Effects of isotope disorder on energies and lifetimes of phonons in germanium

The effects of isotope disorder on phonon energies and lifetimes have been measured by means of high-resolution inelastic neutron scattering on a nearly maximally disordered Ge-70/76 single crystal (mass variance g=1.52x10(-3)). Two theoretical approaches have been used to understand the observed effects: calculations based on the coherent potential approximation and on large supercells, respectively. For the mass variance of our sample, the two methods lead to rather similar results that agree with the experimental data. This agreement is marginally better for the supercell calculations. Calculations performed for larger mass variances g revealed, however, that the two approaches yield significantly different results when g is larger than 3x10(-3) [S0163-1829(98)02938-5].