Large field-induced gap of Kitaev-Heisenberg paramagnons in $\hat{I}\pm$-RuCl${}_3$

Abstract

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, $\hat{I}\pm$-RuCl${}_3$ recently emerged as a prime candidate. Here we unveil highly unusual low-temperature heat conductivity $\hat{I}º$ of $\hat{I}\pm$-RuCl${}_3$: beyond a magnetic field of $B_c\approx$ 7.5 T, $\hat{I}º$ 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 $\hslash \ddot{I}{‰}_0/k_B\approx$ 50 K at 18 T.

9 pages, 4 figures, supplementary information included