Microscopic dynamics in liquid metals: The experimental point of view

Abstract

The experimental results relevant for the understanding of the microscopic dynamics in liquid metals are reviewed, with special regards to the ones achieved in the last two decades. Inelastic Neutron Scattering played a major role since the development of neutron facilities in the sixties. The last ten years, however, saw the development of third generation radiation sources, which opened the possibility of performing Inelastic Scattering with X rays, thus disclosing previously unaccessible energy-momentum regions. The purely coherent response of X rays, moreover, combined with the mixed coherent/incoherent response typical of neutron scattering, provides enormous potentialities to disentangle aspects related to the collectivity of motion from the single particle dynamics. If the last twenty years saw major experimental developments, on the theoretical side fresh ideas came up to the side of the most traditional and established theories. Beside the raw experimental results, therefore, we review models and theoretical approaches for the description of microscopic dynamics over different length-scales, from the hydrodynamic region down to the single particle regime, walking the perilous and sometimes uncharted path of the generalized hydrodynamics extension. Approaches peculiar of conductive systems, based on the ionic plasma theory, are also considered, as well as kinetic and mode coupling theory applied to hard sphere systems, which turn out to mimic with remarkable detail the atomic dynamics of liquid metals. Finally, cutting edges issues and open problems, such as the ultimate origin of the anomalous acoustic dispersion or the relevance of transport properties of a conductive systems in ruling the ionic dynamic structure factor are discussed.

Polarized neutron scattering investigation of excitations at low momentum transfer in liquid Ga: The mystery continues

“…As expected, on very short length scales one observes the collision dominated behavior of an ion trapped in the cage formed by its neighbors. In addition, at low momentum transfers q (that is, at long probing wave lengths λ =2π/ q) the indirect effects of cage diffusion on the decay mechanism of density fluctuations could be inferred [2]. Surprisingly however, it was reported [7] that cage diffusion could be observed directly in neutron scattering experiments at low momentum transfer as a contribution separate from the standard three hydrodynamic modes and extensions thereof to finite q.…”

“…The last decades in particular have seen a wealth of new results [2]. Benefitting from the increased capabilities of neutron and X-ray scattering facilities, it is now possible to get highly accurate data on the microscopic origins of the dynamic response unique to liquid metals.…”

“…The spectra of liquid metals that have been measured through X-ray scattering on a range of metals all show that they can be very well described using the extended viscoelastic model [1,2]. In a nutshell, a density fluctuation decays by giving rise to a velocity disturbance, which in turn decays because of gradients in the velocity.…”

“…Not only has the propagation of short wavelength sound modes been investigated in a series of liquid metals such as Ga [3][4][5][6], Hg [7][8][9][10] and alkali metals [11][12][13], the details of the relaxation mechanism of density fluctuations in general have been probed and modeled [1,2,5]. One of the main results to come out of these experiments was the role that cage diffusion plays in liquid metals for fluctuations over all length scales [1,2]. As expected, on very short length scales one observes the collision dominated behavior of an ion trapped in the cage formed by its neighbors.…”

High-frequency dynamics of liquid and supercritical nitrogen

“…In the intermediate (visco-elastic) regime the acoustic propagation is strongly coupled with the active relaxation. The sound velocity strongly depends on the frequency of the probed acoustic wave, reflected in an upwards bending of the sound dispersion relation with respect to the linear behaviour characteristic of the viscous regime (positive sound dispersion) [4][5][6][7][8][9].…”