# Realisation of Symmetry Enforced Two-Dimensional Dirac Fermions in Nonsymmorphic $\alpha$-Bismuthene

## TL;DR

This paper demonstrates how nonsymmorphic symmetry in 2D materials like $ extalpha$-bismuthene enforces robust Dirac fermions, confirmed by experiments and simulations, opening new avenues for discovering symmetry-protected electronic states.

## Contribution

The study reveals a symmetry-based mechanism to realize stable Dirac cones in 2D materials with nonsymmorphic lattices, validated by experimental and theoretical methods.

## Key findings

- Observation of Dirac-like dispersions in $ extalpha$-bismuthene via $ extmu$-ARPES
- Confirmation of nonsymmorphic symmetry through $ extmu$-LEED and STM
- Theoretical demonstration of symmetry-enforced Dirac states in 2D materials

## Abstract

Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene. 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. 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. Through $\mu$-ARPES measurements we observe Dirac-like band dispersions in $\alpha$-bismuthene. The nonsymmorphic lattice symmetry is confirmed by $\mu$-LEED and STM. Our first-principles 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.