Revealing topological hinge states in the second order topological insulator Bi₄Br₄

Among newly discovered Topological Insulators (TIs), Bi₄Br₄ appears to be a very promising material, with a large bulk gap (~ 230 meV), and experimental indications of a Second Order Topological Insulator (SOTI) character. Bulk-edge correspondence predicts 1D helical states to arise at the hinges of a 3D SOTI, but their expected topological protectection against elastic disorder has turned out to be less robust than anticipated.

Our work has been focused on evidencing these hinge states in low-temperature transport experiments by investigating the modulation of quantum interferences with magnetic field and gate voltage. We have found signatures of phase coherence in µm-sized samples with surprisingly large characteristic fields, and a strongly anisotropic behavior. These results suggest that transport in the Bi₄Br₄ flakes is mediated by 1D ballistic channels. Our results thus support the SOTI nature of Bi₄Br₄.