Dynamics of liquid Au from neutron Brillouin scattering and ab initio simulations: Analogies in the behavior of metallic and insulating liquids

We performed a neutron Brillouin scattering determination of the dynamic structure factor of liquid gold in the wave-vector range 6<Q<16 nm -1. Despite the experimental difficulties due to the considerable neutron absorption and the high melting temperature of the sample, a non-negligible coherent signal could successfully be extracted and revealed the presence of underdamped ion density-fluctuation modes in the whole Q range of the experiment. The quality of the data further enabled a significant comparison with the results of ab initio simulations for liquid gold. The agreement found between neutron and simulation data not only provides a necessary test of ab initio methods still limited to the use of few hundreds atoms, but also reasonably justifies a thorough analysis of the simulated dynamics in the extended Q range that can be more comfortably accessed by calculations. A viscoelastic modeling of the simulated spectra proves to be appropriate and, surprisingly, the so-derived salient dynamic features of liquid gold show an overall Q behavior globally similar to that already found for insulating liquids. The present and recent results on the collective properties of simple liquids thus lead us to embrace the concept that simple liquid metals and insulators, at the nanometer length scale, are dynamically much less different than expected.