Abstract
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 (L) and transverse (T) 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.
Highlights
Collective dynamics in simple liquid metals is well understood on macroscopic spatial and temporal scales only, where all the contributions from relaxation and propagating collective modes to the measured in Brillouin light scattering experiments dynamic structure factors are well known
The velocity autocorrelation functions ψ(t) were calculated with the purpose to analyze their time-Fourier transform Z (ω), which outside the small-frequency region reflects the vibrational density of states in the simulated system
The collective dynamics was analyzed from the calculated L- and T-current spectral functions CL/T(k, ω), peaks of which are connected with the frequencies of corresponding collective propagating modes
Summary
Collective dynamics in simple liquid metals is well understood on macroscopic spatial and temporal scales only, where all the contributions from relaxation and propagating collective modes to the measured in Brillouin light scattering experiments dynamic structure factors are well known. On microscopic scale with atomic resolution, the collective dynamics in liquids can be studied by inelastic X-ray or neutron scattering experiments, as well as by molecular dynamics simulations (MD). Many new features appear on the atomic scales in dynamics of liquids and in liquid metals, which are not described by hydrodynamic theory. They can be represented as effects of non-hydrodynamic collective modes like structural relaxation, shear waves, stress relaxation, heat waves, etc.[3]. Recent findings of pressure-induced emergence of unusual high-frequency contribution to transverse current spectral functions in liquid Li [4], Na [5], and Fe [6] at high pressures opened
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