  We examine the nature of topological currents in black phosphorus when its
inversion symmetry is deliberately broken. Here, the conduction and valence
band edges are located at the $\Gamma$ point of the rectangular Brillouin zone,
and they exhibit strong anisotropy along its two crystal axes. We will show
below that these salient features lead to a linear transverse neutral
topological currents, accompanied also by a non-linear transverse charge
current at the Fermi surface. These topological currents are maximal when the
in-plane electric field is applied along the zigzag crystal axes, but zero
along the armchair direction.
