Breakdown of linear spin-wave theory and existence of spinon bound states in the frustrated kagome-lattice antiferromagnet

The spin dynamics of the spin-1/2 kagome-lattice antiferromagnet Cs2Cu3SnF12 is studied using high-resolution, time-of-flight inelastic neutron scattering. The flat mode, a characteristic of the frustrated kagome antiferromagnet, and the low-energy dispersive mode, which is dominated by magnons, can be well described by the linear spin-wave theory. However, the theory fails to describe three weakly dispersive modes between 9 and 14 meV. These modes could be attributed to two-spinon bound states, which decay into free spinons away from the zone center and at a high temperature, giving rise to continuum scattering.