Qihang Liu
Magnonic systems provide a fertile playground for bosonic topology, leading to plenty of exotic phenomena such as charge-free topologically protected boundary modes, magnon thermal Hall effect and magnon spin Nernst effect. However, their comprehension 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 groups (MSGs), thus referred to as unconventional magnons. In this talk, we introduce the recent developments of spin group theory, including extensive enumeration of over 100000 spin space groups, identification of spin groups for collinear, coplanar, and noncoplanar configurations, and irreducible co-representations in momentum space leading to more energy degeneracies that are disallowed by magnetic groups [1-3]. We then apply spin space group theory to collinear magnetic configurations [4], providing a comprehensive tabulation of unconventional magnons, such as duodecuple points, octuple nodal lines, and charge-4 octuple points. Based on the MAGNDATA database, we identified 498 collinear magnets with unconventional magnons, among which over 200 magnon band structures were obtained by using first-principles calculations and linear spin wave theory. Additionally, we evaluated the influence of the spin-orbit coupling-induced exchange interaction in these magnets and found that more than 80% are predominantly governed by the Heisenberg interactions, indicating that SSG serves as an ideal framework for describing magnon band nodes in most 3d, 4d and half-filled 4f collinear magnets.
References:
[1] Liu et al. Phys. Rev. X 12, 021016 (2022).
[2] Chen et al. Phys. Rev. X 14, 0031038 (2024).
[3] Zhu et al. Nature 626, 523 (2024).
[4] Chen et al. arXiv:2307.12366 (2023) Nature in press.