Observation of two types of fractional excitation in the Kitaev honeycomb magnet

Quantum spin liquid is a disordered but highly entangled magnetic state with fractional spin excitations. The ground state of an exactly solved Kitaev honeycomb model is perhaps its clearest example. Under a magnetic field, a spin flip in this model fractionalizes into two types of anyon, a quasiparticle with more complex exchange statistics than standard fermions or bosons: a pair of gauge fluxes and a Majorana fermion. Here, we demonstrate this kind of fractionalization in the Kitaev paramagnetic state of the honeycomb magnet α-RuCl₃. The spin excitation gap determined by nuclear magnetic resonance consists of the predicted Majorana fermion contribution following the cube of the applied magnetic field, and a finite zero-field contribution matching the predicted size of the gauge flux gap. The observed fractionalization into gapped anyons survives in a broad range of temperatures and magnetic fields, which establishes α-RuCl₃ as a unique platform for future investigations of anyons.