Single to multiparticle 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 in this material using triple-axis neutron spectroscopy and observe sharp spin waves at low energies consistent with previous reports and a nearest neighbour exchange of \textasciitilde 1 meV [1]. At higher energies, the fluctuations are heavily damped where the single-quasiparticle excitations are replaced by a momentum and energy-broadened continuum constrained by kinematics of energy and momentum conservation [2]. The delicate energy balance between localized and itinerant characters results in the breakdown of the single-quasiparticle picture in CeRhIn$_{5}$. [1] P. Das \textit{et al.} Phys. Rev. Lett \textbf{113}, 246403 (2014). [2] C Stock \textit{et al.} Phys. Rev. Lett. \textbf{114}, 247005 (2015). [3] T. Park \textit{et al.} PNAS \textbf{105}, 6825 (2008).