Symmetry-Enforced Dirac and Nodal-Line States in Nonsymmorphic α-Bismuthene/Antimonene*

Nonsymmorphic crystal symmetries can enforce the formation of Dirac and nodal line states, providing a new route to establishing symmetry-protected states in 2D materials. Here we will discuss our recent work on the realization of the symmetry-enforced Dirac and nodal-line states in nonsymmorphic α-bismuthene/Antimonene (Bi/Sb monolayer). The bismuthene/antimonene was synthesized by the method of molecular beam epitaxy. The symmetry-protected band structure was observed by angle-resolved photoemission experiments. The Dirac and nodal-line states are located at high-symmetry momentum points and lines in the 2D Brillouin zone. The locations are entirely determined by the lattice symmetry. 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 materials with symmetry-protected states and extend “graphene” physics into new territory where strong spin-orbit coupling is present.