Collective ionic dynamics in a molten binary alloy

A recent experiment @Phys. Rev. Lett. 80, 2141 ~1998!# showed heavily damped excitations in molten Li4Pb, within kinematic scales well beyond those of hydrodynamic sound. These findings pointed to the presence of short-lived out-of-phase atomic motions as the underlying microscopic phenomenon. A series of computer molecular dynamics studies are performed to investigate the details of the atomic motions. From an analysis of the simulated structure factors for molten Li4Pb, as well as by a comparison with those of liquid Li under different thermodynamic conditions, it is found that the high-frequency excitation found in the alloy shows characteristics remarkably different from those of pure Li. The relative phases of the atoms partaking in such motions, as well as the remarkably short excitation lifetimes, portray it as a fairly localized mode, with a frequency dependent polarization. @S1063-651X~98!07210-9#