Dynamical Crossover at the Liquid-Liquid Transformation of a Compressed Molten Alkali Metal

Bryk, Taras; Panfilis, Simone De; Gorelli, Federico A.; Gregoryanz, Eugene; Krisch, M.; Ruocco, G.; Santoro, Mario; Scopigno, T.; Seitsonen, Ari P.

doi:10.1103/physrevlett.111.077801

Cited by 32 publications

(41 citation statements)

References 47 publications

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“…The change of c p at T g is considered as the trum of collective modes. Recently, the evidence for this has started to come from experiments and modeling [152].…”

Section: Liquid-glass Transitionmentioning

confidence: 99%

Collective modes and thermodynamics of the liquid state

2015

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Strongly interacting, dynamically disordered and with no small parameter, liquids took a theoretical status between gases and solids, with the historical tradition of hydrodynamic description as the starting point. We review different approaches to liquids as well as recent experimental and theoretical work, and propose that liquids do not need classifying in terms of their proximity to gases and solids or any categorizing for that matter. Instead, they are a unique system in their own class with a notably mixed dynamical state in contrast to pure dynamical states of solids and gases. We start with explaining how the first-principles approach to liquids is an intractable, exponentially complex problem of coupled non-linear oscillators with bifurcations. This is followed by a reduction of the problem based on liquid relaxation time τ representing non-perturbative treatment of strong interactions. On the basis of τ , solid-like high-frequency modes are predicted and we review related recent experiments. We demonstrate how the propagation of these modes can be derived by generalizing either hydrodynamic or elasticity equations. We comment on the historical trend to approach liquids using hydrodynamics and compare it to an alternative solid-like approach. We subsequently discuss how collective modes evolve with temperature and how this evolution affects liquid energy and heat capacity as well as other properties such as fast sound. Here, our emphasis is on understanding experimental data in real, rather than model, liquids. Highlighting the dominant role of solid-like high-frequency modes for liquid energy and heat capacity, we review a wide range of liquids: subcritical low-viscous liquids, supercritical state with two different dynamical and thermodynamic regimes separated by the Frenkel line, highly-viscous liquids in the glass transformation range and liquid-glass transition. We subsequently discuss the fairly recent area of liquid-liquid phase transitions, the area where the solid-like properties of liquids have become further apparent. We then discuss gas-like and solid-like approaches to quantum liquids and theoretical issues that are similar to the classical case. Finally, we summarize the emergent view of liquids as a unique system in a mixed dynamical state, and list several areas where interesting insights may appear and continue the extraordinary liquid story.

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“…The change of c p at T g is considered as the trum of collective modes. Recently, the evidence for this has started to come from experiments and modeling [152].…”

Section: Liquid-glass Transitionmentioning

confidence: 99%

Collective modes and thermodynamics of the liquid state

2015

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“…63 and used for the case of liquid Li at ambient pressure and liquid Rb in a wide range of pressures. 49 Note that the existing methodology of the ab initio estimation of the adiabatic speed of sound for fluids via the numerical calculation of the Landau-Placzek ratio 67 is quite sophisticated and requires smooth dynamic structure factors S(k,ω) and a methodology how to separate contributions to the central and side peaks. In Fig.…”

Section: Thermo-viscoelastic Model For Longitudinal Dynamicsmentioning

confidence: 99%

“…47 So far, no studies have been performed to clarify how the pressure-induced changes in structure affect the collective dynamics in liquid metals. Only recently there appeared a report on a dynamical crossover in liquid Rb at pressures ∼13 GPa; 49 however, a detailed analysis of the spectra of collective excitations was not performed. For the case of solids, the studies of the phonon spectra at high pressures are of great interest, especially for iron at the earth's core pressures.…”

Section: Introductionmentioning

confidence: 99%

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

Bryk

Ruocco

Scopigno

et al. 2015

The Journal of Chemical Physics

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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...

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“…Here cðkÞ is the wavenumber-dependent ratio of specific heats which easily can be expressed via correlators used in the GCM approach [20,52]. To date this is the most precise methodology of calculations of adiabatic speed of sound from classical [35,48] and ab initio [53][54][55] simulations. A comparison of the calculated adiabatic speed of sound for supercritical Ar with the NIST database showed almost perfect agreement in the whole density range, see [48].…”

Section: ð4:29þmentioning

confidence: 99%

Collective Excitations in Supercritical Fluids

Bryk

Gorelli

Ruocco

et al. 2015

Springer Proceedings in Physics

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Recent progress in theoretical and simulation studies of collective excitations in supercritical fluids is reviewed. We discuss a methodology of fit-free estimation of dispersion of longitudinal and transverse excitations in simple fluids. The issue of vanishing positive sound dispersion-a viscoelastic increase of the speed of sound from adiabatic one to its high-frequency (elastic) value-with reduction of density, as it was observed in inelastic X-ray scattering experiments on supercritical Ar, is discussed from the point of view of finding distinctions in collective dynamics of low-and high-density supercritical fluids. On the basis of several theoretical models within the extended hydrodynamic description of density-density correlations in liquids analytical expressions for positive sound dispersion are obtained and applied for analysis of time correlation functions obtained from molecular dynamics simulations. A location of a crossover from the liquid-like to gas-like types of collective dynamics is discussed based on general findings for spectra of collective excitations in supercritical Ar and soft sphere fluids.

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Dynamical Crossover at the Liquid-Liquid Transformation of a Compressed Molten Alkali Metal

Cited by 32 publications

References 47 publications

Collective modes and thermodynamics of the liquid state

Collective modes and thermodynamics of the liquid state

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

Collective Excitations in Supercritical Fluids

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