Abstract
The strongly enhanced electron–electron interactions in semiconducting moiré superlattices formed by transition metal dichalcogenide heterobilayers have led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type II aligned heterobilayer moiré superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and suggest that tunable correlated bosonic quasiparticles with a valley degree of freedom could be realized. Here we determine the spatial extent of interlayer excitons and the band hierarchy of correlated states that arises from the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moiré superlattice. We also find evidence that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moiré cell. Furthermore, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates that the WS2/WSe2 moiré superlattice is a promising platform for engineering and exploring new correlated states of fermion, bosons and a mixture of both.
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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 twisted bilayer structure construction, atomic relaxations and electronic band structure calculations presented in this paper were carried out using publicly available codes. Our findings can be fully reproduced by the use of these codes and by following the procedure outlined in the paper.
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Acknowledgements
We thank C. Jin for the helpful discussions. 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 at Rensselaer Polytechnic Institute (RPI). S.-F.S. also acknowledges the support from NSF (Career Grant DMR-1945420, DMR-2104902 and ECCS-2139692). X.H., Q.W. and Y.-T.C. acknowledge support from NSF under awards DMR-2104805 and DMR-2145735. The optical spectroscopy measurements were supported by the Air Force Office of Scientific Research (AFOSR) under DURIP awards through Grants FA9550-20-1-0179 and FA9550-23-1-0084. I.M. acknowledges funding from the European Union’s Horizon 2020 Research and Innovation programme under the Marie Skłodowska-Curie Grant agreement no. 101028468. This work used the ARCHER2 UK National Supercomputing Service (https://www.archer2.ac.uk). S.T. acknowledges support from DOE-SC0020653, Applied Materials Inc., NSF CMMI 1825594, NSF DMR-1955889, NSF CMMI-1933214, NSF DMR-1904716, NSF 1935994 and NSF ECCS 2052527 and DMR 2111812. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant Numbers JPMXP0112101001 and JSPS KAKENHI, Grant Numbers 19H05790 and JP20H00354.
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S.-F.S. conceived the project. Y.M., Z.L. and D.C. fabricated heterostructure devices. Z.L., L.M., L.Y. and Y.M. performed the optical spectroscopy measurements. M.B. and S.T. grew the TMDC crystals. T.T. and K.W. grew the BN crystals. I.M. and J.L. performed the DFT calculations. S.-F.S., Y.-T.C., Z.L., Y.M., Xiaotong Chen and Xinyue Chen analysed the data. S.-F.S. and Y.-T.C. wrote the paper with input from all authors.
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Extended data
Extended Data Fig. 1 Zoom-ins of the doping-dependent PL spectra.
(a) and (b) are the Zoom-ins of doping-dependent PL spectra in Fig. 1c,d, respectively.
Extended Data Fig. 2 PL spectra of an H-stacked device showing more fractional fillings.
(a) and (b) are the doping-dependent PL spectra measured on device H5 using an excitation power of 0.2 µW and 300 µW, respectively. A CW laser with photon energy centered at 1.70 eV was used for the optical excitation, and all data were taken at a temperature of 5.0 K.
Extended Data Fig. 3 Power-dependent PL spectra for different R-stacked and H-stacked devices around n = 0.
(a), (b) and (c) are the power-dependent PL spectra of R-stacked devices R1, R2 and R3, respectively. (d), (e) and (f) are the power-dependent PL spectra of H-stacked devices H2, H3 and H4, respectively. Black, red, and blue line show PL spectra under low, medium, and high excitation power. A CW laser with photon energy centered at 1.96 eV was used for the optical excitation, and all data were taken at a temperature of 4–10 K.
Extended Data Fig. 4 Helicity-resolved doping-dependent PL spectra for R-stacked WS2/WSe2 moiré bilayers under different excitation powers.
(a) and (b) are the measured right (σ+) and left circularly (σ-) polarized doping-dependent PL spectra of device R1, under a σ+ excitation with the excitation power of 2 µW. (c) and (d) are σ+ and σ- PL spectra under a σ+ excitation with an excitation power of 390 µW. A CW laser with photon energy centered at 1.70 eV was used for the optical excitation, and all data were taken at a temperature of 5.0 K.
Extended Data Fig. 5 Helicity-resolved doping-dependent PL spectra for H-stacked moiré bilayers under different power.
(a) and (b) are the measured right (σ+) and left circularly (σ-) polarized doping-dependent PL spectra of device H1, under a σ+ excitation with an excitation power of 2 µW. (c) and (d) are σ+ and σ- PL spectra under a σ+ excitation with an excitation power of 390 µW. A CW laser with photon energy centered at 1.70 eV was used for the optical excitation, and all data were taken at a temperature of 5.0 K.
Extended Data Fig. 6 Helicity-resolved PL spectra of device R1 and H1 for various filling factors and under different excitation powers.
(a) and (b) are the PL spectra of device R1 at n = 0 under excitation powers of 2 µW and 390 µW, respectively. The red and blue lines represent measured right (σ+) and left circularly (σ-) polarized PL spectra, respectively, under the σ+ excitation. (c) and (d) are the PL spectra of device R1 at n = 3 under the excitation power of 2 µW and 390 µW, respectively. (e) and (f) are the PL spectra of device H1 at n = 0 with excitation power of 2 µW and 390 µW, respectively. These data are extracted from Extended Data Figs. 4 and 5. The data in (c) are offset for the clarity of presentation.
Supplementary information
Supplementary Information
Supplementary Sections 1–12, Figs. 1–11 and Table 1.
Source data
Source Data Fig. 1
Numerical source data for Fig. 1c–f.
Source Data Fig. 2
Numerical source data for Fig. 2a,b.
Source Data Fig. 3
Numerical source data for Fig. 3a,b,d,e.
Source Data Fig. 4
Numerical source data for Fig. 4a–d.
Source Data Extended Data Fig. 2
Numerical source data for Extended Data Fig. 2a,b.
Source Data Extended Data Fig. 3
Numerical source data for Extended Data Fig. 3a–f.
Source Data Extended Data Fig. 4
Numerical source data for Extended Data Fig. 4a–d.
Source Data Extended Data Fig. 5
Numerical source data for Extended Data Fig. 5a–d.
Source Data Extended Data Fig. 6
Numerical source data for Extended Data Fig. 6a–f.
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Lian, Z., Meng, Y., Ma, L. et al. Valley-polarized excitonic Mott insulator in WS2/WSe2 moiré superlattice. Nat. Phys. 20, 34–39 (2024). https://doi.org/10.1038/s41567-023-02266-2
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DOI: https://doi.org/10.1038/s41567-023-02266-2
