  The extraordinary properties of the Kitaev model have motivated an intense
search for new physics in materials that combine geometrical and bond
frustration. In this work, we employ inelastic neutron scattering, spin wave
theory, and exact diagonalization to study the spin dynamics in the perfect
triangular-lattice antiferromagnet (TLAF) CsCeSe$_2$. This material orders into
a stripe phase, which is demonstrated to arise as a consequence of the
off-diagonal bond-dependent terms in the spin Hamiltonian. By studying the spin
dynamics at intermediate fields, we identify an interaction between the
single-magnon state and the two-magnon continuum that causes decay of coherent
magnon excitations, level repulsion, and transfer of spectral weight to the
continuum that are controlled by the strength of the magnetic field. Our
results provide a microscopic mechanism for the stabilization of the stripe
phase in TLAF and show how complex many-body physics can be present in the spin
dynamics in a magnet with strong Kitaev coupling even in an ordered ground
state.
