Abstract
Spontaneous symmetry breaking, driven by competing interactions and quantum fluctuations, is fundamental to understanding ordered electronic phases. Although electrically neutral, optical excitations like excitons can interact through their dipole moment, raising the possibility of optically active ordered phases. The effects of spontaneous ordering on optical properties remains underexplored. The excitonic Mott insulating state recently observed in semiconducting moiré crystals may help clarify this question. Here we present evidence for an in-plane ferroelectric phase of dipolar moiré excitons driven by strong exciton–exciton interactions. We reveal a speed-up of photon emission at late times and low densities in excitonic decay. This counterintuitive behaviour is attributed to collective radiance, linked to the transition between disordered and symmetry-broken ferroelectric phases of moiré excitons. Our findings provide evidence for strong dipolar intersite interactions in moiré lattices, demonstrate collective photon emission as a probe for moiré quantum materials and a path for exploring cooperative optical phenomena in strongly correlated systems.
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Acknowledgements
We thank L. Zhang, M. Hafezi, A. Imamoğlu, M. Claassen, T. Smoleński and M. Kroner for insightful discussions. A.S. acknowledges funding from the NSF Division of Materials Research (award number 1905809) and the State Secretariat for Education, Research and Innovation (SERI)-funded European Research Council Consolidator Grant TuneInt2Quantum (number 101043957). T.P. and J.K. acknowledge support from the European Union’s Horizon Europe research and innovation programme under the Marie Skłodowska-Curie grant agreement number 101106552 (QuLowD), from the Austrian Science Fund (grant number 10.55776/COE1) and the European Union (NextGenerationEU), and from the European Research Council through the ERC Synergy Grant SuperWave (grant number 101071882). Synthesis of WSe2 (S.L. and J.H.) was supported by the NSF MRSEC program through the Columbia University Center for Precision-Assembled Quantum Materials (DMR-2011738). K.W. and T.T. acknowledge support from the JSPS KAKENHI (grant numbers 21H05233 and 23H02052), the CREST (JPMJCR24A5), JST and World Premier International Research Center Initiative (WPI), MEXT, Japan.
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A.S., T.P., L.M.D., Z.H. and D.S.-F. conceived the project. K.W. and T.T. provided the hBN crystals, and S.L. and J.H. provided the WSe2 crystals. L.M.D., B.L., W.L., Z.H. and L.F. prepared the samples. L.M.D., Z.H., R.L., D.S.-F., K.D., B.L., V.V., N.U. and L.F. carried out the measurements. J.K. and T.P. developed the theoretical model and conducted the Monte Carlo simulations. A.S. and T.P. supervised the project. All authors were involved in the analysis of the experimental data and contributed extensively.
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Devenica, L.M., Hadjri, Z., Kumlin, J. et al. Collective photon emission and ferroelectric exciton ordering near Mott insulating state in WSe2/WS2 heterobilayers. Nat. Mater. (2026). https://doi.org/10.1038/s41563-025-02476-4
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DOI: https://doi.org/10.1038/s41563-025-02476-4
