Two-dimensional topological insulator behavior in monolayer WTe2*

The van der Waals layered material WTe₂ is a semimetal, with closely matched electron and hole pockets and very strong spin-orbit coupling, which exhibits gigantic magnetoresistance at low temperatures and has topological features its single-particle band structure. Studying WTe₂ down to the monolayer limit by exfoliation and encapsulation in hexagonal boron nitride, we find that it exhibits a diverse range of phenomena. In particular, monolayers show edge conduction with properties matching those expected for the helical boundary modes of a two-dimensional topological insulator (producing the quantum spin Hall effect). These include no band gap at zero magnetic field, suppression by an in-plane magnetic field, absence in bilayers, and conductance per edge not exceeding e²/h. However, the conductance never actually reaches e²/h, shows large mesoscopic fluctuations, and falls at dilution fridge temperatures, showing a sub-meV bias threshold for current flow of unknown origin. We also discuss the rather peculiar conductivity behavior of the bulk insulating state, in which electron-hole correlations may be an important factor.