Evidence for the coexistence of two density fluctuation modes in molten Li(30)Bi(70) as probed by neutron scattering

We report on a novel simultaneous observation of two inelastic doublets in the dynamic structure factor of the liquid alloy Li(30)Bi(70). This result establishes the coexistence of two density fluctuation modes in a liquid binary system with disparate mass components. The measurements of the dynamic structure factor of the alloy have been performed on the hot neutron three-axis spectrometer IN1 of the Institut Laue-Langevin with an unprecedented energy resolution in the small wave vector domain. Analysis of the dynamical parameters characterizing the two excitations reveals the acoustic nature of the low-frequency mode and suggests an optic-like nature for the high-frequency mode. A comparison with recent neutron scattering results on pure liquid Bi suggests that the low-frequency mode in the alloy is closely related to the heavier component dynamics.