Atomic-scale visualization of edge photocurrent in topological crystalline insulator Pb_1-xSn_xSe

Light-matter interactions in quantum materials have revealed exotic phenomena such as Floquet-Bloch states, chiral phonons, light-induced superconductivity and magnetism. However, interactions at the boundary modes of topological phases have not been explored at the atomic scale. Scanning tunneling microscopy (STM) has been a key tool to visualize boundary states, but it has so far been limited to equilibrium state. Here, we employ our newly developed ultrafast STM to investigate Pb_1-xSn_xSe. For x=0.3, the material is a topological crystalline insulator (TCI) hosting surface states and helical edge modes, whereas x=0.1 corresponds to a trivial semiconductor. Upon excitation by ultrafast near-IR laser pulses in the TCI phase, we observe an enhanced photocurrent localized at atomic step edges that decay within 1 ps. In contrast, the trivial phase exhibits no such edge enhancement, indicating its connection to the topological edge mode in the TCI phase. The bias-dependent measurements further reveal a maximal response near the Dirac point. Based on band-structure considerations, we attribute the edge photocurrent to bulk-to-boundary carrier diffusion. Our results establish a future pathway to visualize boundary modes of light-driven topological phases with atomic-scale and femtosecond resolution.