Disorder effects in topological insulator thin films

Thin films of topological insulators (TI) attract large attention because of expected topological effects from the inter-surface hybridization of Dirac points. However, these effects may be depleted by unexpectedly large energy smearing $\Gamma$ of surface Dirac points by the random potential of abundant Coulomb impurities. We show that in a typical TI film with large dielectric constant $\sim 50$ sandwiched between two low dielectric constant layers, the Rytova-Keldysh modification of the Coulomb potential of a charge impurity slows down the potential decay in space, and allows a larger number of the film impurities to contribute in $\Gamma$. As a result, $\Gamma$ is large and independent on the TI film width for films thicker than 2 nm. We also study the disorder effect on the hybridization gap, the role of gates and the surface conductivity.