Taras Demchuk

We compare extensive simulations and embedded-atom-model molecular dynamics for analysis of collective dynamics in liquid Fe at several experimental pressures and temperatures of 2200 and 2700 K [Y. Kuwayama , ]. Dynamic eigenmodes of the generalized Langevin equation were calculated based on simulation data and analyzed in a wide range of wave numbers with a focus on dispersion of collective excitations. We analyze the origin of positive dispersion of sound excitations in connection with contributions from different types of hydrodynamic and nonhydrodynamic fluctuations to the corresponding eigenvectors. Dispersion of collective excitations and adiabatic speed of sound, estimated from the simulation data at three pressures, is compared to the values obtained from the inelastic x-ray scattering experiments.

Collective dynamics of metallic melts at high pressures is one of the open issues of condensed matter physics. By means of ab initio molecular dynamics simulations, we examine features of dispersions of collective excitations through transverse current spectral functions, as a function of pressure. Typical metallic melts, such as Li and Na monovalent metals as well as Al, Pb and In polyvalent metals are considered. We firmly establish the emergence of a second branch of high-frequency transverse modes with pressure in these metals, that we associate with the pronounced high-frequency shoulder in the vibrational density of states. Similar correlation also exist with the low frequency modes. The origin of the pressure-induced evolution of transverse excitations in liquid metals is discussed.

Ab initio molecular dynamics simulations were performed to analyze changes in the structure and dynamics of molten Pb along the melting line for pressures ranging from ambient to 70 GPa. Common neighbor analysis reveals a local structural order of the underlying crystalline phase at the corresponding pressure, which increasingly competes with the existing icosahedral local order with pressure. Obtained dispersions of longitudinal and transverse collective excitations contain two branches of transverse modes for all pressures. Analysis of the pressure dependence of observed two-peak structure of the Fourier spectra of velocity autocorrelation functions allowed us to identify their peak locations with the frequencies of nonpropagating transverse branches in the second pseudo-Brillouin zone.

Recent findings of pressure-induced emergence of unusual high-frequency contribution to transverse current spectral functions in several simple liquid metals at high pressures raised a question whether similar features can be observed in liquid metals at ambient conditions. We report here analysis of ab initio molecular dynamics-derived longitudinal and transverse current spectral functions and corresponding dispersions of collective excitations in liquid polyvalent metals Al, Tl, Ni. We have not found evidences of the second branch of high-frequency transverse modes in liquid Al and Ni, while in the case of liquid Tl they were clearly present in transverse dynamics. The vibrational density of states for liquid Tl has a pronounced high-frequency shoulder, which is located right in the frequency range of the second high-frequency transverse branch, while for liquid Al and Ni the vibrational density of states has only a weak indication of possible high-frequency shoulder. The origin of specific behavior of transverse excitations in liquid Tl is discussed.

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