Unusual Phonon Heat Transport in α -RuCl 3 : Strong Spin-Phonon Scattering and Field-Induced Spin Gap

The honeycomb Kitaev-Heisenberg model is a source of a quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. Here we unveil the highly unusual low-temperature heat conductivity $\ensuremath{\kappa}$ of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, a prime candidate for realizing such physics: beyond a magnetic field of ${B}_{c}\ensuremath{\approx}7.5\text{ }\text{ }\mathrm{T}$, $\ensuremath{\kappa}$ increases by about one order of magnitude, 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. In particular, an unexpected, large energy gap arises, which increases linearly with the magnetic field, reaching remarkable $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}/{k}_{B}\ensuremath{\approx}50\text{ }\text{ }\mathrm{K}$ at 18 T.