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Excitonic insulator in a heterojunction moiré superlattice

Abstract

Two-dimensional moiré superlattices provide a highly tunable platform to study strongly correlated physics. In particular, the moiré superlattices of two-dimensional semiconductor heterojunctions have been shown to host tunable correlated electronic states such as a Mott insulator and generalized Wigner crystals1,2,3,4. Here we report the observation of an excitonic insulator5,6,7, a correlated state with strongly bound electrons and holes, in an angle-aligned monolayer WS2/bilayer WSe2 moiré superlattice. The moiré coupling induces a flat miniband on the valence-band side only in the first WSe2 layer interfacing WS2. The electrostatically introduced holes first fill this miniband and form a Mott insulator when the carrier density corresponds to one hole per moiré supercell. By applying a vertical electric field, we tune the valence band in the second WSe2 layer to overlap with the moiré miniband in the first WSe2 layer, realizing the coexistence of electrons and holes at equilibrium, which are bound as excitons due to a strong Coulomb interaction. We show that this new bound state is an excitonic insulator with a transition temperature as high as 90 K. Our study demonstrates a moiré system for the study of correlated many-body physics in two dimensions.

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Fig. 1: Moiré superlattice of 1L/2L WS2/WSe2.
Fig. 2: Electric-field tuning of the band alignment in dual-gate 1L/2L WS2/WSe2.
Fig. 3: MIM spectra of single-gate 1L/2L WS2/WSe2 devices with opposite stacking orders.
Fig. 4: Helicity-resolved magnetoreflectance spectra under an out-of-plane magnetic field.

Data availability

Source data are provided with this paper. All other data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

Code availability

The source code for the numerical simulations is available from the corresponding authors upon reasonable request.

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Acknowledgements

Z.L. and S.-F.S. acknowledge support from NYSTAR through Focus Center-NY–RPI contract C150117. The device fabrication was supported by the Micro and Nanofabrication Clean Room (MNCR) at Rensselaer Polytechnic Institute (RPI). S.-F.S. also acknowledges support from National Science Foundation (NSF) (Career Grants DMR-1945420 and DMR-2104902) and AFOSR (FA9550-18-1-0312). X.H. and Y.-T.C. acknowledge support from NSF under award DMR-2104805. The optical spectroscopy measurements are also supported by a DURIP award through grant FA9550-20-1-0179. Y.S. and C.Z. acknowledge support from NSF PHY-2110212 and PHY-1806227, ARO (W911NF17-1-0128) and AFOSR (FA9550-20-1-0220). D.C. acknowledges support from the National Natural Science Foundation of China via grant number 62004032. S.T. acknowledges support from DOE-SC0020653, Applied Materials, NSF CMMI 1825594, NSF DMR-1955889, NSF CMMI-1933214, NSF DMR-1904716, NSF 1935994, NSF ECCS 2052527 and DMR 2111812. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, via grant number JPMXP0112101001 and JSPS KAKENHI, grant numbers 19H05790 and JP20H00354. L.X. and D.S. acknowledge support from the US Department of Energy (no. DE-FG02-07ER46451) for magnetospectroscopy measurements performed at the National High Magnetic Field Laboratory, which is supported by the NSF through NSF/DMR-1644779 and the State of Florida.

Author information

Authors and Affiliations

Authors

Contributions

S.-F.S. and Y.-T.C. conceived the project. D.C. and Z.L. fabricated the heterostructure devices and performed the optical spectroscopy measurements. X.H. performed the MIM measurements. M.R. assisted with the device fabrication. M.B. and S.T. grew the TMDC crystals. T.T. and K.W. grew the BN crystals. C.Z. and Y.S. performed the theoretical calculations. L.M., L.Y. and S.-F.S. performed the magnetoreflectance measurements, with the help of L.X. and D.S. S.-F.S., Y.-T.C., C.Z., Y.S., D.C., Z.L., L.M., L.Y. and X.H. analysed the data. S.-F.S. and Y.-T.C. wrote the manuscript with inputs from all the authors.

Corresponding authors

Correspondence to Zenghui Wang, Chuanwei Zhang, Yong-Tao Cui or Su-Fei Shi.

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The authors declare no competing interests.

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Nature Physics thanks Dmitry Efimkin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1

Optical microscope image of the device D1 presented in the main text.

Source data

Extended Data Fig. 2

Electric field tuning of the band alignment in dual-gated 1L/2L WS2/WSe2 device D1.

Source data

Extended Data Fig. 3 Transition between Mott insulator and EI states at n = −1 probed by MIM spectra.

(a) Schematic of a dual gated 1L/2L WS2/WSe2 device D1, same as the device in Fig. 1, but in a region with a monolayer graphene (MLG) as the top gate. (b) Schematic of expected MIM features in a dual gate MIM map. (c)–(e) are color plots of MIM spectra as a function of top gate and back gate voltages for 1L/2L WS2/WSe2 region at 20 K (c) and at 70 K (d), and for 1L/1L WS2/WSe2 region at 70 K (e), respectively. The top panels are MIM-Im data and the bottom panels are MIM-Re data.

Source data

Extended Data Fig. 4 MIM spectra of single-gated 1L/2L WS2/WSe2 devices with opposite stacking orders.

(a) MIM spectra as a function of the hole doping for the single gated 1L/1L WS2/WSe2 region of device D1, with the device structure schematically shown in (c). (b) MIM spectra as a function of the hole doping for the single gated 1L/2L WS2/WSe2 region in device D1, with the device stacking order schematically shown in (d). (e) and (f) are doping dependent MIM spectra for the 1L/1L and 2L/1L WSe2/WS2 regions in device D5, with the schematic of the stacking shown in (g) and (h), respectively. The MIM spectra in (b) and (f) are equivalent to the red and blue linecuts in the schematic of the dual gate map in (i). Therefore, the n = −1 states in (b) and (f) correspond to the EI and Mott insulator state, respectively.

Source data

Extended Data Fig. 5 Magneto-reflectance spectra and Zeeman shifts of the excitons at the Mott insulator state.

(a) and (b) are helicity-resolved magneto-reflectance spectra of 1L/2L WS2/WSe2 region of device D2 as a function of doping for the negative electric field -69 mV/nm, under an out-of-plane magnetic field of 4T. At n = −1, in contrast to Fig. 4, exciton resonance XIV, is evidently A exciton like. (c) Zeeman shift of XIV of device D3 at the Mott insulator state (reflectance spectra in Supplementary Fig. 6).

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1–7 and Notes 1–3.

Source data

Source Data Fig. 1

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Source Data Extended Data Fig. 1

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Source Data Extended Data Fig. 5

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Chen, D., Lian, Z., Huang, X. et al. Excitonic insulator in a heterojunction moiré superlattice. Nat. Phys. (2022). https://doi.org/10.1038/s41567-022-01703-y

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