Observation of the axion quasiparticle in 2D MnBi₂Te₄

The axion is a hypothetical fundamental particle that is conjectured to correspond to the coherent oscillation of the theta 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 theta, so far studied as a static, quantized value to characterize the topology of materials3, 4-5. Coherent oscillation of theta 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 theta 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.