近日，哈佛大学徐苏杨团队研究了二维MnBi₂Te₄中轴子准粒子的观测。这一研究成果发表在2025年4月16日出版的《自然》杂志上。

轴子是一种假设的基本粒子，被推测对应于量子色动力学中θ场的相干振荡。其存在将解决多个基本问题，包括量子色动力学和暗物质的强CP问题，但轴子从未被探测到。凝聚态系统的电动力学也可以产生类似的θ，到目前为止，θ被研究为一个静态的量化值，用于表征材料的拓扑结构。凝聚态中θ的相干振荡已被提出，导致物理学直接类似于高能轴子粒子——动态轴子准粒子（DAQ）。

研究组报告了MnBi₂Te₄中DAQ的观察结果。通过将二维电子器件与超快泵浦-探测光学器件相结合，观察到θ在约44千兆赫处的相干振荡，这是由其异相反铁磁振子引起的。这代表了DAQ存在的直接证据，在二维MnBi₂Te₄中，DAQ是由Berry曲率的磁振子诱导相干调制引起的。DAQ在光-物质相互作用和相干反铁磁自旋电子学中也有意义，因为它可能导致轴子极化子和超快自旋极化的电控。最后，DAQ可用于检测轴子粒子。他们估计了毫电子伏特范围内的检测频率范围和灵敏度，到目前为止，这方面的研究还很少。

附：英文原文

Title: Observation of the axion quasiparticle in 2D MnBi2Te4

Author: Qiu, Jian-Xiang, Ghosh, Barun, Schtte-Engel, Jan, Qian, Tiema, Smith, Michael, Yao, Yueh-Ting, Ahn, Junyeong, Liu, Yu-Fei, Gao, Anyuan, Tzschaschel, Christian, Li, Houchen, Petrides, Ioannis, Brub, Damien, Dinh, Thao, Huang, Tianye, Liebman, Olivia, Been, Emily M., Blawat, Joanna M., Watanabe, Kenji, Taniguchi, Takashi, Fong, Kin Chung, Lin, Hsin, Orth, Peter P., Narang, Prineha, Felser, Claudia, Chang, Tay-Rong, McDonald, Ross, McQueeney, Robert J., Bansil, Arun, Martin, Ivar, Ni, Ni, Ma, Qiong, Marsh, David J. E., Vishwanath, Ashvin, Xu, Su-Yang

Issue&Volume: 2025-04-16

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 44gigahertz, 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.

DOI: 10.1038/s41586-025-08862-x

Source: https://www.nature.com/articles/s41586-025-08862-x