A Gate‐Tunable Ambipolar Quantum Phase Transition in a Topological Excitonic Insulator

Abstract Coulomb interactions among electrons and holes in 2D semimetals with overlapping valence and conduction bands can give rise to a correlated insulating ground state via exciton formation and condensation. One candidate material in which such excitonic state uniquely combines with non‐trivial band topology are atomic monolayers of tungsten ditelluride (WTe 2 ), in which a 2D topological excitonic insulator (2D TEI) forms. However, the detailed mechanism of the 2D bulk gap formation in WTe 2 , in particular with regard to the role of Coulomb interactions, has remained a subject of ongoing debate. Here, it shows that WTe 2 is susceptible to a gate‐tunable quantum phase transition, evident from an abrupt collapse of its 2D bulk energy gap upon ambipolar field‐effect doping. Such gate tunability of a 2D TEI, into either n ‐ and p ‐type semimetals, promises novel handles of control over non‐trivial 2D superconductivity with excitonic pairing.