Bulk defects and surface state dynamics in topological insulators: The effects of electron beam irradiation on the ultrafast relaxation of Dirac fermions in Bi ₂ Te ₃

One of the most important challenges in the study of topological insulators is the realization of materials that are really insulating in the bulk, in order to emphasize quantum transport in the protected surface states. Irradiation with electron beams is a very promising approach toward this goal. By studying a series of samples of the prototype 3D topological insulator Bi 2 Te 3, we show that while the topological properties of Dirac surface states are preserved after electron irradiation, their relaxation dynamics are very sensitive to the related modifications of the bulk properties. Using time- and angle-resolved photoelectron spectroscopy, we can reveal two distinct relaxation regimes after optical excitation for non-irradiated and irradiated samples. While the faster regime, corresponding to the first few picoseconds, presents a similar temporal evolution of the photoexcited population for all studied samples, the slower regime is strongly influenced by the controlled generation of defects in the bulk lattice. By adjusting the irradiation parameters in this class of materials, one can thus not only change the bulk transport properties but also tune the ultrafast response of the topological surface states.