Why is the bulk resistivity of topological insulators so small?

As-grown topological insulators (TIs) are typically heavily-doped $n$-type crystals. Compensation by acceptors is used to move the Fermi level to the middle of the band gap, but even then TIs have a frustratingly small bulk resistivity. We show that this small resistivity is the result of band bending by poorly screened fluctuations in the random Coulomb potential. Using numerical simulations of a completely compensated TI, we find that the bulk resistivity has an activation energy of just $0.15$ times the band gap, in good agreement with experimental data. At lower temperatures activated transport crosses over to variable range hopping with a relatively large localization length. \textbf{Reference:} B. Skinner, T. Chen, B. I. Shklovskii, \textit{Phys. Rev. Lett.} \textbf{109}, 176801 (2012).