Conventional and planar thermal Hall effect in a Kagome lattice antiferromagnet

The quantum spin liquid (QSL) is an exotic state of matter where spins remain highly entangled and disordered even at zero temperature. The recently discovered Kagome lattice antiferromagnet YCu₃(OH)₆Br₂[Br_x(OH)_1−x] (YCOB) has emerged as a promising candidate. We present a detailed investigation of the thermal conductivity and the thermal Hall effect in single crystals of YCOB. Despite being a large bandgap insulator, YCOB exhibits a positive thermal Hall effect across a wide temperature range, even though the heat carriers in this material are chargeless. Notably, as the temperature drops below ~ 4 K, the thermal Hall effect undergoes a sign reversal. Furthermore, we observed a pronounced planar thermal Hall effect as the external magnetic field was rotated from the out-of-plane direction to the direction aligned with the thermal current. These findings provide deeper insights into the thermal transport properties of the QSL candidate YCOB and point toward potential phonon-mediated chirality in this intriguing Kagome lattice antiferromagnet.