The discovery of graphene has stimulated enormous interest in two-dimensional (2D) electron gas with linear band dispersion. However, to date, 2D Dirac semimetals are still very rare due to the fact that 2D Dirac states are generally fragile against perturbations such as spin-orbit coupling. Nonsymmorphic crystal symmetries can enforce the formation of Dirac nodes, providing another route to establishing symmetry-protected Dirac states in 2D materials. Here, we report the symmetry-protected Dirac states in nonsymmorphic $\alpha$-antimonene (Sb monolayer). The antimonene was synthesized by the method of molecular beam epitaxy. 2D Dirac states with large anisotropy were observed by angle-resolved photoemission spectroscopy. The Dirac states in $\alpha$-antimonene are spin-orbit coupled in contrast to the spinless Dirac states in graphene. The result extends the graphene physics into a family of 2D materials where spin-orbit coupling is present.

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• Electronic structure
• Topological materials
• Angle-resolved photoemission spectroscopy