Disorder-induced broadening of transverse acoustic phonons in Si _xGe_1-x mixed crystals

Inelastic neutron-scattering measurements of the dispersion and width of transverse acoustic branches are reported for Si_xGe_1-x, particularly for x=0.51. These alloys form high-quality single crystals. They are strongly disordered dynamically owing to the large Ge/Si mass ratio. This allows to study in function of disorder (x) the relations between the inhomogeneous broadening of acoustic waves, the Boson peak, and low-temperature thermal anomalies common to glasses. For x≤0.51 we find no boson peak, apparently no plateau in the thermal conductivity, but at large x a Rayleigh law on the phonon width, Γ∝Q⁴, where Q is the scattering vector.