A high-resolution inelastic neutron-scattering experiment has been carried out on a saturated solution of lithium in deuterated ammonia to investigate the vibrational dynamics at low wave-vector transfer. The response function of the system, dominated by the deuterium dynamics, showed clear coherent contributions arising from collective excitations. Density-fluctuation modes were visible up to the highest measured wave- vector transfer, 0.8Å-1, with an associated dispersion relation characterized by a remarkably anomalous behavior around twice the Fermi momentum. The nature of the anomaly is ascribed to electron correlation effects and closely resembles the Kohn anomaly as observed in the phonon spectrum of solid metals.
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