The mixed longitudinal–transverse nature of collective modes in water

Cimatoribus, Andrea; Saccani, Sebastiano; Bencivenga, Filippo; Gessini, Alessandro; Izzo, Maria Grazia; Masciovecchio, C.

doi:10.1088/1367-2630/12/5/053008

Cited by 31 publications

(37 citation statements)

References 30 publications

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“…Only in water, the coupling between longitudinal and transverse collective excitations and their manifestations in both L and T current spectral functions were studied by MD simulations 44 which were supported in recent experimental study. 45 Hence, the case of L-T coupling in single-component liquid metals is very exotic and so far, there are neither models for the analysis nor explanations of its origin.…”

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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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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“…The origin of the S HF (Q, ω) term has not been clarified in the few observations available in literature. 9,28 However, it seems reasonable to infer a transverse origin for this mode, since a weakly dispersive excitation at similar frequencies was observed in the transverse current correlation spectrum computed via Molecular Dynamics (MD) simulations. 28 The S T (Q, ω) and S HF (Q, ω) profiles in Eq.…”

Section: Discussionmentioning

confidence: 92%

“…The line-shape (I(Q, ω)), measured either by INS or IXS, was analyzed using a best fitting procedure based on a χ 2 minimization of the distance between the experimental spectra and the following model: [8][9][10]18 …”

Section: Discussionmentioning

confidence: 99%

Shear propagation in the terahertz dynamics of water-glycerol mixtures

Cunsolo

Kodituwakku

Bencivenga

et al. 2013

The Journal of Chemical Physics

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Inelastic X-ray and neutron scattering techniques were jointly used to investigate the dynamics of water-glycerol mixtures at different concentrations and temperatures. It was observed that even relatively low concentrations of glycerol increase the damping of shear modes, as a consequence of the known ability of glycerol to disrupt the hydrogen bond network of water. A similar trend was observed when increasing the temperature, which suggests the presence of a locus in the concentration-temperature plane marking a crossover in the shear modulus.

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“…In the spectra, we can see the shoulders correspond to the longitudinal acoustic mode at around the energy of ±2, ±8, ±14 and ±20 for Q equals to 1.58, 4.24, 6.56 and 9.25 nm −1 , respectively. In pure water, it was concluded that IXS spectrum has two collective modes (longitudinal and transverse acoustic excitations) [10,15]. Thus we analyzed the spectrum by a damped harmonic oscillator (DHO) model which has two DHO components correspond to these two modes in addition to a Lorentzian component corresponds to quasi-elastic scattering as shown in the following equation.…”

Section: Experimental Details and Resultsmentioning

confidence: 99%

Inelastic x-ray scattering measurements of liquid waterglycerol mixtures

et al. 2017

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Abstract.We have carried out inelastic x-ray scattering measurements on liquid water-glycerol mixtures. The data are analyzed by a damped harmonic oscillator model with two excitations, longitudinal and transverse modes. The sound velocity of the longitudinal mode is almost constant (about 3.1 km/s), being independent of the composition. Thus the strength of 'fast sound' which we define the ratio of this IXS sound velocity to ultrasonic one is largest at pure water and gradually decrease with increasing mole fraction of glycerol. This result indicates that the relaxation phenomena of pure water gradually reduce with increasing the fraction of the solute: the scenario which we proposed for water-monohydric alcohol mixtures hold true for this water-trihydric alcohol mixtures.

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The mixed longitudinal–transverse nature of collective modes in water

Cited by 31 publications

References 30 publications

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

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

Shear propagation in the terahertz dynamics of water-glycerol mixtures

Inelastic x-ray scattering measurements of liquid waterglycerol mixtures

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