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 ${\ensuremath{\kappa}}_{ab}$ of the Kitaev quantum magnet $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ for large in-plane magnetic fields up to 33 T. Our data reveal, for fields larger than the critical field ${B}_{c}\ensuremath{\approx}8\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, at which the magnetic order is suppressed, a dramatic increase of ${\ensuremath{\kappa}}_{ab}$ 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 ${B}_{c}$ 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 $\mathrm{\ensuremath{\Gamma}}$ 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.