Metallicity at the Ioffe-Regal limit in a topological insulator thin film*

The topological surface states of time-reversal invariant 3D topological insulators have applications for spintronics, metrology, and the realization of topological superconductivity. Initial excitement around these surface states, however, focused on the prediction that they should be robust against Anderson localization, even under strong disorder. This prediction has never been directly confirmed by transport measurements. We present low-temperature transport measurements of a gate-tunable Sb₂Te₃ topological insulator thin film that features high mobility and low carrier density. We find that metallicity is preserved at conductivities well below e²/h, where two dimensional electron systems conventionally scale to an insulating state. Near the charge neutrality point, we observe an unusual magnetoconductance response, which we associate with known weak localization/anti-localization behavior, but beyond the Ioffe-Regal limit k_Fl ~ 1.