Roman Melnyk

Liquid/vapor coexistence and surface tension of the Sutherland fluid with a variable range of interaction: computer simulation and perturbation theory studies.

The liquid-vapor phase diagram and surface tension for the Sutherland fluids with a variable range of interaction have been determined by canonical Monte Carlo simulations and compared with the augmented van der Waals analytic theory. The theory, based on a short-range Yukawa reference, performs well for medium-range models but its accuracy deteriorates with the shortening range due to deteriorating accuracy of the underlined analytic mean spherical approximation solution for the reference Yukawa potential. The simulation results are also analyzed from the point of the extended principle of corresponding states and it is shown that the surface tension and also, to a certain degree of accuracy, the vapor-liquid coexistence curve satisfy the principle.

EXP6 fluids at extreme conditions modeled by two-Yukawa potentials

A two-Yukawa representation of the EXP6 fluids at supercritical temperatures and high pressures has been developed and examined using molecular simulations. A uniquely defined mapping of the repulsive part of the EXP6 potential curve onto the two-Yukawa potential is used. Two ranges of temperatures, one encountered in geochemical applications (T(geo) range) and the other at conditions of detonations (T(det) range), are considered and it is shown that the local structures of both fluids are practically identical. Deviations between the EXP6 and two-Yukawa potential functions at intermediate separations lead to differences in the thermodynamic properties of the two fluids at lower temperatures of the T(geo) range; at higher temperatures and in the high T(det) temperature range both the structural and thermodynamic properties of the EXP6 and two-Yukawa fluids are practically identical.

Vapour/liquid coexistence in long-range Yukawa fluids determined by means of an augmented van der Waals approach

The vapour–liquid phase diagram of long-range Yukawa fluids with the decay parameter zσ ≤ 1 is calculated by means of an augmented van der Waals theory based on the reference system that includes both the repulsive and short-range attractive interaction. Comparison with Monte Carlo simulation data, as well as with the traditional van der Waals approach based on a purely repulsive hard-sphere reference system and other semi-analytical theoretical approaches, is made.

Mean-spherical approximation for the Lennard-Jones-like two Yukawa model: Comparison against Monte Carlo data

Monte Carlo simulation studies are performed for the Lennard-Jones-like two Yukawa (LJ2Y) potential to show how properties of this model fluid depend on the replacement of soft repulsion by hard-core repulsion. Different distances for the positioning of hard-core have been explored. We have found that for temperatures slightly lower and slightly higher than the critical point temperature for the Lennard-Jones fluid, the placement of the hard-core at distances shorter than zero-potential energy is well justified by thermodynamic properties that are practically the same as in the original LJ2Y model without hard-core. However, going to extreme conditions with the high temperature one should be careful since the presence of the hard-core provokes changes in the properties of the system. The later is extremely important when the mean-spherical approximation (MSA) theory is applied to the treatment of the Lennard-Jones-like fluid.

Structure factor of a hard-core fluid with short-range Yukawa attraction: analytical FMSA theory against Monte Carlo simulations

ABSTRACT Analytical solution of the first-order Ornstein–Zernike equation known as the first-order mean spherical approximation (FMSA) theory due to Tang and Lu [J. Chem. Phys. 99, 9828 (1993)] is used to write down a closed equation for the static structure factor of the hard-sphere fluid with a short-range Yukawa attraction. Calculations are performed for a Yukawa decay exponent that corresponds to a range of attraction that does not exceed the first coordination shell of Lennard-Jones-like simple fluids. By comparison with Monte Carlo simulation data it is shown that the analytical FMSA equation for the static structure factor is of the same or even of superior accuracy as that within the seminumerical mean spherical approximation theory.

Photoluminescence Spectroscopy of Neutron-Irradiated Cubic SiC Crystals

Photoluminescence (PL) spectroscopy has been used to characterize neutron-irradiated cubic silicon carbide crystals. The effects of thermal annealing (600-1100OC) on the PL bands have been studied. Several PL bands consisting of a sharp line and its phonon replicas have been observed in the 9-80 K temperature range. Certain of them like the D1 spectrum doublet with 1.975 eV and 1.977 eV zero-phonon lines (ZPL) at 9 K and the L2 spectrum with ZPL at 1.121 eV were reported previously for ion-implanted and electron irradiated 3C-SiC crystals, respectively. Besides, some new bands with ZPL at 2.027, 1.594, 0.989 and 0.844 eV and a broad band at 1.360 eV have been found. A correlation of PL and EPR spectra intensities of these neutron-irradiated and annealed cubic SiC crystals is briefly discussed.

An EPR Study of Defects in Neutron-Irradiated Cubic SiC Crystals

EPR spectroscopy has been used to characterize neutron-irradiated cubic SiC samples after thermal annealing in the 200-1100OC temperature range. Three new paramagnetic defects named Ky6, Ky7 and Ky8 have been revealed. Based on the present results, these defects have been tentatively attributed to the negatively charged carbon vacancy-carbon antisite pair, negatively charged divacancy and neutral carbon <100> split interstitial, respectively. Furthermore, the finding of practically isotropic hyperfine splitting for EPR lines of the T6 center confirms its assignment as a carbon vacancy-interstitial pair.