Neutron scattering and ab initio simulation: Two essential and inseparable tools for studying liquids dynamics. The case of shear waves in silver

Assessing the experimental visibility of shear-wave excitations has recently become a hot topic in the field of liquids dynamics. Experimental investigations, mostly focused on liquid metals, have reported contrasting results in different systems. Inelastic neutron scattering is a powerful technique for such studies, particularly when high energy and low momentum transfer are exploited simultaneously as made possible by the unique performances of the Brillouin spectrometer BRISP. However, a careful analysis of both experimental and simulation data reveals that a well-founded modelling of the dynamical response of the liquid is crucial for a correct interpretation of the results. This last point has not always been given due consideration in a research field where the analysis is mostly based on model fitting, sometimes used as a heuristic method to simply locate lines, shoulders, or other spectral features. The joint use of BRISP data and ab initio molecular dynamics simulations of liquid silver has allowed us both to detect transverse-wave excitations in this liquid, in addition to the well-known longitudinal sound waves, and to highlight the importance of a critical assessment of the various dynamical contributions to the spectral shape. In fact, all excitations and relaxations must be modelled in such a way that the total system response complies with a few fully general, theoretical properties of the dynamic structure factor. This goal has been attained through the consistent application of the well-tested and efficient multi-exponential analysis of the relevant time correlation functions. The most valuable indications coming out of this work are that: (i) more than one correlation function have to be studied, with simulations paralleling experimental work; (ii) rigorous methods of analysis have to be applied for a reliable interpretation of spectra; and (iii) the multi-exponential analysis allows for an in-depth assessment of the dynamical processes. This work proofs that future progress in the general understanding of the dynamics of disordered systems with high energy excitations needs for new appropriate neutron spectrometers able to span the (most interesting) low-Q range of the dynamic structure factor, Brisp having been the first working prototype in the world of such kind of instruments.