Magnetic Weyl Semimetal in Quasi Two-dimensional Half Metallic Co$_3$Sn$_2$Se$_2$

We have found a ferromagnetic Weyl semimetal (WSM) in half metallic Co$_3$Sn$_2$Se$_2$. The three pairs of Weyl points near Fermi level (E$_F$) are derived from nodal lines gapped by spin-orbit coupling (SOC). Though the Weyl points are 0.11 eV above the charge neutral point, Fermi arc related states in the cleaved surface can range from E$_F$ -0.15 to E$_F$ +0.11 eV in energy space, due to the surface bands dispersion. Hence, Weyl points related physics should be detected by surface measurements, such as ARPES and STM. Because of the large Berry curvature deriving from the gapped nodal lines and Weyl points, the anomalous Hall conductivity of Co$_3$Sn$_2$Se$_2$ can keep above 620 S/cm in a large energy window. Beside magnetic WSM, Co$_3$Sn$_2$Se$_2$ also possesses a Z$_2$ topological semimetal phase as the temperature is above Curie temperature. Owing to the quasi two-dimensional lattice structure, it is also possible to realize quantum anomalous Hall effect in its 2D limit via the size effect. Therefore, Co$_3$Sn$_2$Se$_2$ provides a good platform for the interplay of different topological states and magnetic states.