Anisotropic in-plane heat transport of Kitaev magnet Na2Co2TeO6

We report a study on low -temperature heat transport of Kitaev magnet Na2Co2TeO6, with the heat current and magnetic fields along the honeycomb spin layer (the ab plane). The zero -field thermal conductivity of kappa xax and kappa a* xx display similar temperature dependence and small difference in their magnitudes, whereas their magnetic field (parallel to the heat current) dependence are quite different and are related to the field -induced magnetic transitions. The kappa xax(B) data for BIla at very low temperatures have an anomaly at 10.25-10.5 T, which reveals an unexplored magnetic transition. The planar thermal Hall conductivity kappa xa y and kappa xa*yshow very weak signals at low fields and rather large values with sign change at high fields. This may point to a possible magnetic structure transition or the change of the magnon band topology that induces a radical change of magnon Berry curvature distribution before entering the spin polarized state. These results put clear constraints on the high -field phase and the theoretical models for Na2Co2TeO6.