Planar Thermal Hall Effects in Kitaev Spin Liquid Candidate Na2Co2TeO6

We investigate both the longitudinal thermal conductivity (\(\kappa_{xx}\)) and the planar thermal Hall conductivity (\(\kappa_{xy}\)) in the Kitaev spin liquid candidate of Co-based honeycomb antiferromagnet Na\(_2\)Co\(_2\)TeO\(_6\) in a magnetic field (\(B\)) applied along the \(a\) and \(a^*\) axes. A finite \(\kappa_{xy}\) is resolved for both field directions in the antiferromagnetic (AFM) phase below the Néel temperature of 27 K. The temperature dependence of \(\kappa_{xy}/T\) shows the emergence of topological bosonic excitations. In addition, the field dependence of \(\kappa_{xy}\) 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 \(\kappa_{xy}\) is observed in \(B \parallel a^*\) between the first-order transition field in the AFM phase and the saturation field, 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.