Peculiar behavior in two-dimensional semimetals such as WTe2

In two-dimensional materials that, according to band-structure calculations, should contain both electron and hole pockets, there is every reason to expect that electron-hole correlations will be strong and have significant consequences, such as formation of an excitonic insulator state. This is the situation in monolayer and bilayer WTe2, which exhibit peculiar properties near charge neutrality. As these materials are cooled a thermodynamic gap opens up at the charge-neutral point, and the variation of the electrical conductivity with doping and temperature is impossible to reconcile with a single-particle band picture incorporating disorder. Calculations show that exciton binding here should be very strong, and it is natural to ask whether formation and condensation of equilibrium excitons can explain the observed behavior. While the opening of a gap appears consistent with formation of an excitonic insulator, understanding the conductivity when the electron and hole densities are unbalanced is beyond the excitonic insulator model and calls for new experimental approaches as well as theoretical insights. We are attacking this question by modifying the WTe2 and by searching for other two-dimensional semimetals exhibiting similar behavior.