Thin flakes of black phosphorus (BP) are a two-dimensional (2D) semiconductor whose energy gap is predicted being sensitive to the number of layers and external perturbations. Recently, it was found that a simple method of potassium (K) doping on the surface of BP closes its band gap completely, producing a Dirac semimetal state with a linear band dispersion in the armchair direction and a quadratic one in the zigzag direction . Here, based on first-principles density functional calculations, we predict that beyond the critical K density of the gap closure, 2D massless Dirac fermions (i.e. Dirac cones) emerge in K-doped few-layer BP, with linear band dispersions in all momentum directions . The electronic states around Dirac points have chiral pseudospins and Berry's phase which are robust with respect to the spin-orbit interaction. We also investigate the gap-closing mechanism and the origin of the Dirac semimetal phase through a theoretical analysis of mono- and bilayer BP . We find analytic conditions for the band-gap closing in mono- and bilayer BP, revealing roles of intra- and interlayer interactions in the size of the band gap. Protected by the glide-reflection symmetry, the Dirac points are robust regardless of the gap-closing mechanism and the number of layers. Our findings provide basic properties of the Dirac semimetal phase in BP layers which can play a guiding role in high-performance device applications and search for a new class of topological materials. This work was supported by NRF of Korea (Grant No. 2011-0018306). Computational resources were provided by the KISTI Supercomputing Center (Project No. KSC-2014-C3-070).
 J. Kim, S. S. Baik, S. H. Ryu, Y. Sohn, S. Park, B.-G. Park, J. Denlinger, Y. Yi, H. J. Choi, and K. S. Kim, Science 349 , 723 (2015).
 S. S. Baik, K. S. Kim, Y. Yi, and H. J. Choi, Nano Letters 15 , 7788 (2015).
 H. Doh and H. J. Choi, http://arxiv.org/abs/1606.04264.