Evidence of a Room-temperature Quantum Spin Hall Edge State in a Higher-order Topological Insulator

Room-temperature realization of macroscopic quantum phases is one of the major pursuits in fundamental physics. A topological insulator is a material that behaves as an insulator in its interior but  whose  surface  contains  protected  conducting  states.  A  two-dimensional  (2D)  topological insulator features time-reversal symmetry-protected helical edge states residing in an insulating bulk gap  and  accordingly  exhibits  the  quantum  spin  Hall  effect.  The  helical  edge  state  features dissipationless electron channels along the sample edges, which is of great interest in energy-saving technology and quantum information science. Among topological insulator candidates, Bi4Br4has a layered structure with van der Waals–like bonding and has been proposed to feature a large insulating gap and weak inter-layer coupling; thus, monolayer Bi4Br4 has the potential to realize a high-temperature  quantum  spin  Hall  state.  Although  previous  work  using  the  angle-resolved photoemission technique set out to resolve the topological boundary mode from crystalline steps, it remains  elusive  whether  the  observed  boundary  mode  signal  arises  from  the  side  surfaces  or atomic step edges of the crystal. The magnetic-field response and temperature robustness of the predicted quantum spin Hall state are also largely unexplored, which will provide indispensable information for the underlying quantum topology and future applications of this quantum material. Therefore, a real-space experimental investigation of the nature of the edge state with atomic-layer spatial resolution, magnetic-field tunability and temperature control is highly desirable. In this work, vector field and variable temperature-based scanning tunnelling microscopy was used to evidence and elucidate its insulating gap and topological edge state.

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