Multiple electronic phase transitions in room-temperature second-order topological insulator

Two-dimensional magnetic materials with coupled valley and topological properties provide an attractive platform for next-generation spintronic and valleytronic applications. Here, using first-principles calculations, the novel monolayer SeVPH is predicted as a dynamically, thermally, and mechanically stable magnetic semiconductor. Its Berezinskii–Kosterlitz–Thouless transition temperature is 382 K. Spin–orbit coupling generates a valley splitting of 68 meV in the valence band with magnetization oriented along the +z direction, resulting in the anomalous valley Hall effect. Moreover, the monolayer SeVPH is identified as a second-order topological insulator (SOTI) hosting robust corner states. Biaxial tensile strain modulates the valley degree of freedom to induce successive topological phase transitions, driving the system from the SOTI to valley-polarized quantum anomalous Hall phase and then to a trivial normal insulator. These findings demonstrate that monolayer SeVPH is a promising multifunctional quantum material for tunable ferrovalley, spintronic, and topological devices.