Tetragonal Mexican-hat dispersion and switchable half-metal state with multiple anisotropic Weyl fermions in penta-graphene

Due to the paired valence electrons configuration, all known two-dimensional (2D) carbon allotropes are intrinsically nonmagnetic. Based on the reported 2D carbon structure database and first-principles calculations, herein we demonstrate that inherent ferromagnetism can be obtained in the prominent allotrope, penta-graphene, which has a unique Mexican-hat valence band edge, giving rise to van Hove singularities and electronic instability. Induced by modest hole-doping that is achievable in electrolyte gate, the semiconducting penta-graphene can be transformed into different ferromagnetic half-metals with room-temperature stability and switchable spin directions. In particular, multiple anisotropic Weyl states, including type-I and type-II Weyl cones and hybrid quasi Weyl nodal loop, can be found in a sizable energy window of spin-down half-metal under proper strains. These findings not only identify a promising carbon allotrope to obtain the inherent magnetism for carbon-based spintronic devices, but highlight the possibility to realize different Weyl states by combining the electronic and mechanical means as well.