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

Honeycomb cobalt oxides containing 3d Co^2+ ions might realize frustrated magnetism and novel quantum phases. Among candidate materials, Na₃Co₂SbO₆ 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 Na₃Co₂SbO₆ subject to in-plane vector fields. We find that the thermal conductivity 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.