Tunable topological edge states in black-phosphorus-like Bi(110)

We have investigated the structures and electronic properties of ultrathin Bi(110) films grown on an $s$-wave superconductor substrate using low-temperature scanning tunneling microscopy and spectroscopy. Remarkably, our experimental results validate the theoretical predictions that the manipulation of Bi(110) surface atom buckling can control the topological phase transition. Notably, we have observed robust unreconstructed edge states at the edges of both three- and four-bilayer Bi(110) films, with the four-bilayer film displaying stronger edge state intensity and a smaller degree of atomic buckling. First-principles calculations further substantiate these findings, demonstrating a gradual reduction in buckling as the film thickness increases, with average height differences between two Bi atoms of approximately 0.19, 0.10, 0.05, and 0.00 \AA{} for the one-, two-, three-, and four-bilayer Bi(110) films, respectively. When Bi films are larger than two layers, the system changes from a trivial to a nontrivial phase. This research sets the stage for the controlled realization of topological superconductors through the superconducting proximity effect, providing a significant platform for investigating Majorana zero modes and fabricating quantum devices.