COLLECTIVE EXCITATIONS IN CRYSTALS COMPOSED OF DISPARATE MASS PARTICLES

The calculated structure factor of a disordered fee crystal formed by a disparate-mass mixture shows at low wave vectors (k) a broad high-fr equency band in addition to hydrodynamic phonons. The former shows a p ronounced k dependence. The average phase velocity of such excitations is lower than that of the longitudinal sound in a crystal of light pa rticles. At larger k, the hydrodynamic frequencies decrease with incre asing k, well below the linear dispersion regime, and finally disappea r. The findings are rationalized on the basis of a ''two-solid model.' '