Quantum confinement and coherent transport in ultrathin [Formula: see text] nanoribbons

In recent years much progress has been made in realizing topological insulator (TI) nanostructures where the reduced dimensions should help to diminish the contributions from bulk carriers and enhance quantum confinement. Though nm thick 3D-TI nanoribbons exhibiting topological properties are still difficult to reproducibly synthesize. Here we demonstrate the growth of ultrathin [Formula: see text] nanoribbons by a simple catalyst-free physical-vapour deposition, where the tuning of the material evaporation time plays a crucial role in determining the ultimate thickness of the nanoribbons. Magnetotransport and Hall effect measurements show that at thicknesses close to 10 nm the transport features are affected by Altshuler-Aronov-Spivak like coherent orbits at low magnetic fields, while Shubnikov-de Haas oscillations take over at high fields. The observed phenomena originate from the topological surface states and dominate the nanoribbon transport. Ultrathin nanoribbons also show pronounced conductance oscillations as a function of gate voltage, that can be attributed to ballistic transport and quantized sub-bands. The results highlight the importance of material growth to exploit the unique properties of topological surface states, establishing 3D-TI nanoribbons as a promising platform for a variety of novel applications.