Dirac fermions in blue-phosphorus

We propose that Dirac cones can be engineered in phosphorene with fourfoldcoordinated phosphorus atoms. The key is to separate the energy levels of the inplane (s, px, and py) and out-of-plane (pz) oribtals through the sp2 configuration, yielding respective σ-and φ-character Dirac cones, and then quench the latter. As a proof-of-principle study, we create σ-character Dirac cones in hydrogenated and fluorinated phosphorene with a honeycomb lattice. The obtained Dirac cones are at K-points, slightly anisotropic, with Fermi velocities of 0.91 and 1.23 times that of graphene along the ?K and KM direction, and maintain good linearity up to 2 eV for holes. A substantive advantage of a σ-character Dirac cone is its convenience in tuning the Dirac gap via in-plane strain. Our findings pave the way for development of high-performance electronic devices based on Dirac materials.