Strain-driven valley states and phase transitions in Janus VSiGeN4 monolayer

Manipulating the valley degree of freedom is an important target of valleytronics development, which provides remarkable opportunities for both fundamental research and practical applications. Here, based on first-principles calculations, we demonstrate the intrinsic valley-polarized quantum anomalous Hall effect in a monolayer ferrovalley material: Janus VSiGeN4, of which the edge states are chiral-spin-valley locking. Furthermore, a small tensile or compressive strain can drive phase transition in the material from the valley-polarized quantum anomalous Hall state to the half-valley-metal state. With the increase in the strain, the material turns into a ferrovalley semiconductor with the valley anomalous Hall effect. The origin of the phase transition is the sequent band inversion of the V d orbital at the K valleys. Moreover, we find that phase transition causes the sign reversal of the Berry curvature and induces different polarized light absorption in different valley states. Our work provides an ideal material platform for practical applications and experimental exploration of the interplay among topology, spintronics, and valleytronics.