  Spin dynamics of the spin-1/2 kagome lattice antiferromagnet
Cs$_2$Cu$_3$SnF$_{12}$ was 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.
