The dynamical structure factor for liquid aluminium

Pressure evolution of transverse collective excitations in liquid Al along the melting line

Evolution of structure and dynamics of liquid Al with pressure along the melting line up to 300 GPa has been studied by means of ab initio molecular dynamics simulations. An analysis of structural properties shows that liquid Al undergoes uniform compression with pressure associated with a competition of the existing icosahedral local order with bcc ordering above 200 GPa. Dispersion of collective excitations indicates the presence of two branches of transverse nonpropagative modes in the second pseudo-Brillouin zone. Under pressure, the second high-frequency branch manifests as the second peak position in transverse current correlation functions, while, for ambient pressure, it corresponds to a smeared-out high-frequency shoulder. We report a correspondence of the peak locations in vibrational density of states with these two transverse collective excitations as well as their linear evolution with density.

Experimental evidence for a dynamical crossover in liquid aluminium

The temperature dependence of the dynamic structure factor at next-neighbour distances has been investigated for liquid aluminium. This correlation function is a sensitive parameter for changes in the local environment and its Fourier transform was measured in a coherent inelastic neutron scattering experiment. The zero frequency amplitude decreases in a nonlinear way and indicates a change in dynamics around 1.4 ∙ Tmelting. From that amplitude a generalized viscosity can be derived which is a measure of local stress correlations on next-neighbour distances. The derived generalized longitudinal viscosity shows a changing slope at the same temperature range. At this temperature the freezing out of degrees of freedom for structural relaxation upon cooling sets in which can be understood as a precursor towards the solid state. That crossover in dynamics of liquid aluminium shows the same signatures as previously observed in liquid rubidium and lead, indicating an universal character.

Time-scale invariance of relaxation processes of density fluctuation in slow neutron scattering in liquid cesium

The realization of the idea of time-scale invariance for relaxation processes in liquids has been performed by the memory functions formalism. The best agreement with experimental data for the dynamic structure factor S(k,omega) of liquid cesium near melting point in the range of wave vectors (0.4 A(-1) < or = k < or = 2.55 A(-1)) is found with the assumption of concurrence of relaxation scales for memory functions of third and fourth orders. Spatial dispersion of the first four points in the spectrum of the statistical parameter of non-Markovity epsilon(i)(k,omega) at i=1,2,3,4 has allowed us to reveal the non-Markov nature of collective excitations in liquid cesium, connected with long-range memory effect.

Dynamic properties of Lennard-Jones fluids and liquid metals

The dependence of the dynamic properties of liquid metals and Lennard-Jones fluids on the characteristics of the interaction potentials is analyzed. Molecular-dynamics simulations of liquids in analogous conditions but assuming that their particles interact either through a Lennard-Jones or a liquid-metal potential were carried out. The Lennard-Jones potentials were chosen so that both the effective size of the particles and the depth of the potential well were very close to those of the liquid-metal potentials. In order to investigate the extent to which the dynamic properties of liquids depend on the short-range attractive interactions as well as on the softness of the potential cores, molecular-dynamics simulations of the same systems but assuming purely repulsive interactions with the same potential cores were also performed. The study includes both single-particle dynamic properties, such as the velocity autocorrelation functions, and collective dynamic properties, such as the intermediate scattering functions, the dynamic structure factors, the longitudinal and transverse current correlations, and the transport coefficients.