Planar Thermal Hall Effects in Kitaev Spin Liquid Candidate Na₂Co₂TeO₆

The Co-based honeycomb antiferromagnet Na₂Co₂TeO₆ has attracted attention as a new Kitaev quantum spin liquid (KQSL) candidate, because it shows a field-induced phase transition from an antiferromagnetic to a spin disordered phase similar to the major candidate of KQSL, α-RuCl₃. In this presentation, we present the experimental results of our thermal transport measurements on Na₂Co₂TeO₆. A finite thermal Hall conductivity is resolved in the antiferromagnetic phase below the Néel temperature of 27 K under the magnetic fields applied in the ab plane. The temperature dependence of this planar Hall conductivity suggests the emergence of topological bosonic excitations. The field dependence of the Hall conductivity shows sign reversals at the critical fields in the AFM phase, suggesting the changes in the Chern number distribution of the topological magnons. Remarkably, a finite Hall conductivity is observed in the field applied along the a* axis, which is prohibited in a disordered state by the two-fold rotation symmetry around the a* axis of the honeycomb lattice, showing the presence of a magnetically ordered state that breaks the two-fold rotation symmetry. Our results demonstrate the presence of topological magnons in this compound in the whole field range below the saturation field.