Room-temperature quantum spin Hall edge state in a higher-order topological insulator Bi${}_4$Br${}_4$

Abstract

Room-temperature realization of macroscopic quantum phenomena is one of the major pursuits in fundamental physics. The quantum spin Hall state, a topological quantum phenomenon that features a two-dimensional insulating bulk and a helical edge state, has not yet been realized at room temperature. Here, we use scanning tunneling microscopy to visualize a quantum spin Hall edge state on the surface of the higher-order topological insulator Bi4Br4. We find that the atomically resolved lattice exhibits a large insulating gap of over 200meV, and an atomically sharp monolayer step edge hosts a striking in-gap gapless state, suggesting the topological bulk-boundary correspondence. An external magnetic field can gap the edge state, consistent with the time-reversal symmetry protection inherent to the underlying topology. We further identify the geometrical hybridization of such edge states, which not only attests to the Z2 topology of the quantum spin Hall state but also visualizes the building blocks of the higher-order topological insulator phase. Remarkably, both the insulating gap and topological edge state are observed to persist up to 300K. Our results point to the realization of the room-temperature quantum spin Hall edge state in a higher-order topological insulator.

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• Transport response of topological hinge modes in $\alpha$-Bi${}_4$Br${}_4$
• Discovery of a hybrid topological quantum state in an elemental solid
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• Quantum transport evidence of the boundary states and Lifshitz transition in Bi${}_4$Br${}_4$
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• Majorana vortex phases in time-reversal invariant higher-order topological insulators and topologically trivial insulators