近日，南方科技大学刘奇航团队研究了由自旋空间群决定的共线磁体中的非常规磁振子。2025年3月12日，《自然》杂志发表了这一成果。

磁振子系统为玻色子拓扑提供了一个肥沃的游乐场，例如狄拉克和外尔磁振子，导致了各种奇特的现象，如无电荷拓扑保护的边界模、磁振子热霍尔效应和磁振子自旋能斯特效应。然而，由于缺乏主要由各向同性海森堡相互作用控制的磁几何和自旋哈密顿量的基本对称性描述，其理解受到了阻碍。随后的磁振子色散使无间隙磁振子带节点超越了磁空间群的表示理论，因此被称为非常规磁振子。

研究组发展了自旋空间群理论来阐明共线磁结构，将1421个共线自旋空间群分为4种类型，构建了它们的能带表示，并提供了非常规磁振子的综合表格，如十二倍点、八重节点线和电荷-4八重点。基于MAGNDATA数据库，研究组识别出498个具有非常规磁振子的共线磁体，其中200多个磁振子带结构是通过第一性原理计算和线性自旋波理论获得的。

此外，研究组评估了这些磁体中自旋轨道耦合诱导的交换相互作用的影响，发现超过80 百分比主要由海森堡相互作用决定，这表明自旋空间群是描述大多数3d、4d和半填充4f共线磁体中磁振子带节点的理想框架。

附：英文原文

Title: Unconventional magnons in collinear magnets dictated by spin space groups

Author: Chen, Xiaobing, Liu, Yuntian, Liu, Pengfei, Yu, Yutong, Ren, Jun, Li, Jiayu, Zhang, Ao, Liu, Qihang

Issue&Volume: 2025-03-12

Abstract: Magnonic systems provide a fertile playground for bosonic topology1, for example, Dirac2,3,4,5,6 and Weyl7,8 magnons, leading to a variety of exotic phenomena such as charge-free topologically protected boundary modes6,7, the magnon thermal Hall effect9 and the magnon spin Nernst effect10. However, their understanding has been hindered by the absence of fundamental symmetry descriptions of magnetic geometries and spin Hamiltonians primarily governed by isotropic Heisenberg interactions. The ensuing magnon dispersions enable gapless magnon band nodes that go beyond the scenario of representation theory of the magnetic space groups11,12, thus referred to as unconventional magnons. Here we developed spin space group13,14,15,16,17 theory to elucidate collinear magnetic configurations, classifying the 1,421 collinear spin space groups into 4 types, constructing their band representations and providing a comprehensive tabulation of unconventional magnons, such as duodecuple points, octuple nodal lines and charge-4 octuple points. On the basis of the MAGNDATA database18, we identified 498 collinear magnets with unconventional magnons, among which more than 200 magnon band structures were obtained by using first-principles calculations and linear spin wave theory. In addition, we evaluated the influence of the spin–orbit-coupling-induced exchange interaction in these magnets and found that more than 80per cent are predominantly governed by the Heisenberg interactions, indicating that the spin space group serves as an ideal framework for describing magnon band nodes in most 3d, 4d and half-filled 4f collinear magnets.

DOI: 10.1038/s41586-025-08715-7

Source: https://www.nature.com/articles/s41586-025-08715-7