Symmetry-Protected Dirac Semimetal Phase in Surface-Doped Black Phosphorus

Two-dimensional (2D) black phosphorus has attracted growing interest owing to its tunable band structure with external parameters such as field and strain. In particular, the effect of strong vertical electric field can be induced by in-situ surface doping of alkali metal atoms on the surface of black phosphorus, leading to the widely tunable band gap. In this study, we experimentally demonstrate the giant band inversion in the surface of black phosphorus by means of angle-resolved photoemission spectroscopy (ARPES). Our ARPES spectra reveal that the band-inverted state of black phosphorus has a pair of Dirac cones that are separated along the zigzag direction. Unlike graphene, the Dirac points of black phosphorus are stable even in the presence of spin-orbit coupling owing to space-time inversion symmetry, a combination of glide-mirror symmetry and time-reversal symmetry.