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.

References

Understanding the puzzling results regarding interactions between excitons in TMDs mediated by electrons or other excitons is also a largely open and important problem. This is complicated by several factors such as the composite nature of the excitons leading to phase space filling effects, the non-trival nature of the exciton-electron and exciton-exciton interactions, and the inherent non-equilibrium nature of these systems, which may require the development of new theoretical frameworks.

— Interactions mediated by atoms, photons, electrons, and excitons  (2406.13795 - Paredes et al., 2024) in Section Conclusions and Outlook

Although the microscopic origin of XU cannot yet be identified unambiguously, its pronounced absorption signature, weak and spectrally narrow emission, finite valley polarization, spatial delocalization, and sub-linear excitation-power dependence are collectively consistent with a weakly allowed excitonic transition, likely arising from a COC-induced modification of the local excitonic landscape.

— Scalable Cyclic Olefin Copolymer Encapsulation for High Optical Quality of TMD Monolayers  (2608.20973 - Das et al., 21 Aug 2026) in Section Conclusions

While it is not known exactly for excitonic systems, it can be estimated to logarithmic accuracy:

— Multi-dimensional spectroscopy of mobile excitons in two-dimensional semiconductors  (2608.27951 - Wheaton et al., 28 Aug 2026) in Section “Mobile versus localized excitons,” subsection “Mobile excitons,” immediately following Eq. (mobileselfenergy)