Observation of a macroscopic topological insulator phase in an assembly of coupled topological insulator nanocrystals

We study an assembly of tunnel coupled topological insulator (TI) nanocrystals. We demonstrate experimentally that a macroscopic topological insulator phase can emerge in this system. Electrical transport measurements on thin films of Bi$_2$Se$_3$ nanocrystals reveal the presence of decoupled top and bottom topological surface states above a certain film thickness. The surface state penetration depth is found to be unusually large, $\sim$ 30nm at 2K, and decreases with increasing temperature. For samples with low film thickness, we observe deviations of the surface state Berry phase from $\pi$ due to hybridization of opposite surface states. This weakens the effective spin-orbit coupling field to as low as $\sim$ 30T at 2K. Remarkably, the topological insulating behavior becomes more pronounced with increasing temperature. Our work exhibits a model TI that is distinct from bulk/single crystal TIs and also displays phenomena that are expected, but normally not accessible in the latter systems.