Planar thermal Hall effect of topological bosons in the Kitaev magnet alpha-RuCl3

The honeycomb magnet alpha-RuCl3 has attracted considerable interest because it is proximate to the Kitaev Hamiltonian whose excitations are Majoranas and vortices. The thermal Hall conductivity k(xy) of Majorana fermions is predicted to be half-quantized. Half-quantization of k(xy)/T (T, temperature) was recently reported, but this observation has proven difficult to reproduce. Here, we report detailed measurements of k(xy) on alpha-RuCl3 with the magnetic field B parallel to a (zigzag axis). In our experiment, k(xy)/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 similar to 1. The bosonic character is incompatible with half-quantization of k(xy)/T.