Simultaneous manifestations of the 2D van Hove singularity and the Fermi surface nesting in the acoustic soft phonon mode of alloys

The experimentally observed composition and uniaxial-stress dependences of the soft phonon mode in alloy have been interpreted using numerical calculations of the electronic band structure, elastic shear constant , generalized susceptibility and different sections of Fermi surface as a basis. It is shown that the main features of these dependences are attributable to two different band structure peculiarities, namely, to Fermi surface nesting and 2D van Hove singularity in the density of states. Whereas the nesting vector fixes the position of a dip (Kohn anomaly) in the phonon branch, the separation between the Fermi level and the energy of the 2D van Hove singularity governs the softening (hardening) of this branch as a whole.