Large field-induced gap of Kitaev-Heisenberg paramagnons in $\alpha$-RuCl$_{3}$

Understand

The honeycomb Kitaev-Heisenberg model is a source of a quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles.

• In the quest of finding a pertinent material, $\alpha$-RuCl$_{3}$ recently emerged as a prime candidate.
• Here we unveil highly unusual low-temperature heat conductivity $\kappa$ of $\alpha$-RuCl$_{3}$: beyond a magnetic field of $B_c\approx$ 7.5 T, $\kappa$ increases by about one order of magnitude, resulting in a large magnetic field dependent peak at about 7 K, both for in-plane as well as out-of-plane transport.
• This clarifies the unusual magnetic field dependence unambiguously to be the result of severe scattering of phonons off putative Kitaev-Heisenberg excitations in combination with a drastic field-induced change of the magnetic excitation spectrum.