Disorder broadening of the acoustic branches in SixGe1-x mixed crystals

Complete text of publication follows. Si and Ge both crystallize in the diamond structure, forming mixed crystals over the entire range of composition x. The dispersion curves of the end members are almost identical in shape, but differ by a large scale factor, mostly resulting from the large mass difference (MGe/MSi ∼ 2.6) [1]. The Bragg reflections being very sharp, this is a convenient system to investigate the effect of disorder on acoustic phonons, and its dependence on the momentum exchange Q. This question is of relevance to acoustic excitations in glasses. The acoustic branches for various compositions x has been measured. The spectra were analyzed with a 4d Monte Carlo procedure, taking into account the curvature of the branches calculated with the Bond Charge model [2]. The results exhibit a very large and strongly Q-dependant broadening of the acoustic branches at intermediate compositions. They also reveal the presence of local modes, and of resonant modes interacting with the phonons. Interestingly, the scale factor for intermediate compositions does not follow a simple law. (author)