Abstract
Moiré superlattices can be used to engineer strongly correlated electronic states in two-dimensional van der Waals heterostructures, as recently demonstrated in the correlated insulating and superconducting states observed in magic-angle twisted-bilayer graphene and ABC trilayer graphene/boron nitride moiré superlattices1,2,3,4. Transition metal dichalcogenide moiré heterostructures provide another model system for the study of correlated quantum phenomena5 because of their strong light–matter interactions and large spin–orbit coupling. However, experimental observation of correlated insulating states in this system is challenging with traditional transport techniques. Here we report the optical detection of strongly correlated phases in semiconducting WSe2/WS2 moiré superlattices. We use a sensitive optical detection technique and reveal a Mott insulator state at one hole per superlattice site and surprising insulating phases at 1/3 and 2/3 filling of the superlattice, which we assign to generalized Wigner crystallization on the underlying lattice6,7,8,9,10,11. Furthermore, the spin–valley optical selection rules12,13,14 of transition metal dichalcogenide heterostructures allow us to optically create and investigate low-energy excited spin states in the Mott insulator. We measure a very long spin relaxation lifetime of many microseconds in the Mott insulating state, orders of magnitude longer than that of charge excitations. Our studies highlight the value of using moiré superlattices beyond graphene to explore correlated physics.
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Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
We thank S. Li for discussions, S. Wang for help with device fabrication, and C. Stansbury and C. Wang for assistance with figure design. This work was supported primarily by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract number DE-AC02-05-CH11231 (van der Waals heterostructures programme, KCWF16). The device fabrication was also supported by the US Army Research Office under MURI award W911NF-17-1-0312. E.C.R. acknowledges support from the Department of Defense through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. C.J. acknowledges support from a Kavli Postdoctoral Fellowship. S.T. acknowledges support from NSF DMR-1552220 and 1838443.
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F.W. conceived the research. E.C.R., D.W. and C.J. carried out optical measurements. D.W., E.C.R., C.J. and F.W. performed data analysis. E.C.R., D.W., B.G., X.W., M.I.B.U., S.Z., W.Z., Z.Z., J.D.C., M.C. and A.Z. contributed to the fabrication of van der Waals heterostructures. K.Y., M.B. and S.T. grew WSe2 and WS2 crystals. K.W. and T.T. grew hBN crystals. All authors discussed the results and wrote the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 WSe2 A exciton gate behaviour.
a, Reflection contrast spectra for the lowest-energy WSe2 A exciton resonance in region 2 of device D1 when the local top-gate voltage Vtop is tuned from 0.5 V to –3.5 V. Region 2 is not affected when the hole concentration is tuned in region 1 by Vtop. b, Reflection contrast spectra for the WSe2 A exciton in region 2 when the global back gate is tuned from −0.95 V to −1.05 V. The inset shows a zoomed-in view of the exciton peak. The spectral change is monotonic and approximately linear with carrier concentration.
Extended Data Fig. 2
ODRC signal measured at very low frequencies for a range of modulation voltages. Vmod, modulation voltage.
Extended Data Fig. 3 Calibration of hBN dielectric constant.
a, MoSe2 A exciton peak intensity measured while tuning the voltages of the top graphite gate and back Si gate. b, The extracted charge-neutral points (CNP, dots) for each Si back gate, corresponding to the top graphite gate voltages that bring the system to zero net charge. The black line is a linear fit to the data, from which the relative gate efficiency is determined.
Extended Data Fig. 4 Determination of WSe2 and WS2 flake alignment.
Polarization-dependent SHG signal on monolayer WSe2 (red circles) and WS2 (black circles) regions of device D1 and corresponding fittings (red and black curves, respectively).
Extended Data Fig. 5 ODRC signal for other aligned WSe2/WS2 heterostructures.
a, b, ODRC signal at low (grey) and high (black) frequency from charge-neutral to moderate hole doping in devices D2 (a) and D3 (b). The dashed lines are guides to the eye at hole concentrations of n = 0 (purple), n = n0/3 (orange), n = 2n0/3 (green) and n = n0 (blue).
Extended Data Fig. 6 ODRC signal for a large-twist-angle WSe2/WS2 heterostructure.
a, Normalized ΔOC for a large-twist-angle heterostructure (D4, blue) and an aligned heterostructure (D1, black). The misaligned heterostructure does not show any insulating features. b, The frequency dependence of the large-twist-angle heterostructure signal shows a characteristic RC circuit fall-off with increasing frequency.
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Regan, E.C., Wang, D., Jin, C. et al. Mott and generalized Wigner crystal states in WSe2/WS2 moiré superlattices. Nature 579, 359–363 (2020). https://doi.org/10.1038/s41586-020-2092-4
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DOI: https://doi.org/10.1038/s41586-020-2092-4
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