Topological currents in black phosphorus with broken inversion symmetry

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