  We study the electric-field tunable electronic properties of phosphorene thin
films, using the framework of density functional theory. We show that
phosphorene thin films offer a versatile material platform to study two
dimensional Dirac fermions on application of a transverse electric field.
Increasing the strength of the transverse electric field beyond a certain
critical value in phosphorene leads to the formation of two symmetry protected
gapless Dirac fermions states with anisotropic energy dispersion. The
spin-orbit coupling splits each of these Dirac state into two spin- polarized
Dirac cones which are also protected by non-symmorphic crystal symmetries. Our
study shows that the position as well as the carrier velocity of the spin
polarized Dirac cone states can be controlled by the strength of the external
electric field.
