NMR observation of field induced fractionalized spin excitations in the Kitaev spin liquid candidate

Quantum spin liquid involves fractionalized quasipariticles such as spinons and visons respectively corresponding to itinerant Majorana fermions and Z₂ gauge fluxes in an exactcly soluble Kitaev model with frustrated bond-dependent exchange interactions on a honeycomb spin lattice. The observation has recently attracted attention for a candidate of the Kitaev model, α-RuCl₃, showing spin liquid behaviour under magnetic field. Owing to the entanglement of these quasiparticles in magnetic observables, the identification remains challenging. Here we report nuclear magnetic and quadrupole resonance measurements down to 0.4 K [1]. Through the dynamical spin susceptibility in a low-energy limit, we found low-lying gapped excitations in two energy scales across the critical field where the magnetic order vanishes. The higher energy branch grows with a cubic field dependence, as expected for itinerant Majorana fermions, whereas the lower branch in an energy scale of the flux condensation appears above the critical field. The result suggests that the field induced mixing and redistribution of fractionalized excitations.

[1] Y. Nagai, T. Jinno, J. Yoshitake, J. Nasu, Y. Motome, M. Itoh, Y. Shimizu, preprint http://arxiv.org/abs/1810.05379.