Demchuk T. Features of one-particle and collective dynamics in metal melts at ambient and high pressures

The thesis is devoted to the study of single-particle and collective dynamics in liquid metals at normal and high pressures via computer simulations within ab initio molecular dynamics. A number of metals in the liquid state, namely, the molten Tl at normal pressure near the melting temperature, liquid Pb, In and Al along the melting lines, and liquid Si and Na along some isotherms are investigated by first-principle computer modelling. In the methods of ab initio molecular dynamics the electron density is considered explicitly within the density functional theory. For each ionic configuration, one estimates one-electron wave functions and the electron density from them, which allows one to calculate the Hellman-Feynman forces acting on the ions. This approach gives more accurate results for metallic systems due to account for the instantaneous configurations of ions and electron density. The advantages of application ab initio computer modelling are especially noticeable in the study of liquid metals at high pressures. Based on the data obtained from the computer experiment, the features of the collective dynamics, its correlation with single-particle one, the features of the structure and their effect on the dynamics in considered melts are investigated. From the trajectories and velocities of the particles determined during the modelling, current-current and density-density time correlation functions were calculated. The latter are related to the dynamic structure factor that can be measured by inelastic X-ray or neutron scattering experiments. It is found that the frequency spectrum of the correlation function of the transverse current shows the presence of two peaks for all investigated melts with different thermodynamic conditions. Besides, this effect is observed only at large wave numbers, outside the first pseudo-Brillouin zone. The study of the presence of intramolecular bonds in liquid Pb along the melting curve in the pressure range 0-70 GPa via the common neighbours analysis showed that there are various structure clusters in the melt with similar to the crystalline short-range structures. Analysis of the topological structure of such clusters shows that different predominant configuration emerges in system at different pressures. Moreover, this predominant configuration corresponds to the structure of crystalline Pb at the appropriate pressure. It was found that almost 40% of all detected structural clusters have the icosahedral configuration appropriate to liquid and glass phases. Liquid Si was investigated in the pressure range of 10.2-27.5 GPa. Pair distribution functions for liquid Si along the isotherm at different pressures were calculated. Curves show the constant value of the mean distance between the two nearest neighbours. Same result was previously obtained in the study of the Si melt along the melting curve in the pressure range of 4-23 GPa by X-ray diffraction. Besides, the calculated static structural factors for all pressures demonstrate the presence of the shoulder near the first maximum. Such data indicate the presence of covalent bonds between Si atoms and partially tetrahedral ordering in a wide range of pressures. The study of the bond-angle distribution functions showed that there is a tetrahedral ordering in molten Si, which decreases with increasing pressure, while the icosahedral ordering increases with increasing pressure. The frequency spectra of the autocorrelation velocity function were calculated in order to study the features of the single-particle dynamics of the investigated melts. It was found that such spectra may contain two peaks simultaneously with the presence of two branches in the transverse collective excitations dispersion. In addition, the temperature in the system does not make an effect on the peak maxima positions. The analysis of single-particle and collective modes frequencies showed that for all investigated metals with different thermodynamic conditions the positions of the maxima on the frequency spectrum of the velocity autocorrelation function coincide with the characteristic frequencies of transverse collective modes. This result indicates a direct manifestation of the transverse collective dynamics in the single-particle dynamics of liquid metals and contradicts with the previous hypotheses, where one peak was considered as a consequence of longitudinal modes and the second as a consequence of transverse modes. The study of the collective dynamics of liquid metals at different thermodynamic points revealed that the value of the characteristic frequency of the high-frequency transverse collective modes dispersion branch increases linearly with increasing density of the system. It is shown that such dependence is universal for all metallic melts. Moreover, it was found that the slope of such a linear dependence is the same for all polyvalent metals.