Topological nonmagnetic impurity states in topological Kondo insulators

We examine the presence of nonmagnetic impurities in a hybridization gap model of a Kondo insulator which has band inversion. The model has been used to predict that ${\mathrm{SmB}}₆$ is a topological insulator. We show that there are two types of nonmagnetic impurity states in a Kondo insulator. The type of states can be categorized as deep impurity states, similar to the impurity states in an ordinary metal, and impurity states which have energies within the hybridization gap. Unlike the deep impurity states which only form if the impurity potential exceeds a critical value of the order of the conduction bandwidth, the in-gap impurity states form for exceptionally small values of the impurity potential comparable to the hybridization gap. This result may explain why Kondo insulators are found to be exceptionally sensitive to impurities. We show that these in-gap states are caused by band inversion and have properties similar to those expected for impurities in a topological insulator.