Instabilities of heavy magnons in an anisotropic magnet

Abstract

The search for new elementary particles is one of the most basic pursuits in physics, spanning from subatomic physics to quantum materials. Magnons are the ubiquitous elementary quasiparticle to describe the excitations of fully-ordered magnetic systems. But other possibilities exist, including fractional and multipolar excitations. Here, we demonstrate that strong quantum interactions exist between three flavors of elementary quasiparticles in the uniaxial spin-one magnet FeI2. Using neutron scattering in an applied magnetic field, we observe spontaneous decay between conventional and heavy magnons and the recombination of these quasiparticles into a super-heavy bound-state. Akin to other contemporary problems in quantum materials, the microscopic origin for new physics in FeI2 is the quasi-flat nature of excitation bands and the presence of Kitaev anisotropic magnetic exchange interactions.

31 pages, 3 main figures

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• Revealing Quadrupolar Excitations with Non-Linear Spectroscopy
• Noncentrosymmetric Triangular Magnet CaMnTeO${}_6$: Strong Quantum Fluctuations and Role of s0 vs. s2 Electronic States in Competing Exchange Interactions
• Formulation of spin Nernst effect for spin-nonconserving insulating magnets
• High-accuracy evaluation of non-thermal magnetic states beyond spin-wave theory: applications to higher-energy states
• Longitudinal magnons in large-$S$ easy-axis magnets
• Atomic imaging of 2D transition metal dihalides
• Tunable multi-magnon Floquet topological edge states