Nature of the insulating and superconducting states of monolayer WTe₂

Although in the bulk WTe₂ is a semimetal, exfoliated to a monolayer it becomes a topological insulator that undergoes a transition to a metallic state on electron doping of 5x10¹² cm^-2, and furthermore this metallic state becomes superconducting at temperatures below about 1 K. This proximity, in terms of doping, of a small-gap insulator to a superconductor is surprising, and the insulator exhibits a number of other unusual properties. We will summarize and compare these properties and assess the possibility that the insulating state has a many-body character akin to an excitonic insulator. In addition, we will present and evaluate evidence that the helical edge states remain throughout the insulator to superconductor transition.