Perfect Transmission of Topological Surface States Through Surface Barriers on Sb(111)^*

Topological surface states are unconventional two-dimensional electronic states that are protected from backscattering and overcome surface barriers. Previous experiments have shown that topological surface states have an enhanced transmission and a reduced reflection at the common bilayer step edges on the (111) surface of topological semimetal antimony (Sb), indicating the presence of backscattering against bilayer step edges. We use scanning tunneling microscopy and spectroscopy to investigate Sb(111) surface, which exhibits novel monolayer step edges. We demonstrate that topological surface states transmit through these monolayer step edges perfectly without backscattering. Furthermore, we discover localized states on these edges resembling the helical edge states originating from higher-order topology.