Observation of 4- and 6-magnon bound-states in the spin-anisotropic frustrated antiferromagnet FeI${}_2$

Abstract

Spin-waves e.g. magnons are the conventional elementary excitations of ordered magnets. However, other possibilities exist. For instance, magnon bound-states can arise due to attractive magnon-magnon interactions and drastically impact the static and dynamic properties of materials. Here, we demonstrate a zoo of distinct multi-magnon quasiparticles in the frustrated spin-1 triangular antiferromagnet FeI${}_2$ using time-domain terahertz spectroscopy. The energy-magnetic field excitation spectrum contains signatures of one-, two-, four- and six-magnon bound-states, which we analyze using an exact diagonalization approach for a dilute gas of interacting magnons. The two-magnon single-ion bound states occur due to strong anisotropy and the preponderance of even higher order excitations arises from the tendency of the single-ion bound states to themselves form bound states due to their very flat dispersion. This menagerie of tunable interacting quasiparticles provides a unique platform in a condensed matter setting that is reminiscent of the few-body quantum phenomena central to cold-atom, nuclear, and particle physics experiments.

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• Multimagnon dynamics and thermalization in the $S=1$ easy-axis ferromagnetic chain
• High-angular momentum excitations in collinear antiferromagnet FePS${}_3$
• Chiral phonons induced from spin dynamics via magnetoelastic anisotropy
• Transverse and longitudinal magnons in strongly anisotropic antiferromagnet FePSe3
• Revealing Quadrupolar Excitations with Non-Linear Spectroscopy
• Refined Spin Wave Model and Multi-magnon Bound States in $L{i}_{2}Cu{O}_2$
• Quadrupole moments and their interactions in the triangular lattice antiferromagnet FeI${}_2$
• Excitonic transverse and amplitude fluctuations in the noncollinear and charge-ordered RbFe${}^{2+}$Fe${}^{3+}$F${}_6$
• Longitudinal magnons in large-$S$ easy-axis magnets
• High-accuracy evaluation of non-thermal magnetic states beyond spin-wave theory: applications to higher-energy states
• Spin amplitude wave due to dipole-quadrupole hybridization in spin-1 pyrochlore magnets