Pressure-induced emergence of unusually high-frequency transverse excitations in a liquid alkali metal: Evidence of two types of collective excitations contributing to the transverse dynamics at high pressures

Abstract

Unlike phonons in crystals, the collective excitations in liquids cannot be treated as propagation of harmonic displacements of atoms around stable local energy minima. The viscoelasticity of liquids, reflected in transition from the adiabatic to elastic high-frequency speed of sound and in absence of the long-wavelength transverse excitations, results in dispersions of longitudinal (L) and transverse (T) collective excitations essentially different from the typical phonon ones. Practically, nothing is known about the effect of high pressure on the dispersion of collective excitations in liquids, which causes strong changes in liquid structure. Here dispersions of L and T collective excitations in liquid Li in the range of pressures up to 186 GPa were studied by ab initio simulations. Two methodologies for dispersion calculations were used: direct estimation from the peak positions of the L/T current spectral functions and simulation-based calculations of wavenumber-dependent collective eigenmodes. It is found that at ambient pressure, the longitudinal and transverse dynamics are well separated, while at high pressures, the transverse current spectral functions, density of vibrational states, and dispersions of collective excitations yield evidence of two types of propagating modes that contribute strongly to transverse dynamics. Emergence of the unusually high-frequency transverse modes gives evidence of the breakdown of a regular viscoelastic theory of transverse dynamics, which is based on coupling of a single transverse propagating mode with shear relaxation. The explanation of the observed high-frequency shift above the viscoelastic value is given by the presence of another branch of collective excitations. With the pressure increasing, coupling between the two types of collective excitations is rationalized within a proposed extended viscoelastic model of transverse dynamics. Unlike phonons in crystals, the collective excitations in liquids cannot be treated as propagation of harmonic displacements of atoms around stable local energy minima. The viscoelasticity of liquids, reflected in transition from the adiabatic to elastic high-frequency speed of sound and in absence of the long-wavelength transverse excitations, results in dispersions of longitudinal (L) and transverse (T) collective excitations essentially different from the typical phonon ones. Practically, nothing is known about the effect of high pressure on the dispersion of collective excitations in liquids, which causes strong changes in liquid structure. Here dispersions of L and T collective excitations in liquid Li in the range of pressures up to 186 GPa were studied by ab initio simulations. Two methodologies for dispersion calculations were used: direct estimation from the peak positions of the L/T current spectral functions and simulation-based calculations of wavenumber-dependent collective eigenmodes. It is found that at ambient pressure, the longitudinal and transverse dynamics are well separated, while at high pressures, the...

A Search for Two Types of Transverse Excitations in Liquid Polyvalent Metals at Ambient Pressure: An Ab Initio Molecular Dynamics Study of Collective Excitations in Liquid Al, Tl, and Ni

“…The second high-frequency branch of transverse excitations in liquid Tl becomes visible only for wave numbers higher than the first pseudo-Brillouin zone boundary, i.e., for k > K p /2, where K p is the position of the main peak of static structure factor S(k). For smaller wave numbers the high-frequency transverse excitations were not found, in total agreement with the case of liquid Li [4] at high pressure; iii. For the case of liquid Tl we observed a pronounced highfrequency shoulder and a small peak on the high-frequency side of the spectrum of velocity autocorrelation function.…”

“…As a matter of fact, analysis of inelastic X-ray scattering experiments indicated a possible coupling between longitudinal (L) and transverse (T) collective excitations and consequently two contributions of coupled propagating modes at relatively small wave numbers [7]. In contrast, ab initio MD (AIMD) simulations [4] clearly indicated the emergence of the second transverse branch only for wave numbers outside the first pseudo-Brillouin zone. Moreover, it has been reported earlier for polyvalent liquid metals such as Ga [7] and Sn [8] the existence of two contributions from propagating modes as measured by IXS experiments.…”

“…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 a new discussion on possible two types of transverse propagating modes coexisting on the spatial scale of the cage of nearest neighbors. The cage of nearest neighbors means the configuration of nearest atoms in the first coordination shell, which for dense liquids (especially in supercooled state) can be stable over some period of time, while diffusion of particles destroy the instantaneous cages.…”

A search for manifestation of two types of collective excitations in dynamic structure of a liquid metal: Ab initio study of collective excitations in liquid Na

“…17 and 13, the case of liquid Sn was studied, and ab initio simulations performed for three temperatures of 573 K, 873 K, and 1273 K gave evidence of several propagating modes contributing to the shape of C L/T (k,ω). In the case of liquid Li at temperature 1000 K and ambient pressure, 18 it was observed in ab initio MD simulations just a standard one-peak behaviour of C L/T (k,ω). However, the applied pressure up to 126 GPa showed an emergence of a weak high-frequency maximum on the transverse current spectral function, which was resolved for large wave numbers in the longitudinal C L (k,ω) too.…”

Structure and dynamics of liquid Zn: an analysis of ab-initio simulations

“…The appearance of the second high frequency peak in C T (q; !) had indeed been observed before, but only for liquids under high pressure [14], so it is rather surprising to nd them at ambient pressure as we do here. However, we note that this behaviour is also observed in other metals at ambient pressure, such as liquid Ni [15].…”