Dirac Fermions in Blue-Phosphorus

We propose that Dirac cones can be engineered in phosphorene with fourfold-coordinated phosphorus atom. The key is to separate in energy the in-plane ($s$, $p_x$ and $p_y$) and out-of-plane ($p_z$) oribtals through the $sp^2$ configuration, yielding respective $\sigma$- and $\pi$-character Dirac cones, and then quench the latter. As a proof-of-principle study, we realize $\sigma$-character Dirac cone in hydrogenated/fluorinated phosphorene with the honeycomb lattice. The obtained Dirac cones are at $K$-points, slightly anisotropic, with Fermi velocities of 0.91/1.23 times that of graphene along $\Gamma$K/KM direction, and maintain a good linearity up to $\sim$2 eV for holes. One substantive advantage of $\sigma$-character Dirac cone is its convenience to tune the Dirac gap via in-plane strain. Our findings pave a new way for development of high performance electronic devices based on Dirac materials.