Optic-like excitations in binary liquids: transverse dynamics

The generalized collective mode approach is applied for the study of transverse dynamics in binary mixtures. The scheme is based on simultaneous treatment of the conserved total mass current and the mutual mass-concentration currents, as well as their time derivatives. The condition for existence of optic-like transverse modes in a binary system is derived. It is shown that high mutual diffusion and a tendency towards demixing prevent the emergence of transverse optic-like modes. Optic-like excitations are found in a Lennard-Jones Kr-Ar fluid and the liquid-metallic alloys Mg70Zn30 and Li4Pb, while in the 'fast-sound' dense gas mixture He75Ar25 they do not appear for small, wavenumbers.