The protected surface conductivity of topological insulators, carried by ultra-relativistic Dirac fermions, is in high demand for the next generation of electronic devices. Progress in the unambiguous identification of this surface contribution and, in a second step, its control are needed to move forward. Here we demonstrate both, with a combined transport and spectroscopy study of highquality single crystals and mesoscopic devices of the topological insulator TlBiSe 2−δ 1-5 . We show how various external stimuli-from thermal radiation, via low-intensity light, to high-intensity laser pumping and current driving-can boost the surface contribution, thereby making it both unambiguously detectable and potentially exploitable for applications. Once switched on, the extra surface contribution is persistent, with lifetimes of hundreds of years at low temperatures. We understand this effect in terms of the well-known concept of surface charge accumulation 6,7 via a Schottky barrier formation 8,9 , and propose that the same mechanism underlies also the slow relaxations seen with spectroscopic probes in our and other 10-16 materials, which might thus also be persistent. We expect our technique to be readily transferable to other materials and probes, thereby shedding light on unexplained slow relaxations in transport [17][18][19] and beyond.

The protected surface conduction of topological insulators is in high demand for the next generation of electronic devices. What is needed to move forward are robust settings where topological surface currents can be controlled by simple means, ideally by the application of external stimuli. Surprisingly, this direction is only little explored and the role of topological states in such processes has remained obscure. In this work we demonstrate that we can boost the surface conduction of a topological insulator by light and/or electric field. This happens in a fully controlled way, and the additional Dirac carriers exhibit ultra-long live times. We provide a comprehensive understanding—carriers injection from the bulk to the surface states across an intrinsic Schottky barrier—and expect this mechanism to be at play in a broad range of materials and experimental settings.