Observation of Topological Edge States on α-Bi4Br4 Nanowires Grown on TiSe2 Substrates.

A time-reversal invariant two-dimensional (2D) topological insulator (TI) is characterized by the gapless helical edge states propagating along the perimeter of the system. However, the small band gap in the 2D TIs discovered so far hinders their applications. Recently, we predicted that single-layer Bi4Br4 is a 2D TI with a remarkable band gap and that α-Bi4Br4 crystals can host topological edge states at the step edges. Here we report the growth of α-Bi4Br4 nanowires with (102)-oriented top surfaces on the TiSe2 substrates and the direct observation of the predicted topological edge states at the step edges of the nanowires using scanning tunneling microscopy. The coupling between the edge states leads to the formation of surface states at the (102) top surfaces of the nanowires. Our work demonstrates the existence of topological edge states in α-Bi4Br4 and paves the way for developing α-Bi4Br4-based devices for a high-temperature quantum spin Hall effect.