Planar thermal Hall effects in the Kitaev spin liquid candidate Na2Co2TeO6

Abstract

We investigate both the longitudinal thermal conductivity (κxx) and the planar thermal Hall conductivity (κxy) in the Kitaev spin liquid candidate of the cobalt-based honeycomb antiferromagnet Na2Co2TeO6 in a magnetic field (B) applied along the a- and a∗-axes. A finite κxy is resolved for both field directions in the antiferromagnetic (AFM) phase below the Néel temperature of 27 K. The temperature dependence of κxy/T shows the emergence of topological bosonic excitations. In addition, the field dependence of κxy 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 κxy is observed in Ba∗ 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.