  The archetypical 3D topological insulators Bi2Se3, Bi2Te3 and Sb2Te3 commonly
exhibit high bulk conductivities, hindering the characterization of the surface
state charge transport. The optimally doped topological insulators Bi2Te2Se and
Bi2-xSbxTe2S, however, allow for such characterizations to be made. Here we
report the first experimental comparison of the topological surface states and
bulk conductances of Bi2Te2Se and Bi1.1Sb0.9Te2S, based on
temperature-dependent high-pressure measurements. We find that the surface
state conductance at low temperatures remains constant in the face of orders of
magnitude increase in the bulk state conductance, revealing in a
straightforward way that the topological surface states and bulk states are
decoupled at low temperatures, consistent with theoretical models, and
confirming topological insulators to be an excellent venue for studying charge
transport in 2D Dirac electron systems.
