*

Spin waves (e.g. magnons) are the conventional elementary excitations of a magnetically ordered phase. However, other possibilities exist. For instance, magnon bound states can arise at low energy due to attractive magnon-magnon interactions and modify the low-temperature properties of the system. The most common case corresponds to two-magnon bound states (n = 2) in quasi-one-dimensional quantum magnets [1]. Here we study the frustrated spin-1 triangular antiferromagnetic lattice FeI₂ [2] using time-domain terahertz spectroscopy. The spectra reveal a zoo of distinct multi-magnon quasiparticles at low energies, including up to six-magnon bound states, along with interactions between them. The energy-magnetic field excitation spectrum provides valuable information on the Hamiltonian of this peculiar material and is well reproduced by exact diagonalization calculations of a low-energy Hamiltonian for a dilute gas of interacting quasiparticles. The existence of these high order magnon bound states provides a novel platform to study multiparticle interactions and decays in a condensed matter setting.

[1] Torrance and Tinkham, Phys. Rev. 187, 595 (1969)
[2] Bai et al., arXiv:2004.05623 (2020)