Phonon Anomalies in VN and Their Electronic Origin

Phonon dispersion and density-of-states measurements are presented for VN, a cubic transition-metal compound with ten valence electrons. The acoustic phonons differ strongly from those observed in nine-valence-electron compounds. In particular, the prominent soft-mode region has moved from the interior to the boundary of the Brillouin zone. Using the theory of Varma and Weber, we find that these changes arise primarily from a rigid-band-like increase in the Fermi energy.