Electronic and magnetic properties of honeycomb transition metal monolayers: first-principles insights

p-Electron-based monolayer materials have dominated the research of Dirac fermions since the first exfoliation of graphene. In the present work, the electronic and magnetic properties of d-electron-based Dirac systems are studied by combining first-principles with mean field theory and Monte Carlo approaches. From first-principles calculations, we demonstrate that transition-metal (TM) monolayers (TM = Ti, Zr, Hf, V, Nb, or Ta), d-electron-based materials, could also hold Dirac cones and not only p-electron-based materials as known before. This may shed light on the breakthrough of new nanomaterials with d-type Dirac points. Moreover, the carrier mobility near the Dirac points of these materials can be tuned regularly by isotropic strains from -5% to 5%, without breaking the Dirac cones. However, the Dirac points would disappear under anisotropic strains, indicating that a rigorous honeycomb lattice may be the main precondition for Dirac points in TM-monolayers. Furthermore, some TM-monolayers (TM = Ti, Zr, or Hf) exhibit ferromagnetic couplings simultaneously. In addition, by mean field theory and Monte Carlo methods, it is found that Curie temperatures of TM-monolayers can be higher than 580 K even to 1180 K. Our findings significantly expand the Dirac systems.