  In ideal topological insulator (TI) films the bulk state, which is supposed
to be insulating, should not provide any electric coupling between the two
metallic surfaces. However, transport studies on existing TI films show that
the topological states on opposite surfaces are electrically tied to each other
at thicknesses far greater than the direct coupling limit where the surface
wavefunctions overlap. Here, we show that as the conducting bulk channels are
suppressed, the parasitic coupling effect diminishes and the decoupled surface
channels emerge as expected for ideal TIs. In Bi2Se3 thin films with fully
suppressed bulk states, the two surfaces, which are directly coupled below ~10
QL, become gradually isolated with increasing thickness and are completely
decoupled beyond ~20 QL. On such a platform, it is now feasible to implement
transport devices whose functionality relies on accessing the individual
surface layers without any deleterious coupling effects.
