High-field thermal transport properties of the Kitaev quantum magnet alpha-RuCl3: evidence for low-energy excitations beyond the critical field

TL;DR

This study explores how high magnetic fields affect thermal conductivity in alpha-RuCl3, revealing low-energy excitations beyond the critical field that suggest complex magnetic phenomena and potential quantum spin liquid behavior.

Contribution

It provides experimental evidence of low-energy magnetic excitations in alpha-RuCl3 beyond the critical field, challenging existing spin wave theory predictions and proposing alternative scattering mechanisms.

Findings

Thermal conductivity increases dramatically above the critical field.

Identification of a small-energy mode with square-root field dependence.

Evidence for magnetic excitations beyond conventional magnon models.

Abstract

We investigate the phononic in-plane longitudinal low-temperature thermal conductivity kappa_ab of the Kitaev quantum magnet alpha-RuCl3 for large in-plane magnetic fields up to 33 T. Our data reveal for fields larger than the critical field Bc ~ 8 T, at which the magnetic order is suppressed, a dramatic increase of kappa_ab at all temperatures investigated. The analysis of our data shows that the phonons are not only strongly scattered by a magnetic mode at relatively large energy which scales roughly linearly with the magnetic field, but also by a small-energy mode which emerges near Bc with a square-root-like field dependence. While the former is in striking agreement with recent spin wave theory (SWT) results of the magnetic excitation spectrum at the Gamma point, the energy of the latter is too small to be compatible with the SWT-expected magnon gap at the M point, despite the matching field dependence. Therefore, an alternative scenario based on phonon scattering off the thermal excitation of random-singlet states is proposed.