Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides

Abstract

Monolayer transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) semiconductors for application in optoelectronics. Their optical properties are dominated by two series of photo-excited exciton states—A (XA) and B (XB)1,2—that are derived from direct interband transitions near the band extrema. These exciton states have large binding energies and strong optical absorption3,4,5,6, and form an ideal system to investigate many-body effects in low dimensions. Because spin–orbit coupling causes a large splitting between bands of opposite spins, XA and XB are usually treated as spin-polarized Ising excitons, each arising from interactions within a specific set of states induced by interband transitions between pairs of either spin-up or spin-down bands (TA or TB). Here, by using monolayer MoS2 as a prototypical system and solving the first-principles Bethe–Salpeter equations, we demonstrate a strong intravalley exchange interaction between TA and TB, indicating that XA and XB are mixed states instead of pure Ising excitons. Using 2D electronic spectroscopy, we observe that an optical excitation of the lower-energy TA induces a population of the higher-energy TB, manifesting the intravalley exchange interaction. This work elucidates the dynamics of exciton formation in monolayer TMDCs, and sheds light on many-body effects in 2D materials.

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Fig. 1: Exciton state mixing from the intravalley exchange interaction.
Fig. 2: 2D electronic spectroscopy measurement of monolayer MoS2.
Fig. 3: Simulation of the rephasing amplitude 2D spectra including the intravalley exchange interaction.

Data availability

The data that support the findings of this study are available from the corresponding authors on reasonable request.

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Acknowledgements

We thank G. Moody and K. Hao for helpful discussion. This material is based on work supported by the National Science Foundation under grant no. CHE-1362830, grant no. DMR-1508412 and grant no. EFMA-1542741. D.M.M. received a National Science Foundation Graduate Research Fellowship under grant no. DGE-1106400. Advanced codes were provided by the Center for Computational Study of Excited-State Phenomena in Energy Materials (C2SEPEM) at LBNL, which is funded by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05CH11231, as part of the Computational Materials Sciences Program. Computational resources were provided by the DOE at Lawrence Berkeley National Laboratory’s NERSC facility and the NSF through XSEDE resources at NICS. Y.-H. L. acknowledges support from the Ministry of Science and Technology (MoST-106-2119-M-007-023-MY3; MoST-105-2112-M-007-032-MY3), the Frontier Research Center on Fundamental and Applied Sciences of Matters, and the Center for Quantum Technology of National Tsing Hua University.

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L.G., M.W., T.C. and D.M.M. contributed equally to this work. L.G., M.W. and T.C. conceived the concept. Supervised by G.R.F., L.G. and D.M.M. led the experiments by designing the optical system and acquiring the data. M.W. and T.C. performed the theoretical studies under the supervision of S.G.L. Y.-H.L. provided the samples. L.G., M.W. and T.C. wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to Steven G. Louie or Graham R. Fleming.

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Supplementary Figures 1–11; Supplementary Tables 1–4; Supplementary References 1–24

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Guo, L., Wu, M., Cao, T. et al. Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides. Nat. Phys. 15, 228–232 (2019). https://doi.org/10.1038/s41567-018-0362-y

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