Magnon bound states vs. anyonic Majorana excitations in the Kitaev honeycomb magnet $\alpha$-RuCl$_3$

TL;DR

This paper uses Raman spectroscopy to investigate the nature of excitations in the Kitaev honeycomb magnet $ ext{α-RuCl}_3$ under magnetic fields, revealing evidence of bound states of fractionalized excitations distinct from conventional magnons.

Contribution

It provides experimental evidence of bound states of fractionalized excitations in $ ext{α-RuCl}_3$, helping to distinguish between competing theoretical scenarios.

Findings

Detection of low-energy quasiparticles emerging from fractionalized excitations

Observation of temperature-dependent formation of bound states

Contrast between fractionalized excitations and conventional spin-wave modes

Abstract

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as $\alpha$-RuCl$_3$. Currently, two competing scenarios exist for the intermediate field phase of this compound ($B=7-10$ T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.