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Realizing macroscopic quantum phenomena at room temperature is a major pursuit in physics. The quantum spin Hall (QSH) state, a prototypical quantum phenomenon that features a two-dimensional insulating bulk and a topologically protected helical edge state at zero magnetic field, has not been realized at room temperature. Here, using scanning tunneling microscopy, we directly visualize a QSH 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 while an atomically sharp monolayer step edge hosts an in-gap gapless state, which is the hallmark of topological bulk-boundary correspondence. An external magnetic field can gap the edge state, consistent with the time-reversal symmetry protection of the underlying topology. Furthermore, via directly identifying the geometrical hybridization of such edge states, we show the manifestation of the Z₂ topology of the QSH state and visualize the building blocks of the higher-order topological insulator phase in Bi₄Br₄. Most notably, both the insulating gap and topological edge state persist up to 300K, pointing to the room temperature realization of the QSH state.

Reference: N. Shumia, Md. S. Hossain et al., arXiv:2110.05718 (2021).