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 θ field in quantum chromodynamics^1,2. 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 materials^3-5. 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)^6-23. 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 polarization^6,15-20. Finally, the DAQ could be used to detect axion particles^21-23. We estimate the detection frequency range and sensitivity in the millielectronvolt regime, which has so far been poorly explored.