Mobile interlayer excitons at the Mott transition in Moiré-free heterostructures

Vertically stacked heterostructures of transition metal dichalcogenides (TMDCs) present an exciting platform to study electronic many-body phenomena. The type-II band alignments, commonly encountered in TMDC heterobilayers and the presence of strong Coulomb interactions results in the formation of tightly bound and mobile interlayer excitons. What remains barely explored, however, is the high-density regime between excitons and dense plasma in the context of exciton propagation. Moreover, the heterostructures can exhibit substantial complexity due to formation of Moiré-type superlattices. It motivates investigation of high-density exciton transport phenomena in the absence of such potentials, to disentangle the effects of dipolar excitons from those stemming from Moiré effects. This is the main topic of our study, taking advantage of hBN-encapsulated WSe2/MoSe2 heterostructures studied in the Moiré-free limit of large, atomically reconstructed domains. Using ultrafast microscopy, we show that the interlayer excitons propagate freely even at cryogenic temperatures and low densities. At elevated exciton densities, we demonstrate that in addition to broadly assumed exciton-exciton repulsion, the non-linear increase of the diffusion coefficient also originates from efficient exciton-exciton annihilation. Remarkably, at the exciton ionization threshold of the Mott transition and beyond, we reveal a highly unusual regime of negative effective diffusion that persist for many 100's of ps after the excitation. This observation presents a particularly interesting case of non-equilibrium phenomena in composite many-particle systems, highlighting the rich physics of optical excitations in van der Waals heterostructures.