Real and Virtual Kohn Effect in Palladium by Inelastic Neutron Scattering

A detailed study has been made of the anomalous lattice vibrations in palladium by inelastic neutron scattering. Anomalous shifts in the phonon frequencies are particularly evident along the [0ζζ] T1 branch at reduced wave vectors [Formula: see text] to 0.4, becoming rapidly weaker at 'off symmetry' wave vectors. (Rather similar effects have also been observed in platinum.) The phonon wave vectors corresponding to possible Kohn transitions in the symmetry planes have been computed, using the Fermi surfaces given in the literature. The results indicate that the anomalous frequency shifts arise largely from transitions across the 'heavy' hole Fermi sheet formed from the fifth band electrons. Measurements along the T1 branch at temperatures between 8 and 853 °K show that the anomalous frequency shifts are smeared out in wave vector and perhaps weakened with increasing temperature. The corresponding phonon line widths, obtained by correction of the neutron groups for instrumental resolution, were found to be anomalously broadened as well, particularly at 8 °K where anharmonic effects are expected to be small. This suggests that the anomalous line widths reflect, in large part, the reduction in phonon lifetimes (and in electron lifetimes) arising from real Kohn transitions. This is believed to be the first identification of phonon line broadening because of electron–phonon interaction in a nonsuperconductor.