Coexistence of massless and massive Dirac fermion in surface doped black phosphorus near insulator-semimetal phase transition

The coexistence of massless and massive Dirac fermions has remained elusive to date. Currently, this phenomenal was only realized in graphite, symmetry-broken bilayer graphene and topological crystalline insulator Pb_1-xSn_xSe. Here, we proposed a new candidate – surface doped black phosphorus that preserves space-time inversion symmetry, which guarantees the stability of the Dirac point, leading to an insulator-semimetal phase transition without gapping. Beyond a critical surface doping value, band inversion occurs with two separate Dirac points, which allows the system to exhibit either a massive, semi-Dirac or massless Klein tunnelling. Furthermore, we show that this elusive behaviour can be portrayed in electronic conductance and Fano factor measurements. Strikingly, this phenomenal can be generalized to any system that can be described by the universal type I semi-Dirac Hamiltonian.