Metal phonon-spectrum singularities determined by local geometry of the Fermi surface
Avanesyan G.T., Kaganov M.I., Lisovskaya T.Yu.
The existence of singularities in the angular dependence of the speed s and of the absorption coefficient Γ of sound in metal is predicted. The singularities are due to zero-curvature lines (lines of parabolic points) on the Fermi surface. The parabolic points should lead, besides, to enhancement of the Migdal-Kohn singularities and to singularities in the angular dependence of the Pippard (geometric) oscillations of the sound absorption coefficient.