Two-dimensional ferromagnetic high order topological insulators CrSX(X=Cl, Br, I)

Using density functional theory calculations and nontrivial topological index analyzations, we propose a series of Cr-based monolayer compounds to be magnetic high order topological insulators. The monolayer type-A CrSX(X=Cl, Br, I) are ferromagnetic insulators, with a quantized fractional charge e/2 at the corner in the up-spin state. While the monolayer type-B CrSX have a quantized fractional charge e/3 at the corner in both up-spin and down-spin states. These corner states form a very flat band in the bulk bandgap. Except for the corner states, there are edge states in the bulk bandgap. These edge states are not separated from the bulk bands as the flat band, but submerging into the bulk bands. It's significantly important that the magnetic moments of Cr ions are out of plane in the type-B compounds, which is of great advantage for the spintronic design. Another merit of the type-B monolayer CrSX is the valley polarization, which can be used to realize the spin and chirality related electron transportations.