“Half moons,” distinctive crescent patterns in the dynamical structure factor, have been identified in inelastic neutron scattering experiments for a wide range of frustrated magnets. In an earlier paper [H. Yan et al., Phys. Rev. B 98, 140402(R) (2018)] we have shown how these features are linked to the local constraints realized in classical spin liquids. Here, we explore their implication for the topology of magnon bands. The presence of half moons indicates a separation of magnetic degrees of freedom into irrotational and incompressible components. Where bands satisfying these constraints meet, it is at a singular point encoding Berry curvature of $\pm 2\pi$. Interactions which mix the bands open a gap, resolving the singularity, and leading to bands with finite Berry curvature, accompanied by characteristic changes to half-moon motifs. These results imply that inelastic neutron scattering can, in some cases, be used to make rigorous inference about the topological nature of magnon bands.

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• Magnons
• Chern insulators
• Topological insulators
• Topological materials