Exciton-exciton interactions in heterobilayers of transition-metal dichalcogenides*

Vertically stacked heterostructures of transition metal dichalcogenides (TMDCs) present a versatile platform to study electronic many-body phenomena. In these systems, the commonly encountered type-II band alignment and the presence of strong Coulomb interactions result in the formation of tightly-bound interlayer excitons. The interactions between these excitons are key to understand both non-linear optical and, notably, transport phenomena, as the propagation is strongly affected by the interactions. Here, we address this topic by studying spectrally narrow interlayer excitons in the moiré free limit of atomically reconstructed, hBN-encapsulated MoSe₂/WSe₂ heterobilayers. These samples are shown to host freely propagating interlayer excitons allowing the excitons to efficiently scatter with each other. The measurements are combined with theoretical calculations of the exciton-exciton interaction.

While dipolar repulsion is broadly assumed to determine exciton interaction for TMDC heterobilayer, we find an almost perfect compensation of the resulting spectral shifts by additional many-body effects, most notably screening-induced self-energy correlation. In contrast to the dipolar model predictions, we find blue shifts of the interlayer exciton resonances on the scale of only a few meV even for high injection densities close to the Mott transition. These findings challenge the broadly assumed picture of the dipole repulsion dominating the exciton interaction in van der Waals heterobilayers, highlighting the role of exchange and screening contributions.