Quantum spin liquid involves fractionalized quasipariticles such as spinons and visons. They are expressed as itinerant Majorana fermions and Z2 fluxes in the Kitaev model with bond-dependent exchange interactions on a honeycomb spin lattice. The observation has recently attracted attention for a candidate material α-RuCl3, showing spin liquid behaviour induced by a magnetic field. Since the observable spin excitation is inherently composed of the two quasiparticles, which further admix each other by setting in the magnetic field as well as non-Kitaev interactions, their individual identification remains challenging. Here we report an emergent low-lying spin excitation through nuclear magnetic and quadrupole resonance measurements down to ∼0.4 K corresponding to 1/500 of the exchange energy under the finely tuned magnetic field across the quantum critical point. We determined the critical behaviour of low-lying excitations and found evolution of two kinds of the spin gap at high fields. The two excitations exhibit repulsive magnetic field dependence, suggesting anti-crossing due to the hybridization between fractionalized quasiparticles.
@article{arxiv.1810.05379,
title = {Two-step gap opening across the quantum critical point in a Kitaev honeycomb magnet},
author = {Y. Nagai and T. Jinno and Y. Yoshitake and J. Nasu and Y. Motome and M. Itoh and Y. Shimizu},
journal= {arXiv preprint arXiv:1810.05379},
year = {2020}
}