Granular topological insulators.

We demonstrate experimentally that a macroscopic topological insulator (TI) phase can emerge in a granular conductor composed of an assembly of tunnel coupled TI nanocrystals of dimension ∼10 nm × 10 nm × 2 nm. Electrical transport measurements on thin films of BiSe nanocrystals reveal the presence of decoupled top and bottom topological surface states that exhibit large surface state penetration depths (∼30 nm at 2 K). By tuning the size of the nanocrystals and the couplings between them, this new class of TIs may be readily tuned from a non-topological to a topological insulator phase, that too with designer properties.

Intermediate stages of surface state formation and collapse of topological protection to transport in BiSe.

Surface states consisting of helical Dirac fermions have been extensively studied in three-dimensional topological insulators. Yet, experiments to date have only investigated fully formed topological surface states (TSS) and it is not known whether preformed or partially formed surface states can exist or what properties they could potentially host. Here, by decorating thin films of BiSe with nanosized islands of the same material, we show for the first time that not only can surface states exist in various intermediate stages of formation but they exhibit unique properties not accessible in fully formed TSS.