Magnetic topological insulators with switchable edge and corner states in monolayer VSi2P4

Magnetic topological insulators have been attracting great interest in two dimensions for both fundamental physics and applications in spintronics. Here, we put forward that the topological phase transition between a second-order topological insulator and quantum anomalous Hall insulator with a strikingly different bulk-boundary correspondence is possible in two-dimensional ferromagnets. We employ the intrinsic ferromagnetic ${\mathrm{VSi}}_{2}{\mathrm{P}}_{4}$ monolayer with giant valley polarization as a material candidate and elucidate that the second-order topological insulator emerges, distinguished by the topological indices ${\ensuremath{\chi}}^{(3)}=(\ensuremath{-}3,2)$ and well-localized corner states. Remarkably, under strain engineering, a topological phase transition takes place under a 0.67% tensile strain accompanied by obtaining the quantum anomalous Hall effect with a Chern number $\mathcal{C}=\ensuremath{-}1$ and one chiral edge state. As the tensile strain further increases, another topological phase transition is realized as the ${\mathrm{VSi}}_{2}{\mathrm{P}}_{4}$ monolayer changes into a normal insulator. Our work considerably bridges the higher-order topology and quantum anomalous Hall effect with a high possibility of innovative applications in topotronic devices.