PHONON DISPERSION RELATION OF MASS DEFECT CONCENTRATED ALLOY Ni-Pt AND ITS INTERPRETATION BY CPA

Résumé
Relation de Dispersion pour les Phonons dans les Alliages concentrés Ni-Pt avec défaut de masse. Les vibrations atomiques dans des cristaux simples de l'alliage Ni-Pt ont été étudiées au moyen de la diffusion inélastique de neutrons. Des mesures ont été effectuées pour les branches [00ζ] T. Les résultats sont comparés avec des calculs basés sur la CPA.

Abstract
By means of inelastic scattering of neutrons, the phonon dispersion relations of 00ζ T branch have been measured for Ni-Pt alloys with various concentrations. For Ni₉₅Pt₅ and Ni₇₀Pt₃₀, double peaks were found in the neutron scattering spectra obtained by constant q method at about q = 0.5. The dispersion relation has an energy gap corresponding to the appearance of the double peaks. The gap with is about 3 meV and 6 meV for Ni₉₅Pt₅ and Ni₇₀Ni₃₀, respectively. Taylor has calculated the spectral Green function of phonons in mass defect concentrated alloy AuCu under CPA. From the positions and widths of the peaks of the Green function, one can construct the dispersion curves of phonon mode as well as width. The theoretical dispersion relation, thus obtained has an energy gap corresponding to the localized vibrations. The similarity of the theoretical and experimental dispersion relations indicates that the nature of phonons in highly concentrated mass defect alloys can be well interpreted by the theory under CPA. The reason of the appearance of the double phonons corresponding to a single wave number can be understood by considering that the vibration frequency of an atom changes in time or in space from higher one which is a localized mode around light atoms to lower one which is a localized mode around heavy atoms.