Prediction of a BeP2 monolayer with a compression-induced Dirac semimetal state

We have identified a two-dimensional (2D) beryllium diphosphide (${\mathrm{BeP}}₂$) structure using a global structure search combined with first-principles calculations. Phonon calculations and molecular dynamics simulation confirm that the structure is dynamically and thermally stable. Electronic structure calculations show that the 2D sheet is a direct band gap semiconductor with a small band gap of 0.15 eV, and the intrinsic acoustic-phonon-limited carrier mobility of the structure can reach $\ensuremath{\sim}{10}⁴\phantom{\rule{4pt}{0ex}}{\text{cm}}²{\text{V}}^{\ensuremath{-}1}{\text{s}}^{\ensuremath{-}1}$ for both electrons and holes with anisotropic features in the $x$ and $y$ directions. More interestingly, both mechanical and chemical compression can close the band gap and the structure turns to a Dirac semimetal with the Dirac cones located exactly at the Fermi level. The emerged Dirac semimetal state is direction dependent, with a linear band dispersion in the $x$ direction and a quadratic one in the $y$ direction. Moreover, it is demonstrated that the Dirac point is symmetry protected in the absence of spin-orbit coupling (SOC). In ${\mathrm{BeP}}₂$, the SOC is too weak to alter the semimetal feature except for the cases at extremely low temperatures. The band gap closing mechanism is further clarified by using the tight-binding (TB) method.