Aleksei G Khrapak

There is a revamped interest in the transport properties of quasi-free electrons in dense and cold hydrogen gas because it has recently been shown that multiple scattering effects modify their drift mobility in the same way as they do in helium and neon gases. Owing to the similarities in the electron-atom/molecule scattering cross sections, there is the possibility that also in hydrogen, electrons self-localize in bubbles as they do in dense gaseous helium and neon and in their liquids. A very scant number of experiments suggest that this may happen. In this paper, we investigate this possibility by numerically carrying out the prediction of the optimum fluctuation model. We show that electron self-localization is a very likely process and that the model accurately predicts the density at which quasi-free electrons and electron bubbles coexist in equal proportions, as inferred from the experiments.

The purpose of this paper is to discuss to which extent a microscopic version of the Stokes–Einstein (SE) relation without the hydrodynamic radius applies to liquid water. We demonstrate that the self-diffusion and shear viscosity data for five popular water models, recently reported by Ando [J. Chem. Phys. 159, 101102 (2023)], are in excellent agreement with the SE relation. The agreement with experimental results is also quite impressive. The limitations on the applicability of the SE relation are briefly discussed.

The original Rosenfeld–Tarazona (RT) scaling of the excess energy in simple dense fluids predicts a ∝T3/5 thermal correction to the fluid Madelung energy. This implies that the excess isochoric heat capacity scales as Cvex∝T−2/5. Careful examination performed in this paper demonstrates that the exponent −2/5 is not always optimal. For instance, in the Lennard-Jones fluid in some vicinity of the triple point, the exponent −1/3 turns out to be more appropriate. The analysis of the specific heat data in neon, argon, krypton, xenon, and liquid mercury reveals that no single value of the exponent exists, describing all the data simultaneously. Therefore, we propose a generalized RT scaling in the form Cvex∝T−α, where α is a density- and material-dependent adjustable parameter. The question concerning which material properties and parameters affect the exponent α and whether it can be predicted from general physical arguments requires further investigation.

We present the results of a new analysis of the literature data on electron mobility μ in dense helium gas aimed at determining the existence of a threshold density for electron self-trapping in gaseous helium as a function of temperature. We have investigated the density dependence of μ and, when available, its dependence on the electric field. The experimental data are favorably rationalized by minimizing the excess free energy of the self-localized states within the optimum fluctuation model. It is shown that the formation of electron bubbles via the self-trapping phenomenon is determined by the delicate balance between the electron thermal energy, the density dependence of the electron energy at the bottom of the conduction band in the gas, and the work necessary to expand the bubble. We show that the self-trapping phenomenon is not limited to low temperatures but occurs at any temperatures for large enough densities.

It is shown that the transport coefficients (self-diffusion, shear viscosity, and thermal conductivity) of the Weeks–Chandler–Andersen (WCA) fluid along isotherms exhibit a freezing density scaling (FDS). The functional form of this FDS is essentially the same or closely related to those in the Lennard-Jones fluid, hard-sphere fluid, and some liquefied noble gases. This proves that this FDS represents a quasi-universal corresponding state principle for simple classical fluids with steep interactions. Some related aspects, such as a Stokes–Einstein relation without a hydrodynamic diameter and gas-to-liquid dynamical crossover, are briefly discussed. Simple fitting formulas for the transport coefficients of the dense WCA fluid are suggested.

Structural and dynamical properties of a bilayer binary strongly coupled dusty plasma system in a sheath of an rf discharge are experimentally investigated. Microparticles of two different sizes are aligned into vertical pairs due to ion focusing (plasma wakes) downstream from the upper particles in the ion flow toward the bottom electrode. The pairs are ordered into a 2D hexagonal crystalline structure. The collective mode dispersion relations are measured and analyzed. Splitting of the modes in the bilayer lattice is observed and theoretically described. The relative intensities of the mode branches reflect non-reciprocal features of the wake-mediated interaction between the microparticles. The conditions of stability of vertically aligned bilayer structure are discussed, and relevant plasma parameters are estimated.

We present systematic investigation of sound velocities in various fluids at the fluid–solid phase transition. First, theoretical estimates indicating that quasi-universal values of sound velocities at freezing can be expected are presented. Then, this prediction is verified on three model systems with quite different interactions (inverse power law, screened Coulomb, and Lennard-Jones pairwise potentials) and 15 real atomic and molecular liquids. It is documented that the ratio of the sound velocity to the thermal velocity tends to a quasi-universal value (cs/vT∼10) in many systems considered, but exceptions also exist. In particular, extremely soft interactions can result in indefinitely large ratios cs/vT. Complex hydrocarbon liquids also demonstrate high ratios cs/vT. On the other hand, liquids composed of light elements, such as hydrogen and neon, demonstrate lower ratios cs/vT. For model systems, we discuss relations between the thermodynamic sound velocity and instantaneous longitudinal, transverse, and bulk sound velocities. It is found that these relations are greatly affected by the potential softness. Finally, correlations between the thermal conductivity coefficient and the sound velocity are briefly discussed in the context of Bridgman's formula.

It is demonstrated that self-diffusion and shear viscosity data for the TIP4P/Ice water model reported recently [Baran et al., J. Chem. Phys. 158, 064503 (2023)] obey the microscopic version of the Stokes–Einstein relation without the hydrodynamic diameter.