Instabilities of heavy magnons in an anisotropic magnet

TL;DR

This paper investigates the complex interactions and instabilities of heavy magnons in the anisotropic magnet FeI2, revealing spontaneous decay and bound-state formation driven by quantum interactions and magnetic anisotropy.

Contribution

It demonstrates the existence of strong quantum interactions between multiple quasiparticles and observes novel decay and recombination phenomena in FeI2 using neutron scattering.

Findings

Observation of spontaneous decay between conventional and heavy magnons

Recombination into a super-heavy bound state

Identification of quasi-flat excitation bands and Kitaev interactions

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.