Superdiffusion of Photoexcited Carriers in Topological Insulator Nanoribbons

Understanding the ultrafast dynamics and transport of photoexcited carriers in topological insulators is crucial for the optical manipulation of spins and may shed light on the nature of topological excitons. Here we investigate bulk-insulating Sb-doped Bi₂Se₃ nanoribbons via ultrafast transient photovoltage microscopy. The probe-pulse-induced photovoltage is substantially suppressed by a pump pulse. Recovery time increases from 50 to 1600 ps as the pump fluence increases. We found that the diffusivity of photoexcited carriers increases significantly at lower carrier concentrations, up to 800 cm²/s at 21 K, 2 to 3 orders of magnitude higher than that of band-edge carriers. Remarkably, the photoexcited carriers travel up to 10 μm for hundreds of picoseconds at this high diffusivity. The diffusivity peaks in intrinsic devices and is reduced at high temperatures. We discuss the possible mechanisms of long-range superdiffusion in the frames of hot carriers and exciton condensation.