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

Simulation study of the collective excitations in liquid sodium under high pressure

The dynamic structure of liquid sodium is investigated using classical molecular dynamics simulations over a wide range of densities (from 739 to 4177 kg m^-3). The interactions are described using screened pseudopotential formalism with Fiolhais model of electron-ion interaction. The effective pair potentials obtained are validated by comparing the predicted static structure, coordination number, self-diffusion coefficients and spectral density of the velocity autocorrelation function with results fromab initiosimulations at the same state points. Both longitudinal and transverse collective excitations are computed from the corresponding structure functions and their evolution with density is investigated. The frequency of the longitudinal excitations increases with density, as well as the sound speed, which is extracted from their dispersion curves. The frequency of the transverse excitations also increases with density, but they cannot propagate over macroscopic distances and the propagation gap clearly appears. The values of the viscosity, which are extracted from these transverse functions are in good agreement with available results computed from stress autocorrelation functions.

Quasi-bound atoms in collective dynamics of liquid Sb

We report anab initiosimulation and theoretical study of collective dynamics in liquid Sb at 973 K. An application of the GCM (generalized collective modes) theoretical approach to analysis of simulation-derived time correlation functions resulted in two types of propagating eigenmodes. We found that the almost flat dispersion of the high-frequency branch of propagating modes can be explained by out-of-phase oscillations of nearest neighbors which form quasi-bound atomic pairs for at least 30 ps. We discuss the features of collective dynamics in non-simple metallic melts containing quasi-bound pairs.

Low energy excitation in liquid Sb and liquid Bi observed in inelastic x-ray scattering spectra

The dynamic structure factorS(Q,E), whereQandEare momentum and energy transfer, respectively, has been measured for liquid Sb, using inelastic x-ray scattering. A modified damped harmonic oscillator model function was applied to analyseS(Q,E) of liquid Sb and also to that of liquid Bi by Inuiet al(2015Phys. Rev.B92, 054206). The obtained excitation energy was in fairly good agreement with that predicted byab initiomolecular dynamics simulations on these liquid semi-metals. The excitation energy of the longitudinal acoustic mode in liquid Sb and liquid Bi exhibits flat-toppedQdependence whereas the lower excitation energy below the longitudinal acoustic excitation showsQ-gap behaviour. From the viscosity estimated from theQ-gap experimentally obtained, it is inferred that the lower energy excitation arises from the transverse acoustic excitation in the liquids.

Transverse acoustic phonon excitations in liquid metals

In this article, transverse acoustic (TA) phonon excitations in various liquid metals are reviewed. Low-frequency TA phonon excitations were investigated on liquid Ga, Sn, Fe, Cu, Zn, and Bi, by using high energy resolution inelastic X-ray scattering (IXS). The current correlation functions obtained from the IXS spectra were analyzed by using two Gaussians to evaluate the excitation energies and widths of the longitudinal acoustic and TA excitation modes, which are consistent with ab initio molecular dynamics simulations. The lifetimes and propagating lengths of the TA modes were determined, and may respectively correspond to the lifetimes and sizes of cages instantaneously formed in each liquid metal. The microscopic elastic constants were estimated and characteristic differences from macroscopic polycrystalline values were found in Poisson’s ratios. Future experimental and analytical problems to be solved in the near future are addressed.

Transverse acoustic phonon excitations in liquid metals

In this article, transverse acoustic (TA) phonon excitations in various liquid metals are reviewed. Low-frequency TA phonon excitations were investigated on liquid Ga, Sn, Fe, Cu, Zn, and Bi, by using high energy resolution inelastic X-ray scattering (IXS). The current correlation functions obtained from the IXS spectra were analyzed by using two Gaussians to evaluate the excitation energies and widths of the longitudinal acoustic and TA excitation modes, which are consistent with ab initio molecular dynamics simulations. The lifetimes and propagating lengths of the TA modes were determined, and may respectively correspond to the lifetimes and sizes of cages instantaneously formed in each liquid metal. The microscopic elastic constants were estimated and characteristic differences from macroscopic polycrystalline values were found in Poisson’s ratios. Future experimental and analytical problems to be solved in the near future are addressed.

Anticrossing of Longitudinal and Transverse Modes in Simple Fluids

If interacting modes of the same symmetry cross, they repel from each other and become hybridized. This phenomenon is called anticrossing and is well-known for mechanical oscillations, electromagnetic circuits, waveguides, metamaterials, polaritons, and phonons in crystals, but it still remains poorly understood in simple fluids. Here, we show that structural disorder and anharmonicity, governing properties of fluids, lead to the anticrossing of longitudinal and transverse modes, which is accompanied by their hybridization and strong redistribution of excitation spectra. We combined theory and simulations for noble gases to prove the reliability of mode anticrossing in simple fluids, studied here for the first time. Our results open novel prospects in understanding collective dynamics, thermodynamics, and transport phenomena in various fluids, spanning from noble gas fluids and metallic melts to strongly coupled plasmas and molecular and complex fluids.

Excitation spectra in fluids: How to analyze them properly

Although the understanding of excitation spectra in fluids is of great importance, it is still unclear how different methods of spectral analysis agree with each other and which of them is suitable in a wide range of parameters. Here, we show that the problem can be solved using a two-oscillator model to analyze total velocity current spectra, while other considered methods, including analysis of the spectral maxima and single mode analysis, yield rough results and become unsuitable at high temperatures and wavenumbers. To prove this, we perform molecular dynamics (MD) simulations and calculate excitation spectra in Lennard-Jones and inverse-power-law fluids at different temperatures, both in 3D and 2D cases. Then, we analyze relations between thermodynamic and dynamic features of fluids at (Frenkel) crossover from a liquid- to gas-like state and find that they agree with each other in the 3D case and strongly disagree in 2D systems due to enhanced anharmonicity effects. The results provide a significant advance in methods for detail analysis of collective fluid dynamics spanning fields from soft condensed matter to strongly coupled plasmas.

Static and dynamic structures of liquid Ba8Ga16Sn30: a melt of the thermoelectric clathrate compounds

X-ray diffraction and inelastic x-ray scattering measurements of liquid Ba₈Ga₁₆Sn₃₀ have been carried out to investigate local structure and atomic dynamics in the liquid. The pair distribution function shows shorter and longer interatomic distances in the first coordination shell. The dynamic structure factor exhibits the inelastic excitations on both sides of the quasielastic central peak. The inelastic excitations disperse with increasing the momentum transfer, suggesting the longitudinal acoustic mode. We found a low energy excitation in addition to the longitudinal acoustic excitation in the dynamic structure factor and it reminds us a strong relationship with a rattling motion of a guest (Ba) atom in the solid state. The temperature dependence of the pair distribution function and the longitudinal acoustic excitation energy is very weak in a range from 600 to 900 °C. The result suggests that Ba and other atoms in the melt are located around minimum positions of the effective pair potential approximated as a harmonic one.