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Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe2/MoSe2 bilayers

Abstract

Van der Waals heterostructures obtained via stacking and twisting have been used to create moiré superlattices1, enabling new optical and electronic properties in solid-state systems. Moiré lattices in twisted bilayers of transition metal dichalcogenides (TMDs) result in exciton trapping2,3,4,5, host Mott insulating and superconducting states6 and act as unique Hubbard systems7,8,9 whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures feature atomic reconstruction and domain formation10,11,12,13,14. However, due to the nanoscale size of moiré domains, the effects of atomic reconstruction on the electronic and excitonic properties have not been systematically investigated. Here we use near-0°-twist-angle MoSe2/MoSe2 bilayers with large rhombohedral AB/BA domains15 to directly probe the excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane electric dipole moments in opposite directions. The dipole orientation of ground-state Γ–K interlayer excitons can be flipped with electric fields, while higher-energy K–K interlayer excitons undergo field-asymmetric hybridization with intralayer K–K excitons. Our study reveals the impact of crystal symmetry on TMD excitons and points to new avenues for realizing topologically non-trivial systems16,17, exotic metasurfaces18, collective excitonic phases19 and quantum emitter arrays20,21 via domain-pattern engineering.

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Fig. 1: AB/BA domains in t-MoSe2/MoSe2 devices.
Fig. 2: Electric-field-dependent PL spectra of the XI,1 peaks obtained from D2 at 4 K.
Fig. 3: Electric-field-dependent reflectance spectra of the X0 peaks obtained from D2 at 4 K.
Fig. 4: Electronic band structure of AB-stacked MoSe2/MoSe2.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank B. Urbaszek for helpful discussions. We acknowledge support from the DoD Vannevar Bush Faculty Fellowship (N00014-16-1-2825 for H.P., N00014-18-1-2877 for P.K.), NSF (PHY-1506284 for H.P. and M.D.L.), NSF CUA (PHY-1125846 for H.P. and M.D.L.), AFOSR MURI (FA9550-17-1-0002), ARL (W911NF1520067 for H.P. and M.D.L.), the Gordon and Betty Moore Foundation (GBMF4543 for P.K.), ONR MURI (N00014-15-1-2761 for P.K.), and Samsung Electronics (for P.K. and H.P.). V.I.F. acknowledges EPSRC grants no. EP/S019367/1, EP/S030719/1, EP/N010345/1, ERC Synergy Grant Hetero2D, Lloyd’s Register Foundation Nanotechnology Grant, European Graphene Flagship Project and European Quantum Technologies Project 2D-SIPC. The device fabrication was carried out at the Harvard Center for Nanoscale Systems. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and the CREST (JPMJCR15F3), JST. D.B. acknowledges support from the Summer Undergraduate Research Fellowship at Caltech.

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H.P., P.K., J.S., Y.Z., G.S., H.Y. and D.S.W. conceived the study, and J.S., Y.Z., G.S., T.I.A, A.Y.J., R.J.G., D.B. and A.M.M.V. fabricated the devices and performed the optical spectroscopy. H.P. V.I.F. J.S., Y.Z., G.S., V.Z., T.I.A. and D.S.W. analysed the data. V.I.F., V.Z. and S.J.M. performed the DFT calculations. H.Y. performed electron microscopy measurements. H.H. performed MoSe2 crystal growth. T.T. and K.W. performed h-BN crystal growth. J.S., Y.Z., G.S., T.I.A, M.D.L., P.K., V.I.F. and H.P. wrote the manuscript with extensive input from all authors. H.P., V.I.F., P.K. and M.D.L. supervised the project.

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Correspondence to Vladimir I. Fal’ko or Hongkun Park.

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Sung, J., Zhou, Y., Scuri, G. et al. Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe2/MoSe2 bilayers. Nat. Nanotechnol. 15, 750–754 (2020). https://doi.org/10.1038/s41565-020-0728-z

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