PRESSURE-INDUCED FREQUENCY-SHIFTS OF TRANSVERSE ACOUSTIC PHONONS IN GERMANIUM TO 9.7 GPA

A complete set of transverse acoustic phonon frequencies of Ge has bee n measured at pressures up to 9.7 GPa by inelastic neutron scattering and studied by ab initio calculations using density functional perturb ation theory. At the X and L points, the pressure dependence of the ph onon frequency omega in the cubic structure is shown to be that of a c lassical soft mode with omega(2) strictly linear in pressure P. This f inding is in contrast with previous interpretations that associated th e pressure dependence of the zone boundary modes with the occurrence o f the first order transition at similar to 10 GPa and represented the pressure dependence of its frequencies by a polynomial in P.