Symmetry-Enforced Dirac Fermions in Nonsymmorphic α-Bismuthene

According to recent theoretical works, nonsymmorphic crystal symmetries can enforce the formation of Dirac cones, providing a new route to establishing Dirac states in 2D materials. Here we will discuss our recent work on the realization of the symmetry-enforced Dirac fermions in nonsymmorphic α-bismuthene (Bi monolayer). The bismuthene was synthesized by the method of molecular beam epitaxy (MBE). The Dirac band structure was observed by the micro-angle-resolved photoemission (μ-ARPES) experiment. The Dirac points are located at high-symmetry momentum points which are entirely determined by the lattice symmetry. This correspondence of Dirac states to the nonsymmorphic symmetry group can potentially lead to the discovery of a range wide of new 2D Dirac materials. In addition, the Dirac fermions in α-bismuthene is of spin-orbit type in contrast to the spinless Dirac states in graphene. The result will accelerate the search of 2D Dirac materials and extend “graphene” physics into new territory where strong spin-orbit coupling is present.