PHONON-DISPERSION AND KOHN ANOMALIES IN CU0.84AL0.16

Motivated by the observation of x-ray diffuse satellites, attributed to screening singularities associated with parallel flat sections of the Fermi surface (in analogy with Kohn anomalies), we have made detailed measurements of phonon frequencies along all high-symmetry directions on a large single crystal of Cu0.84Al0.16 at room temperature. Phonon frequencies were ascertained to better than 0.03 meV. Interatomic force constants and a vibrational density of states were calculated by performing a Born von Karman analysis on the complete set of phonon dispersion curves. Near the zone boundaries phonon energies are significantly lower than the corresponding phonons in pure Cu. This is attributed to mode repulsion from high-frequency localized vibrational modes of the Al atom in the Cu host lattice. No strong anomaly, either in the phonon dispersion itself or in the derivative, was observed near the expected wave vector q=2kF. The absence of Kohn anomalies in the present system could be due either to a smeared-out Fermi surface or to the possibility that the electron-electron interaction that screens the interionic potential is not the dominant interaction in the system; i.e., the existence of Kohn anomalies in these alloys may depend mainly on the details of the electron-phonon interaction. © 1990 The American Physical Society.