Anharmonic properties of Li

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Abstract
The energies and lifetimes of phonons in metallic Li between 110 and 424 K are calculated using the self-consistent phonon theory and the effective ion-ion interaction developed by Dagens, Rasolt, and Taylor (DRT). The results are compared with the recent neutron scattering measurements of Beg and Nielson. Li turns out to be surprisingly harmonic, less anharmonic than Na or K, due to the strong Li ion-ion interaction which has a relatively soft repulsive core. The anharmonic corrections do improve agreement with the observed dispersion curves at 110 K, and they provide a good description of the phonon frequency changes with temperature, but the calculated phonon lifetimes are often two or three times the observed values. The remaining discrepancies with experiment suggest (i) the presence of three- or four-body interactions in Li which are not included in the DRT pseudopotential method, and (ii) that the phonons decay significantly via processes higher than those accounted for by cubic anharmonicity.