  A single monolayer of the layered semimetal WTe2 behaves as a two-dimensional
topological insulator, with helical conducting edge modes surrounding a bulk
state that becomes insulating at low temperatures. Here we present evidence
that the bulk state has a very unusual nature, containing electrons and holes
bound by Coulomb attraction (excitons) that spontaneously form in thermal
equilibrium. On cooling from room temperature to 100 K the conductivity
develops a V-shaped dependence on electrostatic doping, while the chemical
potential develops a ~43 meV step at the neutral point. These features are much
sharper than is possible in an independent-electron picture, but they can be
largely accounted for by positing that some of the electrons and holes are
paired in equilibrium. Our calculations from first principles show that the
exciton binding energy is larger than 100 meV and the radius as small as 4 nm,
explaining their formation at high temperature and doping levels. Below 100 K
more strongly insulating behavior is seen, suggesting that a charge-ordered
state forms. The observed absence of charge density waves in this state appears
surprising within an excitonic insulator picture, but we show that it can be
explained by the symmetries of the exciton wave function. Monolayer WTe2
therefore presents an exceptional combination of topological properties and
strong correlations over a wide temperature range.
