Understanding exciton-mediated interactions in transition metal dichalcogenides (TMDs)

Investigate and quantitatively explain the interactions between excitons (and exciton-polaritons) in transition metal dichalcogenide monolayers that are mediated by a two-dimensional electron gas or by other excitons. Specifically, determine a theoretical framework that reconciles puzzling experimental observations—such as the sign and magnitude of polaron-polariton energy shifts—with Landau’s quasiparticle interaction picture, while accounting for phase-space filling due to the composite nature of excitons, the detailed exciton–electron and exciton–exciton interaction potentials, and the inherently non-equilibrium character of the optical pump–probe conditions used in experiments.

Background

The perspective reviews mediated interactions across atomic gases, cavity QED platforms, and two-dimensional semiconductors, highlighting that in TMD monolayers (e.g., MoSe2, WS2) the observed polariton–polaron interactions show puzzling behavior, including large repulsive shifts that conflict with simple expectations from bosonic impurity models and equilibrium theories.

Experimental findings in TMDs involve strong nonlinearities, phase-space filling, and possibly trion-related effects, while theoretical treatments (e.g., Chevy-type variational ansätze, equilibrium Green’s functions) can predict different signs or mechanisms. The authors emphasize that the composite nature of excitons, complex interaction potentials, and non-equilibrium optical conditions complicate a direct mapping to Landau quasiparticle interactions, making a unified, quantitative understanding an open problem.