Bi

Na₃Bi in bulk is a zero-bandgap topological Dirac semimetal (TDS), but when confined to a few layers it is predicted to be a large-gap (~300 meV) topological insulator. Application of an electric field to few-layer Na₃Bi has been predicted to drive a band inversion due to Stark effect and induce a topological phase transition, which could be the basis of a topological transistor. Here we demonstrate the growth of epitaxial few-layer Na₃Bi via MBE, and probe its electronic structure and response to an electric field using scanning probe microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy, and compare with results from DFT.¹ Both monolayer and bilayer Na₃Bi show bandgaps >300 meV in STS, and the observation of an edge state with exponential decay into the bulk confirms their topological nature, consistent with DFT. With application of an electric field via potassium doping or approach of the STM tip the bandgap can be tuned to semi-metallic and then re-opened to greater than 100 meV. The electric fields required to induce this transition are below the breakdown field of many conventional dielectrics, making the creation of a topological transistor based on a few-layer TDS within reach.


¹ J.L. Collins et al., arXiv:1805.08378 (to appear in Nature)