Planar thermal Hall effect of topological bosons in the Kitaev magnet α-RuCl 3

The honeycomb magnet α-RuCl 3 has attracted considerable interest because it is proximate to the Kitaev Hamiltonian whose excitations are Majoranas and vortices. The thermal Hall conductivity κ x y of Majorana fermions is predicted to be half-quantized. Half-quantization of κ x y / T ( T , temperature) was recently reported, but this observation has proven difficult to reproduce. Here, we report detailed measurements of κ x y on α-RuCl 3 with the magnetic field B ∥ a (zigzag axis). In our experiment, κ x y / T is observed to be strongly temperature dependent between 0.5 and 10 K. We show that its temperature profile matches the distinct form expected for topological bosonic modes in a Chern-insulator-like model. Our analysis yields magnon band energies in agreement with spectroscopic experiments. At high B , the spin excitations evolve into magnon-like modes with a Chern number of ~1. The bosonic character is incompatible with half-quantization of κ x y / T .