Ultrafast spectroscopy of nonequilibrium bosonic Mott phase in WSe₂/WS₂ heterobilayers

In semiconducting moiré materials, the bosonic counterpart of Hubbard physics has only recently emerged, highlighted by the realization of a bosonic Mott insulating state, marking the foundation for exploring their intrinsic nonequilibrium dynamics. Here, we demonstrate the nonequilibrium evolution of a bosonic Mott phase in a WSe₂/WS₂ moiré superlattice by employing an isolated 2s excitonic resonance in a WSe₂ monolayer as an ultrafast, high-sensitivity probe of correlated excitonic states. The filling factor of interlayer excitons (ν_ex) is tuned through continuous-wave optical pumping, while femtosecond pump-probe spectroscopy captures the transient evolution of the correlated phase. When ν_ex = 1, corresponding to one exciton per moiré site, we observe a rapid recovery process with a characteristic timescale of approximately 10 ps, associated with a bosonic hopping energy of t ≈ 60 μeV. This recovery occurs faster than the 30 ps thermalization of interlayer excitons, establishing a clear hierarchy of dynamical timescales. It points to the formation of a transient bosonic Mott phase seeded by hot excitons. This phase vanishes above 20 K, where acoustic phonon activation accelerates thermalization to a timescale comparable with the recovery of the bosonic Mott state.