Topological insulators (TIs) show protected metallic states on their boundaries of insulating bulks that can be either edges, surfaces, interfaces or dislocations. Symmetry of the crystal structures further diversifies these boundary states and classifies TIs with different topological index. In three-dimension (3D), the surface states protected by symmorphic mirror lines or non-symmorphic glide lines are proposed and experiments consistent with a part of the proposals are reported. Dislocations in 3D TIs also hold significant importance in revealing such characteristics but their intrinsic embedding structures inside 3D bulk hinder their direct detections that may be possible in lower dimensional TIs. In two-dimensional TIs, however, not only direct observation of topological states on grain boundaries but also interplays between crystal symmetry and topologically protected states have not been reported yet. Here, we present the first direct evidence of one-dimensional metallic states protected by non-symmorphic symmetry of glide reflection grain boundary [1] and a noble method for creation of grain boundaries in a topological insulating two-dimensional transition metal dichalcogenide (TMD), 1T’-MoTe₂. By investigating electronic structures along two different grain boundaries of as-grown MoTe₂ using scanning tunneling microscope (STM) and first-principles calculations, we clearly demonstrate evidences of a topologically protected state along a grain boundary with non-symmorphic symmetry and its absence along the other boundary with symmorphic one.

[1] Hyo Won Kim et al., Nano Lett 20, 5837 (2020).