  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.
