Signatures of low-energy fractionalized excitations in $\alpha$-RuCl$_3$ from field-dependent microwave absorption

TL;DR

This study uses microwave absorption to detect low-energy fractionalized excitations in $ ext{RuCl}_3$, revealing a broad continuum indicative of spin fractionalization and its evolution near a quantum phase transition.

Contribution

First direct microwave evidence of fractionalized excitations in $ ext{RuCl}_3$, demonstrating a continuum extending below sharp modes and persisting at high temperatures.

Findings

Detection of a broad continuum characteristic of fractionalization

Continuum persists above magnetic ordering temperature

Development of a nontrivial energy gap in strong magnetic fields

Abstract

Topologically ordered states of matter are generically characterized by excitations with quantum number fractionalization. A prime example is the spin liquid realized in Kitaev's honeycomb-lattice compass model where spin-flip excitations fractionalize into Majorana fermions and Ising gauge fluxes. While numerous compounds have been proposed to be proximate to such a spin-liquid phase, clear-cut evidence for fractionalized excitations is lacking. Here we employ microwave absorption measurements to study the low-energy excitations in $\alpha$-RuCl$_3$ over a wide range of frequencies, magnetic fields, and temperatures, covering in particular the vicinity of the field-driven quantum phase transition where long-range magnetic order disappears. In addition to conventional gapped magnon modes we find a highly unusual broad continuum characteristic of fractionalization which -- most remarkably -- extends to energies below the lowest sharp mode and to temperatures significantly higher than the ordering temperature, and develops a gap of a nontrivial origin in strong magnetic fields. Our results unravel the signatures of fractionalized excitations in $\alpha$-RuCl$_3$ and pave the way to a more complete understanding of the Kitaev spin liquid and its instabilities.