  A Bose-Einstein condensate is the ground state of a dilute gas of bosons,
such as atoms cooled to temperatures close to absolute zero. With much smaller
mass, excitons (bound electron-hole pairs) are expected to condense at
significantly higher temperatures. Here we study electrically generated
interlayer excitons in MoSe2/WSe2 atomic double layers with density up to 10^12
cm-2. The interlayer tunneling current depends only on exciton density,
indicative of correlated electron-hole pair tunneling. Strong
electroluminescence (EL) arises when a hole tunnels from WSe2 to recombine with
electron in MoSe2. We observe a critical threshold dependence of the EL
intensity on exciton density, accompanied by a super-Poissonian photon
statistics near threshold, and a large EL enhancement peaked narrowly at equal
electron-hole densities. The phenomenon persists above 100 K, which is
consistent with the predicted critical condensation temperature. Our study
provides compelling evidence for interlayer exciton condensation in
two-dimensional atomic double layers and opens up exciting opportunities for
exploring condensate-based optoelectronics and exciton-mediated
high-temperature superconductivity.
