Single to multi quasiparticle excitations in the itinerant helical magnet CeRhIn$_{5}$

CeRhIn$_{5}$ is an itinerant magnet where the Ce$^{3+}$ spins order in a simple helical phase. We investigate the spin excitations and observe sharp spin-waves parameterized by a nearest neighbor exchange $J_{RKKY}$=0.88 $\pm$ 0.05 meV. At higher energies, the spin fluctuations are heavily damped where single quasiparticle excitations are replaced by a momentum and energy broadened continuum constrained by kinematics of energy and momentum conservation. The delicate energy balance between localized and itinerant characters results in the breakdown of the single quasiparticle picture in CeRhIn$_{5}$.
Comment: (main text - 5 pages, 4 figures; supplementary information - 5 pages, 4 figures)