Unconventional magnons in collinear magnets dictated by spin space groups

Magnonic systems provide a fertile playground for bosonic topology1, for example, Dirac2–6 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 group13–17 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 80 per 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. Spin space group theory is applied to identify more than 200 collinear magnets with unconventional magnons; high-throughput calculations with spin–orbit coupling find that most of these unconventional magnets are dictated by the Heisenberg exchange interaction.