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Observation of the axion quasiparticle in 2D MnBi2Te4

Nature volume 641, pages 62–69 (2025)Cite this article

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Abstract

The axion is a hypothetical fundamental particle that is conjectured to correspond to the coherent oscillation of the θ field in quantum chromodynamics1,2. Its existence would solve multiple fundamental questions, including the strong CP problem of quantum chromodynamics and dark matter, but the axion has never been detected. Electrodynamics of condensed-matter systems can also give rise to a similar θ, so far studied as a static, quantized value to characterize the topology of materials3,4,5. Coherent oscillation of θ in condensed matter has been proposed to lead to physics directly analogous to the high-energy axion particle—the dynamical axion quasiparticle (DAQ)6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23. Here we report the observation of the DAQ in MnBi2Te4. By combining a two-dimensional electronic device with ultrafast pump–probe optics, we observe a coherent oscillation of θ at about 44 gigahertz, which is uniquely induced by its out-of-phase antiferromagnetic magnon. This represents direct evidence for the presence of the DAQ, which in two-dimensional MnBi2Te4 is found to arise from the magnon-induced coherent modulation of the Berry curvature. The DAQ also has implications in light–matter interaction and coherent antiferromagnetic spintronics24, as it might lead to axion polaritons and electric control of ultrafast spin polarization6,15,16,17,18,19,20. Finally, the DAQ could be used to detect axion particles21,22,23. We estimate the detection frequency range and sensitivity in the millielectronvolt regime, which has so far been poorly explored.

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Fig. 1: Axion particle and quasiparticle and basic information of 2D MnBi2Te4.
Fig. 2: Probing the static magnetoelectric coupling in 2D MnBi2Te4.
Fig. 3: Observation of the dynamical axion quasiparticle in 2D MnBi2Te4.
Fig. 4: Ultrafast control of Berry curvature and dark-matter detection sensitivity.

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Data availability

All data that support 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

J.-X.Q., J.A., A.G., H.L., M.S., J.M.B., P.P.O., Q.M., R.M., R.J.M., I.M., A.V. and S.-Y.X. were supported through the Center for the Advancement of Topological Semimetals (CATS), an Energy Frontier Research Center (EFRC) funded by the US Department of Energy (DOE) Office of Science, through the Ames National Laboratory under contract DE-AC0207CH11358. The work in S.-Y.X. group was supported by CATS (task that CATS supported) and the Air Force Office of Scientific Research (AFOSR) grant FA9550-23-1-0040 (data analysis and manuscript writing). S.-Y.X. acknowledges the Sloan Foundation and Corning Fund for Faculty Development. S.-Y.X. and D.B. were supported by the National Science Foundation (NSF; career grant number DMR-2143177). Bulk single-crystal growth and characterization at UCLA were supported by the US DOE, Office of Science, Office of Basic Energy Sciences (BES) under award number DE-SC0021117. J.S.-E. was supported by the National Science Foundation under cooperative agreement 202027 and by the by Japan Science and Technology Agency (JST) as part of Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE), grant number JPMJAP2318. K.W. and T.T. acknowledge support from the JSPS KAKENHI (grant numbers 21H05233 and 23H02052), the CREST (JPMJCR24A5), JST and World Premier International Research Center Initiative (WPI), MEXT, Japan. C.F. was supported by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through SFB 1143 (project ID 24731007) and the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter-ct.qmat (EXC 2147, project number 390858490). H.L. acknowledges the support by Academia Sinica in Taiwan under grant number AS-iMATE-113-15. T.-R.C. was supported by National Science and Technology Council (NSTC) in Taiwan (programme number MOST111-2628-M-006-003-MY3 and NSTC113-2124-M-006-009-MY3), National Cheng Kung University (NCKU), Taiwan, and National Center for Theoretical Sciences, Taiwan. This research was supported, in part, by the Higher Education Sprout Project, Ministry of Education to the Headquarters of University Advancement at NCKU. T.-R.C. thanks the National Center for High-performance Computing (NCHC) of National Applied Research Laboratories (NARLabs) in Taiwan for providing computational and storage resources. The work at Northeastern University (A.B. and B.G.) was supported by the National Science Foundation through the Expand-QISE award NSF-OMA-2329067 and benefited from the resources of Northeastern University’s Advanced Scientific Computation Center, the Discovery Cluster, the Massachusetts Technology Collaborative award MTC-22032, and the Quantum Materials and Sensing Institute. For the computational work at S.N. Bose National Center for Basic Sciences (SNBNCBS), B.G. acknowledge National Supercomputing Mission (NSM) for providing computing resources of ‘PARAM RUDRA’ at SNBNCBS, Saltlake, Kolkata-700106, India, which is implemented by C-DAC and supported by the Ministry of Electronics and Information Technology (MeitY) and Department of Science and Technology (DST), Government of India. J.M.B. and R.M.’s experimental activities were performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement number DMR-2128556 and the State of Florida. D.J.E.M. is supported by an Ernest Rutherford Fellowship (ST/T004037/1) and by a Science and Technologies Facilities Council grant (ST/X000753/1). Q.M. also acknowledges support from National Science Foundation (NSF) CAREER award DMR-2143426 and a Sloan Fellowship. O.L., I.P., E.M.B. and P.N. were supported by the Quantum Science Center (QSC), a National Quantum Information Science Research Center of the US Department of Energy (DOE). O.L., I.P., E.M.B. and P.N. also acknowledge support from Gordon and Betty Moore Foundation grants 8048 and 12976, and from the John Simon Guggenheim Memorial Foundation (Guggenheim Fellowship).

Author information

Authors and Affiliations

  1. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA

    Jian-Xiang Qiu, Yu-Fei Liu, Anyuan Gao, Christian Tzschaschel, Houchen Li, Damien Bérubé, Thao Dinh, Tianye Huang & Su-Yang Xu

  2. Department of Physics, Northeastern University, Boston, MA, USA

    Barun Ghosh, Kin Chung Fong & Arun Bansil

  3. Quantum Materials and Sensing Institute, Northeastern University, Burlington, MA, USA

    Barun Ghosh, Kin Chung Fong & Arun Bansil

  4. Department of Condensed Matter and Materials Physics, S. N. Bose National Centre for Basic Sciences, Kolkata, India

    Barun Ghosh

  5. Department of Physics, University of California, Berkeley, Berkeley, CA, USA

    Jan Schütte-Engel

  6. RIKEN iTHEMS, Wako, Saitama, Japan

    Jan Schütte-Engel

  7. Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA, USA

    Tiema Qian & Ni Ni

  8. Materials Science Division, Argonne National Laboratory, Lemont, IL, USA

    Michael Smith & Ivar Martin

  9. Department of Physics, National Cheng Kung University, Tainan, Taiwan

    Yueh-Ting Yao & Tay-Rong Chang

  10. Department of Physics, Harvard University, Cambridge, MA, USA

    Junyeong Ahn, Yu-Fei Liu, Thao Dinh & Ashvin Vishwanath

  11. Department of Physics, University of Zurich, Zurich, Switzerland

    Christian Tzschaschel

  12. College of Letters and Science, University of California, Los Angeles, Los Angeles, CA, USA

    Ioannis Petrides, Olivia Liebman, Emily M. Been & Prineha Narang

  13. Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA, USA

    Olivia Liebman

  14. National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM, USA

    Joanna M. Blawat & Ross McDonald

  15. National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe & Takashi Taniguchi

  16. Department of Electrical and Computer Engineering, Northeastern University, Boston, MA, USA

    Kin Chung Fong

  17. Institute of Physics, Academia Sinica, Taipei, Taiwan

    Hsin Lin

  18. Department of Physics and Astronomy, Iowa State University, Ames, IA, USA

    Peter P. Orth & Robert J. McQueeney

  19. Ames National Laboratory, Ames, IA, USA

    Peter P. Orth & Robert J. McQueeney

  20. Department of Physics, Saarland University, Saarbrücken, Germany

    Peter P. Orth

  21. Electrical and Computer Engineering Department, University of California, Los Angeles, Los Angeles, CA, USA

    Prineha Narang

  22. Max Planck Institute for Chemical Physics of Solids, Dresden, Germany

    Claudia Felser

  23. Center for Quantum Frontiers of Research and Technology (QFort), Tainan, Taiwan

    Tay-Rong Chang

  24. Physics Division, National Center for Theoretical Sciences, Taipei, Taiwan

    Tay-Rong Chang

  25. Department of Physics, Boston College, Chestnut Hill, MA, USA

    Qiong Ma

  26. The Schiller Institute for Integrated Science and Society, Boston College, Chestnut Hill, MA, USA

    Qiong Ma

  27. Theoretical Particle Physics and Cosmology, King’s College London, London, UK

    David J. E. Marsh

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  33. David J. E. Marsh
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S.-Y.X. conceived the experiments and supervised the project. J.-X.Q. fabricated the devices, performed the measurements and analysed data with help from A.G., C.T., H. Li, Y.-F.L., D.B., T.D., T.H., J.M.B., C.F., Q.M., R.M. and R.J.M. T.Q. and N.N. grew the bulk MnBi2Te4 single crystals. B.G., Y.-T.Y., M.S., J.A., I.P., O.L., E.M.B., P.N., T.-R.C., A.B., H. Lin, P.P.O., I.M. and A.V. conducted the theoretical studies including first-principles calculations and effective modelling. K.W. and T.T. grew the bulk hexagonal BN single crystals. J.S.-E., J.-X.Q., K.C.F., D.J.E.M. and S.-Y.X. made the calculation of sensitivity for dark-matter axion detection. S.-Y.X. and J.-X.Q. wrote the paper with input from all authors.

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Correspondence to Su-Yang Xu.

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Extended data figures and tables

Extended Data Fig. 1 Characterization of the magnon frequency in 6-layer MnBi2Te4.

a, Time evolution of out-of-phase magnon. c,\(\widehat{z}\) projection of the out-of-phase magnon, which resembles the antiferromagnetic amplitude mode. b,d, Same as panels (a,c) but for in-phase magnon, which features an oscillation of net magnetization along \(\widehat{z}\). e, We followed the method established in ref. 53 (see detailed discussion in Methods.2). A monolayer WSe2 was stacked on top of MnBi2Te4, which breaks the layer degeneracy, allowing us to selectively probe the top layer information53. Pump-probe Kerr rotation under normal incidence was performed on this heterostructure. The pump laser launches the magnons. The probe Kerr rotation measures the out-of-plane magnetization Mz of the top layer preferentially because of the WSe2. f,g, Pump-induced ΔKerr data and FFT at different B∥.

Extended Data Fig. 2 Experimental setup for measuring the DAQ.

a, The DAQ manifests as a coherent oscillation of the magnetoelectric coupling, which requires us to measure α(t) with fs time-resolution. This was achieved by combining ultrafast pump-probe optics with 2D electronic devices. We built a dual-gated 6L MnBi2Te4 device (no WSe2). The probe beam combined with the gate-applied Ez measures α, whereas the pump beam excites the magnons. By varying the delay time t, we can measure α(t) with fs time-resolution. Experimentally, this was achieved by connecting two lock-in amplifiers. An optical chopper modulated the pump laser at frequency ω1 = 1000 Hz. A functional generator modulated the gate Ez at frequency ω2 = 0.7 Hz. The signal collected by the balanced photodiode detector was first fed into a lock-in at the chopper frequency ω1 = 1000 Hz and then into the second lock-in at the Ez frequency ω2 = 0.7 Hz. The wavelength of the pump beam was set to 1030 nm, and the pump fluence is ~ 160 μJ/cm2. b, Pump-induced Kerr rotation at different AC E field modulation amplitudes. c, Pump fluence dependence of the oscillation amplitude of Δα. d, Coherent oscillation of Δα as a function of pump light polarization. The indifference of pump light polarization suggests excitation mechanism is laser heating induced coherent oscillation of spins30. e, Coherent oscillation of Δα as a function of pump wavelength.

Extended Data Fig. 3 The magnetoelectric coupling and the layer Hall effect in MnBi2Te4.

a, Schematics for the magnetoelectric coupling (\(\alpha =\frac{{M}_{z}}{{E}_{z}}\)). b, Maintext Fig. 4b shows the measured α as a function of the charge density n. By taking a derivative of this data, we get \(\frac{d\alpha }{dn}\) as a function of n. c, Measured α as a function of in-plane magnetic field B∥. d, Schematics for the layer Hall effect. An out-of-plane electric field Ez induces an anomalous Hall effect (finite σxy) in 6L MnBi2Te4. e,Ez induced σxy as a function of carrier density n. This Ez induced σxy directly measures \(\frac{d{\mathcal{D}}}{dn}\) (\({\mathcal{D}}\) is the Berry curvature real space dipole), which is marked in the right axis. f,Ez induced σxy as a function of B∥.

Extended Data Fig. 4 Determining the conversion factor γ.

a, Out-of-plane magnetization as function of out-of-plane magnetic field B⊥ for bulk MnBi2Te4 measured by SQUID. With increasing B⊥, the magnetic order changes from the layered antiferromagnetic state to a spin-flop state. b, In the spin-flop state at B⊥ = 6 T, we measured both the Kerr rotation and the Mz, from which we determined the value of γ. c, In the antiferromagnetic ground state at B⊥ = 0 T, 6L MnBi2Te4 features an electric field induced magnetization. The magneto-electric coupling α is given by \(\alpha =\gamma \frac{dKerr}{d{E}_{z}}\). Therefore, by using the γ determined in the spin-flop state, we converted α of the antiferromagnetic state to the unit of \(\frac{{e}^{2}}{2h}\). In this method, we needed to assume that the spin flop state at B⊥ = 6 T and the antiferromagnetic state at B⊥ = 0 T have the same γ. This is an approximation.

Extended Data Fig. 5 Microscopic mechanism for the magnetoelectric coupling in 6L MnBi2Te4.

a, First-principles band structures of 6L MnBi2Te4 with the Mn 3d orbitals highlighted. b, Calculated αzz from the spin and orbital contributions. The total αzz is the sum of the two contributions. c, Comparison of θ and Txx, which are the trace part and traceless part of αii, respectively (normalized by e2/2h).

Extended Data Fig. 6 Ultrafast Berry curvature oscillation by antiferromagnetic magnons.

a,d, In the even-layer MnBi2Te4, the antiferromagnetic order couples to the Dirac surface states, generating large Berry curvature on the top and bottom surfaces. We study how the Berry curvature responds upon exciting the out-of-phase or the in-phase magnon. b,e, Calculated Berry curvature sum of the top and bottom surfaces at different spin angles during the magnon oscillation under the frozen magnon approximation. The grey area (i.e., the difference of Berry curvature from top and bottom surfaces) is the Berry curvature real space dipole \({\mathcal{D}}\) (\({\mathcal{D}}=\alpha \)). c,f, Calculated θ at different spin angles during the magnon oscillation.

Extended Data Fig. 7 Calculated DAQ strength of 2D even-layer MnBi2Te4.

The strength of DAQ is measured by the change of θ per change of the antiferromagnetic order parameter L, \(\frac{\delta \theta }{\delta L}\)6. In 2D even-layer MnBi2Te4, the top and bottom surface state wavefunction can overlap and hybridize. This hybridization gap competes with with magnetism induced Zeeman gap, which leads to a large but non-quantized θ. We theoretically study the \(\frac{\delta \theta }{\delta L}\) by calculating θ as a function of L for different thicknesses. a,b, Wavefunction hybridization for 6SL and 14SL. c, Calculated θ vs. AFM order Lz for different thicknesses. d, δθ/δLz as a function of thickness.

Extended Data Fig. 8 Kerr effect scheme of dark matter axion detection using DAQ.

a, The Kerr effect scheme: a dark matter axion resonantly excite an axion polariton inside the DAQ material under an external B∥ field (B∥ = 5 T). The axion polariton is essentially a coherent oscillation of θ(ω), where \(\omega =\sqrt{{m}_{{\rm{DAQ}}}^{2}+{b}^{2}}\). By applying an out-of-plane electric field Ez, such a coherent oscillation of θ(ω) will lead to an oscillating magnetization Mz(ω) = θ(ω)Ez. We propose to use MOKE with to measure this oscillating magnetization. b, Dark matter detection sensitivity (gaγ) as a function of the axion mass using the Kerr scheme (see details in Methods.6).

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Supplementary sections 1–6, including Supplementary Figs. 1–41, Table 1 and References.

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Qiu, JX., Ghosh, B., Schütte-Engel, J. et al. Observation of the axion quasiparticle in 2D MnBi2Te4. Nature 641, 62–69 (2025). https://doi.org/10.1038/s41586-025-08862-x

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