Perfect Coulomb drag and exciton transport in an excitonic insulator

Strongly coupled electron-hole bilayers can host quantum states of
interlayer excitons, such as high-temperature exciton condensates at zero
magnetic field. This state is predicted to feature perfect Coulomb drag,
where a current in one layer is accompanied by an equal but opposite
current in the other. Here we use an optical technique to probe the
electrical transport of correlated electron-hole bilayers based on
MoSe2/hBN/WSe2 heterostructures. We observe perfect Coulomb drag in the
excitonic insulator phase at low temperatures; the counterflow resistance
of interlayer excitons remains finite. These results indicate the
formation of an exciton gas that does not condense into a superfluid. Our
work demonstrates that dynamic optical spectroscopy provides a powerful
tool for probing exciton transport behavior in correlated electron-hole
fluids.