Ring-exchange interaction effects on magnons in the Dirac magnet CoTiO3

The magnetic interactions that determine magnetic order and magnon energies typically involve only two spins. While rare, multispin interactions can also appear in quantum magnets and be the driving force in the ground -state selection and in the nature of its excitations. By performing time -domain terahertz and magnetoRaman spectroscopy measurements combined with theoretical modeling, we determine the origin of the magnon excitation gap in Dirac antiferromagnet CoTiO 3 . By adding a ring -exchange interaction in a hexagonal plaquette of the honeycomb lattice to both an XXZ spin model and to a low -energy spin -orbital flavor wave model, a gap is generated in the magnon spectrum at the Brillouin zone center. With this addition, the flavor wave model reproduces a large swath of experimental results including terahertz, Raman, inelastic neutron scattering, and magnetization experiments.