Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene [1-3]. The linear energy dispersion and non-trivial Berry phase play the pivotal role in the remarkable electronic, optical, mechanical and chemical properties of 2D Dirac materials [4]. The known 2D Dirac materials are gapless only within certain approximations, for example, in the absence of SOC. Here we report a route to establishing robust Dirac cones in 2D materials with nonsymmorphic crystal lattice. The nonsymmorphic symmetry enforces Dirac-like band dispersions around certain high-symmetry momenta in the presence of SOC [5, 6]. Through µ-ARPES measurements we observe Dirac-like band dispersions in α-bismuthene. The nonsymmorphic lattice symmetry is confirmed by µ-LEED and STM. Our firstprinciples simulations and theoretical topological analysis demonstrate the correspondence between nonsymmorphic symmetry and Dirac states. This mechanism can be straightforwardly generalized to other nonsymmorphic materials. The results open the door for the search of symmetry enforced Dirac fermions in the vast uncharted world of nonsymmorphic 2D materials.

Using scanning tunneling microscopy, we report the observation of moiré patterns (MPs) on van der Waals heterostructures comprised of various 2D allotropes of bismuth and antimony grown on highly ordered pyrolytic graphite and MoS 2 . The spatial periods of the MPs range from λ ∼ 1 to ∼10 nm. For all the reported cases (α-bismuthene, α-antimonene, β-antimonene, and monolayer bismuthene), we model the observations using a simple superposition model (SSM). Where possible, the results obtained from the SSM are compared to analytical prediction. MPs emerging from mixed symmetry stacking (hexagonal on rectangular) are explained without requiring commensuration of the layers.

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