Magnon Pairing, Interactions, and Decay in the Spin-orbital Magnet FeI₂

One of the scientific frontiers in quantum magnetism is the discovery and understanding of quantum entangled and topologically ordered states in real bulk materials. At the focal point of the experimental investigation of these quantum spin networks is the identification of fractionalized excitations in transport and spectroscopic measurements. Inelastic neutron scattering has proved a powerful technique to reveal such signatures in a variety of systems ranging from quasi-1D magnets to kagome compounds and more. Recent and ongoing developments with neutron scattering instrumentation have allowed the characterization of magnetic excitations in entire volumes of momentum-energy space with high resolution. This has prompted revisiting long-overlooked materials in search of exotic spin dynamics despite seemingly classical magnetically ordered ground-states. In this talk, I will discuss such experiments on a long-known material, FeI2, and show how high-fidelity modeling brings new insights into its spin dynamics [1]. I will describe the mechanism that endows low energy quadrupolar fluctuations in FeI2 with large spectral weight and how these can be completely understood using a SU(3) representation of spin degrees of freedom. I will discuss the consequence of having several quasiparticles as the low-energy degrees of freedom in this system including the formation of heavy bound-states [2] and their mutual decay [3].