High-field thermal transport properties of the Kitaev quantum magnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi><mml:mtext>−</mml:mtext><mml:mi>Ru</mml:mi><mml:msub><mml:mi>Cl</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>: Evidence for low-energy excitations beyond the critical field
Richard Hentrich, Xiaochen Hong, Matthias Gillig, Federico Caglieris, Matija Čulo, M. Shahrokhvand, U. Zeitler, Maria Roslova, Anna Isaeva, Thomas Doert, Lukas Janssen, Matthias Vojta, B. Büchner, C. Heß
Abstract
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.