The calculated structure factor of a disordered fcc crystal formed by a disparate-mass mixture shows at low wave vectors ( k) a broad high-frequency band in addition to hydrodynamic phonons. The former shows a pronounced k dependence. The average phase velocity of such excitations is lower than that of the longitudinal sound in a crystal of light particles. At larger k, the hydrodynamic frequencies decrease with increasing k, well below the linear dispersion regime, and finally disappear. The findings are rationalized on the basis of a ``two-solid model.''
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Enciso et al. (1998) studied this question.
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