  Topological insulators are predicted to present novel surface transport
phenomena, but their experimental studies have been hindered by a metallic bulk
conduction that overwhelms the surface transport. We show that a new
topological insulator, Bi2Te2Se, presents a high resistivity exceeding 1 Ohm-cm
and a variable-range hopping behavior, and yet presents Shubnikov-de Haas
oscillations coming from the surface Dirac fermions. Furthermore, we have been
able to clarify both the bulk and surface transport channels, establishing a
comprehensive understanding of the transport in this material. Our results
demonstrate that Bi2Te2Se is the best material to date for studying the surface
quantum transport in a topological insulator.
