Electrically tunable surface-to-bulk coherent coupling in topological insulator thin films

We study coherent electronic transport in charge-density-tunable microdevices patterned from thin films of the topological insulator (TI) Bi${}_{2}$Se${}_{3}$. The devices exhibit pronounced electric field effect, including ambipolar modulation of the resistance with an on-and-off ratio of 500%. We show that the weak antilocalization correction to conductance is sensitive to the number of coherently coupled channels, which in a TI includes the top and bottom surfaces and the bulk carriers. These are separated into coherently independent channels by the application of gate voltage and at elevated temperatures. Our results are consistent with a model where channel separation is determined by a competition between the phase coherence time and the surface-to-bulk scattering time.