107 research outputs found
Generalised hydrodynamic description of the time correlation functions of liquid metals: ab initio molecular dynamics study
No full textA new methodology of calculation of dispersion and damping of collective excitations from ab initio molecular dynamics (AIMD) of liquid metals is proposed. It is suggested to use for the analysis of AIMD-derived time correlation functions a thermo-viscoelastic dynamic model within an approach of the generalised collective modes. The proposed scheme, in which the viscoelastic matrix elements of the generalised hydrodynamic matrix are directly calculated from AIMD and the matrix elements requiring knowledge of energy density fluctuations in the system are treated as fitting parameters, allows to recover AIMD time correlation functions in a wide range of wave numbers. The dispersion and damping of collective excitations are obtained as a complex-conjugated pair of the generalised hydrodynamic matrix. An issue of sum rules in the proposed scheme is discussed. The methodology is applied for calculations of dispersion and damping of generalised acoustic modes in molten lithium
First-principles computer modeling and statistical theory in dynamics of liquids. On 60-th anniversary of Taras Bryk
No full textIn July 2023, our good friend and colleague Taras Bryk turns 60 years old. Today he is one of the leading experts in Ukraine and throughout the world in the field of first-principles and atomistic modeling of liquid metals and alloys, especially their dynamic properties and behavior under extreme conditions, and his scientific works are well cited and known among experts in the field. Since 2021, Taras Bryk is Director of the Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine
Taras Bryk’s 50th birthday
No full textOur dear friend and colleague, Taras Bryk, turned 50. Our warmest congratulations and best wishes of staying in good health, prosperity and new achievements
Taras Bryk’s 50th birthday
No full textOur dear friend and colleague, Taras Bryk, turned 50. Our warmest congratulations and best wishes of staying in good health, prosperity and new achievements
Landau-Placzek ratio for heat density dynamics and its application to heat capacity of liquids
No full textExact relation for contributions to heat capacity of liquids is obtained from hydrodynamic theory. It is shown from analysis of the long-wavelength limit of heat density autocorrelation functions that the heat capacity of simple liquids is represented as a sum of two contributions due to "phonon-like" collective excitations and heat relaxation. The ratio of both contributions being the analogy of Landau-Placzek ratio for heat processes depends on the specific heats ratio. The theory of heat density autocorrelation functions in liquids is verified by computer simulations. Molecular dynamics simulations for six liquids having the ratio of specific heats. in the range 1.1-2.3, were used for evaluation of the heat density autocorrelation functions and predicted Landau-Placzek ratio for heat processes. The dependence of contributions from collective excitations and heat relaxation process to specific heat on. is shown to be in excellent agreement with the theory. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4774406
Crossover between hydrodynamic and kinetic modes in binary liquid alloys
No full textInelastic x- ray scattering measurements of the dynamic structure factor in liquid Na(57)K(43), which is sensitive to the atomic- scale coarse graining, reveal a sound velocity value exceeding the long wavelength continuum value and indicate the coexistence of two phononlike modes. By applying generalized collective mode analysis scheme, we show that the positive dispersion of the sound velocity occurs in a wavelength region below the crossover from hydrodynamic to atom- type excitations and, therefore, it cannot be explained as sound propagation over the light species ( Na ) network. The present result experimentally proves the existence of positive dispersion in a binary mixture due to a relaxation process, as opposed to fast sound phenomena
Analyzing the dynamic structure of liquid metals and alloys
No full textExperimental and numerical improvements have stimulated a great interest in the dynamic structure of liquids during the last decades. Many unexpected features have been unveiled among which fast sound, positive dispersion and possible coupling between transverse and longitudinal excitations can be mentioned. Models used to analyze these data have to be sound and more and more rigorous. In this study, we discuss the capability of a recently proposed fitting scheme (Wax J.-F. and Bryk T. J. Phys.: Condens. Matter 25 325104 (2013); 26 168002 (2014). Wax J.-F., Johnson M.R., and Bryk T. J. Phys.: Condens. Matter 28 185102 (2016).) to interpret these features of the dynamic structure of liquid metals and alloys
Analyzing the dynamic structure of liquid metals and alloys
No full textExperimental and numerical improvements have stimulated a great interest in the dynamic structure of liquids during the last decades. Many unexpected features have been unveiled among which fast sound, positive dispersion and possible coupling between transverse and longitudinal excitations can be mentioned. Models used to analyze these data have to be sound and more and more rigorous. In this study, we discuss the capability of a recently proposed fitting scheme (Wax J.-F. and Bryk T. J. Phys.: Condens. Matter 25 325104 (2013); 26 168002 (2014). Wax J.-F., Johnson M.R., and Bryk T. J. Phys.: Condens. Matter 28 185102 (2016).) to interpret these features of the dynamic structure of liquid metals and alloys
Collective excitations in supercritical fluids: Analytical and molecular dynamics study of "positive" and "negative" dispersion
No full textThe approach of generalized collective modes is applied to the study of dispersion curves of collective excitations along isothermal lines of supercritical pure Lennard-Jones fluid. An effect of structural relaxation and other nonhydrodynamic relaxation processes on the dispersion law is discussed. A simple analytical expression for the dispersion law in the long-wavelength region of acoustic excitations is obtained within a three-variable viscoelastic model of generalized hydrodynamics. It is shown that the deviation from the linear dependence in the long-wavelength region can be either "positive" or "negative" depending on the ratio between the high-frequency (elastic) and isothermal speed of sound. An effect of thermal fluctuations on positive and negative dispersion is estimated from the analytical solution of a five-variable thermoviscoelastic model that generalizes the results of the viscoelastic treatment. Numerical results are reported for a Lennard-Jones supercritical fluid along two isothermal lines T* = 1.71, 4.78 with different densities and discussed along the theoretical expressions derived. (C) 2010 American Institute of Physics. [doi:10.1063/1.3442412
Comment on “Collective modes and gapped momentum states in liquid Ga. Experiment, theory, and simulation”
No full textWe show that the theoretical expressions presented in Khusnutdinoff et al. [Phys. Rev. B 101, 214312 (2020)] for the longitudinal current spectral function C-l(k, omega) and the dispersion of collective excitations are not correct. Indeed, they are not compatible with the continuum limit and C-L (k, omega -> 0) contradicts the continuity equation
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