  Black phosphorus (BP), a layered van der Waals material, reportedly has a
band gap sensitive to external perturbations and manifests a Dirac-semimetal
phase when its band gap is closed. Previous studies were focused on effects of
each perturbation, lacking a unified picture for the band-gap closing and the
Dirac-semimetal phase. Here, using pseudospins from the glide-reflection
symmetry, we study the electronic structures of mono- and bilayer BP and
construct the phase diagram of the Dirac-semimetal phase in the parameter space
related to pressure, strain, and electric field. We find that the
Dirac-semimetal phase in BP layers is singly connected in the phase diagram,
indicating the phase is topologically identical regardless of the gap-closing
mechanism. Our findings can be generalized to the Dirac semimetal phase in
anisotropic layered materials and can play a guiding role in search for a new
class of topological materials and devices.
