Strong surface scattering in ultrahigh-mobility Bi₂Se₃ topological insulator crystals

While evidence of a topologically nontrivial surface state has been identified in surface-sensitive measurements of Bi₂Se₃, a significant experimental concern is that no signatures have been observed in bulk transport. In a search for such states, nominally undoped single crystals of Bi₂Se₃ with carrier densities approaching 10¹⁶ cm⁻³ and very high mobilities exceeding 2 m² V⁻¹ s⁻¹ have been studied. A comprehensive analysis of Shubnikov-de Haas oscillations, Hall effect, and optical reflectivity indicates that the measured electrical transport can be attributed solely to bulk states, even at 50 mK at low Landau-level filling factor, and in the quantum limit. The absence of a significant surface contribution to bulk conduction demonstrates that even in very clean samples, the surface mobility is lower than that of the bulk, despite its topological protection.