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

TL;DR

This study visualizes a quantum spin Hall edge state in Bi4Br4, a higher-order topological insulator, demonstrating its stability at room temperature and advancing the pursuit of macroscopic quantum phenomena.

Contribution

It provides the first direct visualization of a room-temperature quantum spin Hall edge state in a higher-order topological insulator Bi4Br4.

Findings

Large insulating gap of over 200meV observed

Gapless edge state persists up to 300K

Edge state is protected by time-reversal symmetry

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.