Abstract
The fractional quantum anomalous Hall (FQAH) effect was recently discovered in twisted MoTe2 (tMoTe2) bilayers1,2,3,4. Experiments so far have revealed Chern insulators from hole doping at ν = −1, −2/3, −3/5 and −4/7 (per moiré unit cell)1,2,3,4,5,6. In parallel, theories predict that, between v = −1 and −3, there exist exotic quantum phases7,8,9,10,11,12,13,14,15, such as the coveted fractional topological insulators, fractional quantum spin Hall (FQSH) states and non-Abelian fractional states. Here we use transient optical spectroscopy16,17 on tMoTe2 to reveal nearly 20 hidden states at fractional fillings that are absent in static optical sensing or transport measurements. A pump pulse selectively excites charge across the correlated or pseudogaps, leading to the disordering (melting) of correlated states18. A probe pulse detects the subsequent melting and recovery dynamics by means of exciton and trion sensing1,3,19,20,21. Besides the known states, we observe further fractional fillings between ν = 0 and −1 and a large number of states on the electron doping side (ν > 0). Most importantly, we observe new states at fractional fillings of the Chern bands at ν = −4/3, −3/2, −5/3, −7/3, −5/2 and −8/3. These states are potential candidates for the predicted exotic topological phases7,8,9,10,11,12,13,14,15. Moreover, we show that melting of correlated states occurs on two distinct timescales, 2–4 ps and 180–270 ps, attributed to electronic and phonon mechanisms, respectively. We discuss the differing dynamics of the electron-doped and hole-doped states from the distinct moiré conduction and valence bands.
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Data availability
The data shown in the main figures are available in the Source Data. Data supporting the findings of this study are available from the corresponding author on request. Source data are provided with this paper.
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Acknowledgements
This research, including all time-resolved spectroscopy experiments presented in the main text, was primarily supported by Programmable Quantum Materials, an Energy Frontier Research Center financed by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under award DE-SC0019443. Further support came from DOE-BES under award number DE-SC0024343 (X.-Y.Z.) for pump–probe methodology development, DOE-BES under award number DE-SC0018171 (X.X.) for device fabrication, using the facilities and instrumentation supported by NSF MRSEC DMR- 2308979. US Army Research Office grant number W911NF-23-1-0056 (X.-Y.Z. and X.R.) for the temperature-dependent experiments, Department of Defense Multidisciplinary University Research Initiative (X.-Y.Z.) grant number W911NF2410292 for the development of mechanistic models of electron–phonon coupling and Materials Science and Engineering Research Center through NSF grant DMR-2011738 for facilities used in sample characterizations. Y.W. acknowledges the Max Planck New York Center (MPNYC) for fellowship support. E.A.A. acknowledges support from the Simons Foundation as a Junior Fellow in the Simons Society of Fellows (965526). K.W. and T.T. acknowledge support from the JSPS KAKENHI (grant numbers 21H05233 and 23H02052) and World Premier International Research Center Initiative (WPI), MEXT, Japan. We thank N. Verma and D. M. Segovia for helpful discussions and Y. Guo, J. Pack and S. Ghosh for assistance with sample preparation.
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Y.W., X.-Y.Z. and X.X. conceived this work. Y.W., along with J.C., conducted all spectroscopic measurements and analysed and interpreted the results, with the assistance of E.A.A. and Y.L. and input from D.B., X.R. and J.C.H. E.A. was responsible for the fabrication and characterization of sample D1 and W.L. for sample D2, under the supervision of X.X. T.T. and K.W. provided the h-BN crystal. J.I., R.Q., X.H. and D.X. contributed to mechanistic interpretations. The manuscript was prepared by Y.W. and X.-Y.Z., incorporating input from all co-authors. X.-Y.Z. supervised the project. All authors read and commented on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Device images and characterization.
a,b, 50× microscope image of devices D1 and D2. c, Reflective magnetic circular dichroism signal versus v and perpendicular electric field D at zero magnetic field µ0H = 0 (D1). The phase space with non-vanishing signal corresponds to the ferromagnetic state. d, Reflective magnetic circular dichroism signal versus vertical magnetic field with µ0H swept back and forth at n = −0.4 × 1013 cm−2 and D = 0 V nm−1 (D2). e, D1: PL intensity plot as a function of doping and photon energy. f, D2: PL intensity plot as a function of doping and photon energy. g, Reflection as a function of doping and photon energy (D1). h, Reflection as a function of doping and photon energy (D2).
Extended Data Fig. 2 Pump–probe spectroscopy at T = 7, 20 and 50 K of tMoTe2 device D1 with a twist angle θ = 3.7°.
Transient reflection spectrum as a function of carrier density (n) and probe photon energy E (ħω2) at pump–probe delays of Δt = 4 ps (a), 39 ps (b), 399 ps (c) and 1,399 ps (d) at 7 K, Δt = 4 ps (e), 39 ps (f), 399 ps (g) and 1,399 ps (h) at 20 K and Δt = 4 ps (i), 39 ps (j), 399 ps (k) and 1,399 ps (l) at 50 K. All measurements were conducted at a pump fluence of 54 μJ cm−2 with the transient reflection range ΔR/R0 from −2 × 10−4 to 2 × 10−4. There is no external magnetic field (B = 0) or displacement field (D = 0).
Extended Data Fig. 3 Pump–probe spectroscopy of tMoTe2 device D3 with a twist angle θ = 5.5°.
a, Optical image (approximately 100 µm × 100 µm) of device D3. b, PL spectra as a function doping level (estimated from calculated capacitance). The left and right panels show displacements fields of D = 0 and −0.02 V nm−1; the latter is used to compensate for a small build-in potential. c–e, Transient reflection spectrum as a function of total gate voltage Vg (V) and probe photon energy E (ħω2) at pump–probe delays of Δt = 7 ps (c), Δt = 13 ps (d) and Δt = 500 ps (e). The pump (0.99 eV) and probe fluences are at 42 µJ cm−2 and 22 µJ cm−2, respectively. Sample temperature T = 1.6 K. There is no external magnetic field (B = 0) or displacement field (D = 0). The gate voltage ranges in c–e correspond to the same estimated doping range shown in the PL maps in b. No specific states (at particular Vg or doping levels) are resolved in the pump–probe spectral maps at all three selected Δt values (7, 13 and 500 ps), in agreement with the PL maps in b. The transient spectral maps feature exciton and trion resonances, with energy splitting in certain doping ranges; these splitting features have been observed before and are attributed to exciton/trion fine structures in MoTe2 monolayers44,45 and/or exciton polarons in tMoTe2 bilayers46. The broad contrasts in exciton/trion ΔR/R signal vary slowly with Δt and probably result from the dynamics or hot-carrier relaxation, carrier-phonon scattering, phonon cooling and balances in exciton and trion populations.
Extended Data Fig. 4 Melting dynamics of correlated states.
a,b, Transient reflection spectra at the indicated pump–probe delay times (from top to bottom) of Δt = 1.5, 13, 25, 37, 49, 150, 450 and 1,150 ps for hole doping (a) and the corresponding first derivative (with respect to n) of the transient reflection spectra in a (b). All spectra obtained from tMoTe2 device D2 (θ = 3.1°), at a temperature of T = 2 K, with no external electric or magnetic field.
Extended Data Fig. 5 Time profiles of difference states.
a, Time profiles of all of the electron doping states in D1. b, Electronic melting and recovery time constant of all of the electron-doped states. c, Phonon melting and recovery time constant of electron-doped states. d, Time profiles of hole-doped states. e, Electronic melting and recovery time constant of all of the hole-doped states. f, Time profiles of ν = 0, 1/3 and 1/2. Data normalized to t = 0 ps. g, Time profile difference of ν = 1/3 and 1/2 to ν = 0. All data obtained at a temperature of T = 2 K, with no external electric or magnetic field.
Extended Data Fig. 6 Time profiles of a hole-doped state of −1/7 ≤ ν < 0, which is closest to Vg = 0.
Top line, sample temperature T = 2 K (nominal reading on sample stage 1.58 K). Bottom line, sample temperature T = 70 K, which is above Tc. Electronic and phonon melting processes are observed at 2 K but not 70 K.
Extended Data Fig. 7 Delayed arrival of coherent phonon wavepackets launched at the graphite electrodes.
a, Time profiles of the ν = 1 state with melting and recovery fitting (red line). b, Coherent phonon oscillation after the melting and recovery background is subtracted. c, Fourier transform of the coherent phonons.
Extended Data Fig. 8 Spectral resolved time profile.
a, Transient reflection as a function of delay time and spectral energy for non-correlated state ν = 0 in D1. b, Short time window of a. c, Time profiles of the ν = 0 state at the exciton energy. Only the phonon modulation process observed. d,e, Transient reflection as a function of delay time and spectral energy for ν = 1 (d) and ν = −1 (e).
Extended Data Fig. 9 Pump–probe spectroscopy at T = 2 K of tMoTe2.
a,b, Transient reflection spectrum as a function of carrier density (n) and probe photon energy E (ħω2) at pump–probe delays of device D1 Δt = 300 ps, with the dashed boxes indicating the integration range for the exciton (black) and trion (blue) shown in Fig. 1g (a) and device D2 Δt = 450 ps, with the dashed boxes indicating the integration range for the exciton (black), trion (blue) and trion derivative (red) shown in Fig. 2d (b); trion D2 1.107–1.114 eV, exciton 1.130–1.122 eV, derivative 1.115–1.106 eV; D1: trion electron doping 1.108–1.114 eV, exciton electron doping 1.120–1.125 eV, exciton hole doping 1.126–1.123 eV, trion hole doping: 1.112–1.117 eV.
Extended Data Fig. 10 Pump fluence dependence.
a, Transient reflection in the exciton spectral region for the ν = 1 state at pump fluences of ρ = 7, 14 and 42 µJ cm−2 and a pump–probe delay time of Δt = 7 ps. The magnitude of ΔR/R scales approximately linearly with ρ. b,c, Time profiles of the ν = 1 state at different fluences on two timescales, 0–75 ps (b) and 75–1,875 ps (c). Each profile is obtained from integration of ΔR/R in a probe photon energy window of 1.119–1.125 eV. All profiles in b and c are normalized to ΔR/R at Δt = 7 ps. The electronic melting/recovery processes are independent of r (b), whereas the relative magnitude of the phonon melting processes increases with pump fluence (c).
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Wang, Y., Choe, J., Anderson, E. et al. Hidden states and dynamics of fractional fillings in twisted MoTe2 bilayers. Nature 641, 1149–1155 (2025). https://doi.org/10.1038/s41586-025-08954-8
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DOI: https://doi.org/10.1038/s41586-025-08954-8
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