“…[123] The electronic structure of bismuth (Z = 83) surfaces have been extensively investigated, both theoretically [84,86,87,89,97,103,125,130,134,135] and experimentally. [59,[126][127][128][129][136][137][138][139][140][141][142][143][144][145][146][147] Recently, bismuth has been shown to be a higher order topological insulator (HOTI), [129] possessing helical "hinge" states that consist of 1D gapless Kramers pairs, like the QSH edge, around specific facets of the crystal, which exists even for 3D bulk samples. As a consequence, both monolayers of Bi(111) (buckled honeycomb structure, Figure 4B) and Bi(110) (puckered honeycomb structure, Figure 4C), have been predicted to host gapless edge modes, consistent with those in a quantum spin Hall insulator, tunable by strain and electric field.…”