Neutron Scattering from Low-Frequency Excitations in Amorphous Germanium

The vibrations in amorphous germanium between 160 GHz and 4 THz have been studied by inelastic neutron scattering. From the observed dynamic structure factor, one finds essentially sound waves at low frequencies and essentially bond bending modes akin to the crystalline TA zone boundary modes at 2 THz. In contrast to the crystalline case, these bond bending modes extend down to lower frequencies and coexist with the sound waves below 1 THz. The total vibrational density of states determined at 293 K is in good agreement with measured specific-heat data between 5 and 30 K.