Symmetry dictated grain boundary state in a two-dimensional topological insulator MoTe₂

Structural imperfections such as grain boundaries (GBs) and dislocations are ubiquitous in solids and have been of central importance in understanding the nature of polycrystals. In addition to their classical roles, the advent of topological insulators (TIs) offers a chance to realize distinct topological states bound to them. Although dislocations inside three-dimensional TIs are one of the prime candidates to look for, their direct detection and characterization is challenging. Instead, in two-dimensional (2D) TIs, their creations and measurements are easier and, moreover, topological states at the GBs or dislocations are intimately related to their lattice symmetry. However, such roles of crystalline symmetries of GBs in 2D TIs have not been definitively measured yet. Here, we present the first direct evidence of a symmetry-enforced Dirac type metallic state along a GB in 1T’-MoTe₂, a prototypical 2D TI. Using scanning tunneling microscopy, we show a metallic state along a grain boundary with non-symmorphic lattice symmetry and its absence along another boundary with symmorphic symmetry. Our large-scale atomistic simulations demonstrate hourglass like nodal-line semimetallic in-gap states for the former, whereas the gap opens for the latter, explaining our observation well.