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

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.

References in corpus (12)

• The electronic properties of graphene
• Two-Dimensional Gas of Massless Dirac Fermions in Graphene
• Experimental Observation of Quantum Hall Effect and Berry's Phase in Graphene
• Two and one-dimensional honeycomb structures of silicon and germanium
• Dirac Semimetals in Two Dimensions
• Semimetallic Two-Dimensional Boron Allotrope with Massless Dirac Fermions
• Half-metallic semiDirac point generated by quantum confinement in TiO${}_2$/VO${}_2$ nanostructures
• New classes of topological crystalline insulators with unpinned surface Dirac cones
• Nonsymmorphic-symmetry-protected hourglass Dirac loop, nodal line, and Dirac point in bulk and monolayer $X_3$SiTe${}_6$ ($X=$ Ta, Nb)
• Spin-Orbit Semimetals in the Layer Groups
• Topological semimetals protected by off-centered symmetries in nonsymmorphic crystals
• Spinless hourglass nodal-line semimetals