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

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. In particular, an unexpectedly large energy gap arises, which increases approximately linearly with the magnetic field and reaches a remarkably large \(\hbar\omega_0/k_B\approx \) 50 K at 18 T.