A search for anomalous phonons in potassium

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Abstract
The frequencies of normal modes belonging to the Sigma 4 and G4 branches in potassium at 4.3K have been measured by means of coherent inelastic neutron scattering experiments. No evidence was found in the Sigma 4 branch of the anomaly predicted in a recent theoretical calculation based on a rather sophisticated pseudopotential model. It is likely that this anomaly arises from a particular mathematical problem in calculating the Sigma 4 normal mode frequencies, and that when this problem is avoided, the model provides an excellent description of the observed frequencies. The parameters of a widely used phenomenological model have been re-evaluated in the light of all the phonon frequency data now available for potassium.