Crossover between hydrodynamic and kinetic modes in binary liquid alloys
  Inelastic x-ray scattering (IXS) measurements of the dynamic structure factor in liquid Na57K43, sensitive to the atomic-scale coarse graining, reveal a sound velocity value exceeding the long wavelength, continuum value and indicate the coexistence of two phonon-like modes. Applying Generalized Collective Mode (GCM) 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 can not be explained as sound propagation over the light specie (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. 
Stefano Cazzato (1), Tullio Scopigno (1), Taras Bryk (2), Ihor Mryglod   (2) and Giancarlo Ruocco (1) ((1) INFM CRS-SOFT, c/...