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

## TL;DR

This study reveals a large, field-induced energy gap in the magnetic excitations of $ ext{α-RuCl}_3$, significantly affecting its heat conductivity and indicating complex interactions in the Kitaev-Heisenberg model.

## Contribution

It demonstrates the emergence of a large, linearly increasing energy gap in magnetic excitations of $ ext{α-RuCl}_3$ under magnetic fields, providing new insights into its quantum spin liquid behavior.

## Key findings

- Heat conductivity increases by an order of magnitude beyond 7.5 T.
- A large magnetic field-dependent peak in heat conductivity at 7 K.
- An energy gap reaching approximately 50 K at 18 T.

## 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, $\alpha$-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.