Two-step gap opening across the quantum critical point in a Kitaev honeycomb magnet

TL;DR

This study investigates the evolution of low-lying spin excitations in a Kitaev honeycomb magnet near a quantum critical point, revealing two distinct gaps and their hybridization under magnetic fields through resonance measurements.

Contribution

It provides experimental evidence of two spin gaps and their anti-crossing behavior, advancing understanding of fractionalized quasiparticles in Kitaev materials.

Findings

Identification of two low-lying spin gaps at high magnetic fields.

Observation of anti-crossing behavior indicating hybridization.

Detection of critical behavior of excitations near the quantum critical point.

Abstract

Quantum spin liquid involves fractionalized quasipariticles such as spinons and visons. They are expressed as itinerant Majorana fermions and $Z_2$ 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 $\alpha$-RuCl$_3$, 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 $\sim 0.4$ 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.