Janus VAZ ₃ H (A = Si, Ge; Z = N, P) single layers exhibiting valley polarization, magnetic anisotropy, and topological transition

Using first-principles calculations, we explore the electronic and topological properties of Janus VAZ₃H single layers (A = Si, Ge; Z = N, P) that are dynamically and thermally stable. In the strain-free state, VSiN₃H, VSiP₃H, and VGeN₃H demonstrate direct bandgap ferrovalley (FV) semiconducting properties, while VGeP₃H displays an indirect bandgap. The easy magnetization axis varies among these materials, with VSiN₃H and VGeN₃H preferring in-plane magnetization, whereas VSiP₃H and VGeP₃H favor out-of-plane magnetization. Furthermore, the electronic structure analysis reveals valley polarization at the K and K' points. When subjected to strain, these systems experience phase transitions, such as direct-to-indirect bandgap shift, the evolution from FV semiconducting to half-valley metal (HVM), and the emergence of a quantum anomalous Hall (QAH) phase within certain strain intervals. The QAH phase is identified by chiral edge states and quantized anomalous Hall conductivity (AHC), supported by an integer AHC plateau of 1<i>e</i>²/<i>h</i> and a Chern number of 1. These results highlight the tunability of VAZ₃H SLs through strain engineering, providing a potential platform for valleytronic and topological applications.