.