# Breakdown of Magnons in a Strongly Spin-Orbital Coupled Magnet

## TL;DR

This paper investigates the broad excitation continuum in strongly spin-orbital coupled magnets like $ ext{RuCl}_3$, proposing it arises from magnetic anharmonicity rather than fractional excitations, thus revealing complex nontrivial magnetic states.

## Contribution

It introduces a new explanation for the excitation continuum in $ ext{RuCl}_3$, emphasizing magnetic anharmonicity over fractionalization, and broadens understanding of excitations beyond the Kitaev spin liquid.

## Key findings

- The continuum is explained by incoherent excitations from magnetic anharmonicity.
- The scenario accounts for the inelastic magnetic response of $ ext{RuCl}_3$.
- Nontrivial excitations extend beyond the Kitaev state.

## Abstract

The description of quantized collective excitations stands as a landmark in the quantum theory of condensed matter. A prominent example occurs in conventional magnets, which support bosonic magnons - quantized harmonic fluctuations of the ordered spins. In striking contrast is the recent discovery that strongly spin-orbital coupled magnets, such as $\alpha$-RuCl$_3$, may display a broad excitation continuum inconsistent with conventional magnons. Due to incomplete knowledge of the underlying interactions unraveling the nature of this continuum remains challenging. The most discussed explanation refers to a coherent continuum of fractional excitations analogous to the celebrated Kitaev spin liquid. Here we present a more general scenario. We propose that the observed continuum represents incoherent excitations originating from strong magnetic anharmoniticity that naturally occurs in such materials. This scenario fully explains the observed inelastic magnetic response of $\alpha$-RuCl$_3$ and reveals the presence of nontrivial excitations in such materials extending well beyond the Kitaev state.