Phonon thermal transport shaped by strong spin-phonon scattering in a Kitaev material Na$_2$Co$_2$TeO$_6$

TL;DR

This study investigates low-temperature thermal conductivity in Na$_2$Co$_2$TeO$_6$, revealing phonon-dominated heat transport heavily influenced by magnetic fluctuations, and challenges the presence of itinerant magnetic excitations in this Kitaev candidate.

Contribution

It provides direct experimental evidence that phonons, not magnetic quasiparticles, dominate thermal transport in Na$_2$Co$_2$TeO$_6$, clarifying the nature of excitations in this Kitaev material.

Findings

Severely scattered phonon transport across the phase diagram.

No evidence of mobile quasiparticles other than phonons.

Magnetic fluctuations cause cascades of phase transitions.

Abstract

The recent report of a half-quantized thermal Hall effect in the Kitaev material $\alpha$-RuCl$_3$ has sparked a strong debate on whether it is generated by Majorana fermion edge currents or whether other more conventional mechanisms involving magnons or phonons are at its origin. A more direct evidence for Majorana fermions which could be expected to arise from a contribution to the longitudinal heat conductivity $\kappa_{xx}$ at $T\rightarrow0$ is elusive due to a very complex magnetic field dependence of $\kappa_{xx}$. Here, we report very low temperature (below 1~K) thermal conductivity ($\kappa$) of another candidate Kitaev material, Na$_2$Co$_2$TeO$_6$. The application of a magnetic field along different principal axes of the crystal reveals a strong directional-dependent magnetic-field ($\bf B$) impact on $\kappa$. We show that no evidence for mobile quasiparticles except phonons can be concluded at any field from 0~T to the field polarized state. In particular, severely scattered phonon transport is observed across the $B-T$ phase diagram, which is attributed to prominent magnetic fluctuations. Cascades of phase transitions are uncovered for all $\bf B$ directions by probing the strength of magnetic fluctuations via a precise record of $\kappa$($B$). Our results thus rule out recent proposals for itinerant magnetic excitations in Na$_2$Co$_2$TeO$_6$, and emphasise the importance of discriminating true spin liquid transport properties from scattered phonons in candidate materials.