Bond-dependent anisotropy and magnon breakdown in cobalt Kitaev triangular antiferromagnet

The Kitaev model, a honeycomb network of spins with bond-dependent anisotropic interactions is a rare example of a quantum spin liquid. Although most Kitaev model candidate materials eventually order magnetically due to inevitable non-Kitaev terms, their bond-dependent anisotropy manifests in unusual spin dynamics. It has recently been suggested that bond-dependent anisotropy can stabilize novel magnetic phases and exotic spin dynamics on the geometrically frustrated triangular lattice. Despite the tremendous interest in the synergy of geometric frustration and bond-dependent anisotropy, no example material has yet been found. Here, we report the existence of a frustrated triangular-lattice with bond-dependent anisotropy in the cobalt Kitaev triangular van der Waals antiferromagnet CoI₂. This magnet exhibits substantial magnon breakdown and complex level repulsion in its momentum and energy-resolved spin dynamics, as measured by inelastic neutron scattering. A thorough examination of excitations in both the paramagnetic and magnetically ordered states revealed that the bond-dependent anisotropy is the origin of spiral order and magnon breakdown found in CoI₂. Our result paves the way toward finding and understanding the Kitaev model with geometrical frustration.