Neutron Brillouin scattering and ab initio simulation study of the collective dynamics of liquid silver

We present a thorough investigation of the collective dynamics of liquid Ag combining neutron Brillouin scattering and ab initio molecular dynamics (AIMD) determinations of the dynamic structure factor $S(Q,\ensuremath{\omega})$. The main scope of this work is not only to provide experimental results for some important dynamical properties of this liquid metal in the wave-vector range $4lQl16\phantom{\rule{4pt}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$, but also to inquire about the scarce detectability of shear waves apparently characterizing two elements of group IB, differently from other metals. In fact, as in the case of Au, a transverse-like dynamics is not deducible from the experimental $S(Q,\ensuremath{\omega})$ of Ag, despite the indisputable quality of the neutron data collected on the BRISP spectrometer at the Institut Laue Langevin in Grenoble. However, the significant agreement between experiment and AIMD calculations allowed for an in-depth study of the simulated $S(Q,\ensuremath{\omega})$ in a $Q$ range overlapping and extending the experimental one. A multimode analysis, already proven very successful in the description of various dynamical properties of fluid systems, is shown to be extremely effective also to analyze the intermediate scattering function predicted by AIMD at the various $Q$ values, and eventually enables a reliable determination of both longitudinal and transverse branches in the dispersion curve of this liquid. Throughout the paper we highlight the importance of referring to theoretically well-founded models for $S(Q,\ensuremath{\omega})$ and of imposing physical constraints in a fit-based analysis: These ensure that the used models obey fundamental properties of the dynamic structure factor.