Significant Phonon Chirality Activated by Crystalline Electric Field Excitations in KNdSe_{2}

Chiral phonons, lattice vibrations carrying finite angular momentum, are at the forefront of a fast-developing field for exploring and controlling quantum materials in captivating ways. Phonon chirality originating from topological phonon bands is physically interesting but generally small and limited to a few material classes. Here, we report the observation of significant phonon chirality in the triangular-lattice rare-earth compound KNdSe_{2}. We find that the chirality is activated by a distinct local mechanism-crystalline electric field (CEF) excitations-rather than by a global topology. Using helicity-resolved magneto-Raman spectroscopy, we observe a clear splitting (∼3  cm^{-1} under 9 T, or equivalently ∼0.4μ_{B}) of the degenerate E_{g} phonon mode that exhibits perfect circular polarization selection rules-the unambiguous fingerprint of chirality. The magnitude, field dependence, and thermal evolution of the splitting are quantitatively reproduced by a microscopic theory of CEF-phonon coupling, which we solve by employing the Dyson equation formalism and incorporating a careful analysis of symmetry and angular momentum conservation. Our findings demonstrate a deterministic, CEF-activated mechanism for realizing phononic chirality and identify the rare-earth chalcogenides as a highly tunable playground for investigating the rich physics of coupled electronic and vibrational quasiparticles.