A topological bosonic thermal Hall effect in Kitaev magnet α-RuCl₃

α-RuCl₃ is the most well-studied material candidate for realizing Kitaev’s honeycomb model. However, unlike the idealized model, the system has a complex magnetic phase diagram where a quantum spin liquid phase is thought to be realized at intermediate magnetic fields (7 < B < 12 T). Its thermal transport physics is similarly rich with various anomalous behaviors appearing in both the longitudinal (κ_xx) and Hall (κ_xy) conductivity channels. Our experiments have revealed magnetooscillations in κ_xx that are strikingly reminiscent of Shubnikov-de Haas oscillations in metals. This is highly unexpected as α-RuCl₃ is strongly electrically insulating. We also observe a strongly temperature-dependent planar thermal Hall conductivity (κ_xy/T). The latter effect is well-described by topological bosonic excitations inherent to the high field spin-polarized phase rather than the Majorana fermions suggested by other groups. We quantitatively demonstrate that the excitations responsible for this planar thermal Hall effect are the same excitations studied in earlier spectroscopic measurements. The relation between this observation and the low temperature κ_xx oscillations and alleged half-integer quantized κ_xy/T will be discussed.