Crossover between hydrodynamic and kinetic modes in binary liquid alloys

Inelastic x- ray scattering measurements of the dynamic structure factor in liquid Na(57)K(43), which is sensitive to the atomic- scale coarse graining, reveal a sound velocity value exceeding the long wavelength continuum value and indicate the coexistence of two phononlike modes. By applying generalized collective mode analysis scheme, we show that the positive dispersion of the sound velocity occurs in a wavelength region below the crossover from hydrodynamic to atom- type excitations and, therefore, it cannot be explained as sound propagation over the light species ( Na ) network. The present result experimentally proves the existence of positive dispersion in a binary mixture due to a relaxation process, as opposed to fast sound phenomena.