Double-Weyl fermions in two-dimensional ferromagnetic materials

Double-Weyl fermions with quadratic dispersion, as novel topological states of matter, have attracted a great deal of attention in recent years. However, they are very rare in two-dimensional (2D) magnetic materials. In this work, by performing symmetry analysis and first-principles calculations, we systematically study and screen out all 2D ferromagnetic (FM) materials with double-Weyl points (DWPs). We screened a total of 80 layer groups (LGs) and nearly 3700 materials, finding that there are 32 LGs and 80 2D FM materials that can exist the DWPs. Many of them are half-metal, hosting 100 % spin polarization ratio. For these 2D FM materials, we summarize their basic information, including point group, LG number, high-symmetry points where DWPs are located, generating elements, spin polarization ratio. We also take half-metals materials VSe2, ZrBr3, and Co-PTC (PTC: perthiolated coronene) as examples to support our theory. These materials have DWPs protected by C-4, C-3, and C-6 symmetries, respectively. Applying uniaxial strain to break symmetries can also induce topological phase transition from a DWP to two single-Weyl points (SWPs) with linear dispersion. Our work paves the way to study the DWPs in 2D FM system and provides candidate materials for future physical properties study, experimental research and device application.