Strong potential impurities on the surface of a topological insulator

Topological insulators (TIs) are said to be stable against nonmagnetic impurity scattering due to suppressed backscattering in the Dirac surface states. We solve a lattice model of a three-dimensional TI in the presence of strong potential impurities and find that both the Dirac point and low-energy states are significantly modified: Low-energy impurity resonances are formed that produce a peak in the density of states near the Dirac point, which is destroyed and split into two nodes that move off center. The impurity-induced states penetrate up to ten layers into the bulk of the TI. These findings demonstrate the importance of bulk states for the stability of TIs and how they can destroy the topological protection of the surface.