Probing the Topological Classification of Bismuth with Topological Defects

The growing diversity of topological classes may lead to ambiguity in the classification of materials. Such is the case of Bismuth. While most theoretical models indicate that Bismuth possesses a trivial topological order, other theoretical and experimental studies suggest other classifications such as a strong, a weak, and a higher order topological insulator (TI), all of which host helical modes on their boundaries. We use scanning tunneling microscopy to investigate the response of the topological edge mode, in Bismuth, to a topological defect in the form of a screw dislocation. We find that the edge mode extends over a wider energy range than previously thought, and withstands crystallographic irregularities, without showing any signs of backscattering and gapping. Moreover, it seems to bind to the bulk screw dislocation, as expected for a TI with non-vanishing weak indices. These observations disfavor its recent identification as a hinge mode of a high order TI. We argue that the small scale of the bulk $L$ gap positions Bismuth within the critical region of a topological phase transition to a strong TI with non-vanishing weak indices. Consequently, the observed boundary modes are approximately helical already on the trivial side of the topological phase transition.