Field evolution of magnons in $\alpha$-RuCl$_3$ by high-resolution polarized terahertz spectroscopy

TL;DR

This study uses high-resolution terahertz spectroscopy to investigate magnon excitations in $ ext{α-RuCl}_3$, revealing field-dependent spectral features consistent with spin wave theory and lattice distortions.

Contribution

It provides a detailed characterization of magnon spectra in $ ext{α-RuCl}_3$ under magnetic fields, linking experimental observations with spin wave theory considering lattice distortions.

Findings

Discontinuous changes in magnon resonances at critical field.

Two sharp magnon peaks and broader higher-energy peaks observed.

Linear spin wave theory explains spectral features with lattice distortions.

Abstract

The Kitaev quantum spin liquid (KSL) is a theoretically predicted state of matter whose fractionalized quasiparticles are distinct from bosonic magnons, the fundamental excitation in ordered magnets. The layered honeycomb antiferromagnet $\alpha$-RuCl$_3$ is a KSL candidate material, as it can be driven to a magnetically disordered phase by application of an in-plane magnetic field, with $H_c \sim 7$ T. Here we report a detailed characterization of the magnetic excitation spectrum of this material by high-resolution time-domain terahertz (THz) spectroscopy. We observe two sharp magnon resonances whose frequencies and amplitudes exhibit a discontinuity as a function of applied magnetic field, as well as two broader peaks at higher energy. Below the N\'eel temperature, we find that linear spin wave theory can account for all of these essential features of the spectra when a $C_3$-breaking distortion of the honeycomb lattice and the presence of structural domains are taken into account.