Scattering on Magnetic and Non-Magnetic Impurities on a Surface of a Topological Insulator*

Dirac-like surface states on surfaces of topological insulators have a chiral spin structure that supresses back-scattering and protects the coherence of these states in the presence of potential scatterers. In contrast, magnetic scatterers are expected to open the back- scattering channel via the spin-flip processes and to degrade the state's coherence. We present angle-resolved photoemission spectroscopy studies of the electronic structure and the scattering rates upon adsorption of various magnetic and non-magnetic impurities on the surface of Bi$_2$Se$_3$, a model topological insulator. We uncovered an unusual insensitivity of the topological surface state to both non-magnetic and magnetic impurities. The electrons donated by the impurities fill the topological surface state and pairs of higher lying spin-orbit split surface bands, preserving the non-trivial spin texture of the surface.