Time- and Momentum-resolved Signatures of Edge States in a 2D Topological Insulator

Two-dimensional quantum spin Hall (QSH) insulators are a promising material class for spintronic applications based on dissipationless topologically-protected spin currents in their edges. Yet, they have not lived up to their technological potential, as experimental realizations are scarce and limited to cryogenic temperatures. These constraints have also severely restricted the characterization of their dynamical properties, imperative for the design of QSH devices operating under nonequilibrium conditions. Here, we report on the electron dynamics of the novel room-temperature QSH candidate bismuthene after photoexcitation using time- and angle-resolved photoemission spectroscopy. We map the transiently occupied conduction band and observe a metallic dispersive feature within the bulk band gap - a signature of topological edge states. Our analysis of hot photocarrier lifetimes provides insight into the microscopic scattering processes and the key role of edge states. The demonstration of the large bulk band gap and stability of edge states at room temperature marks a critical step towards QSH applications.