High-field thermal transport properties of the Kitaev quantum magnet α-RuCl3: Evidence for low-energy excitations beyond the critical field

We investigate the phononic in-plane longitudinal low-temperature thermal conductivity κab of the Kitaev quantum magnet α-RuCl3 for large in-plane magnetic fields up to 33 T. Our data reveal, for fields larger than the critical field Bc≈8T, at which the magnetic order is suppressed, a dramatic increase of κab at all temperatures investigated. The analysis of our data shows that the phonons are not only strongly scattered by a magnetic mode at relatively large energy which scales roughly linearly with the magnetic field, but also by a small-energy mode which emerges near Bc with a square-root-like field dependence. While the former is in striking agreement with recent spin-wave theory (SWT) results of the magnetic excitation spectrum at the Γ point, the energy of the latter is too small to be compatible with the SWT-expected magnon gap at the M point, despite the matching field dependence. Therefore, an alternative scenario based on phonon scattering off the thermal excitation of random-singlet states is proposed.