Phonon Heat Transport and Anisotropic Tuning of Quantum Fluctuations in a Frustrated Honeycomb Magnet

TL;DR

This study investigates how magnetic excitations and quantum criticality influence phonon-mediated heat transport in a frustrated honeycomb magnet, revealing anisotropic effects and magnetic fluctuation dynamics at ultra-low temperatures.

Contribution

It provides the first detailed thermal transport measurements under in-plane vector fields in Na$_{3}$Co$_{2}$SbO$_{6}$, highlighting the role of magnetic fluctuations in phonon scattering and quantum critical behavior.

Findings

Thermal conductivity is limited by phonon scattering off magnetic excitations.

Magnetic fluctuations increase near quantum critical points, suppressing heat transport.

Heat transport depends on the magnetic field direction, indicating anisotropic quantum fluctuations.

Abstract

Honeycomb cobalt oxides containing 3$\it{d}$ Co$^{2+}$ ions might realize frustrated magnetism and novel quantum phases. Among candidate materials, Na$_{3}$Co$_{2}$SbO$_{6}$ stands out for its distorted honeycomb lattice and significant in-plane anisotropy, motivating vector-field tuning inside the honeycomb plane. Here we use thermal transport down to the mK regime to study twin-free crystals of Na$_{3}$Co$_{2}$SbO$_{6}$ subject to in-plane vector fields. We find that the thermal conductivity $\kappa$ never exceeds the heat-transport capability of phonons, rendering its suppression primarily due to phonon scattering off magnetic excitations and/or domain boundaries. The system's field-driven quantum criticality manifests itself as an abundance of magnetic fluctuations hindering the heat transport, which further depends on the field direction in an intriguing manner.