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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 kappa_ab of the Kitaev quantum magnet alpha−RuCl3 for large in-plane magnetic fields up to 33 T. Our data reveal, for fields larger than the critical field Bc = 8 T, at which the magnetic order is suppressed, a dramatic increase of 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 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 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.

thermal conductivity ; Kitaev quantum magnet ; alpha-RuCl3 ; high magnetic fields ; random singlet states

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