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The Young's modulus $E$ is the key mechanical property that determines the resistance of solids to tension/compression. In the present work, the correlation of the quantity $E$ with such characteristics as the total molar mass $M$ of alloy components, the number of components $n$ forming an alloy, the yield stress $\sigma_{y}$ and the glass transition temperature $T_{g}$ has been studied in detail based on a large set of empirical data for the Young's modulus of different amorphous metal alloys. It has been established that the values of the Young's modulus of metal alloys under normal conditions correlate with such a mechanical characteristic as the yield stress as well as with the glass transition temperature. As found, the specificity of the ``chemical formula'' of alloy, which is determined by molar mass $M$ and number of components $n$, does not affect on elasticity of the material. The machine learning algorithm identified both the quantities $M$ and $n$ as insignificant factors in determining $E$. A simple non-linear regression model is obtained that relates the Young's modulus with $T_{g}$ and $\sigma_{y}$, and this model correctly reproduces the experimental data for metal alloys of different types. This obtained regression model generalizes the previously presented empirical relation $E\simeq49.8\sigma_{y}$ for amorphous metal alloys., Comment: 14 pages, 5 figures

The Arrhenius crossover temperature, $T_{A}$, corresponds to a thermodynamic state wherein the atomistic dynamics of a liquid becomes heterogeneous and cooperative; and the activation barrier of diffusion dynamics becomes temperature-dependent at temperatures below $T_{A}$. The theoretical estimation of this temperature is difficult for some types of materials, especially silicates and borates. In these materials, self-diffusion as a function of the temperature $T$ is reproduced by the Arrhenius law, where the activation barrier practically independent on the temperature $T$. The purpose of the present work was to establish the relationship between the Arrhenius crossover temperature $T_{A}$ and the physical properties of liquids directly related to their glass-forming ability. Using a machine learning model, the crossover temperature $T_{A}$ was calculated for silicates, borates, organic compounds and metal melts of various compositions. The empirical values of the glass transition temperature $T_{g}$, the melting temperature $T_{m}$, the ratio of these temperatures $T_{g}/T_{m}$ and the fragility index $m$ were applied as input parameters. It has been established that the temperatures $T_{g}$ and $T_{m}$ are significant parameters, whereas their ratio $T_{g}/T_{m}$ and the fragility index $m$ do not correlate much with the temperature $T_{A}$. An important result of the present work is the analytical equation relating the temperatures $T_{g}$, $T_{m}$ and $T_{A}$, and that, from the algebraic point of view, is the equation for a second-order curved surface. It was shown that this equation allows one to correctly estimate the temperature $T_{A}$ for a large class of materials, regardless of their compositions and glass-forming abilities., Comment: 17 pages, 5 figures

Alloys based on TiNi are widely used in various fields of technology and medicine. In the present work, we report on preparation of TiNi-alloy based wire with the shape memory effect, which was used for compression clips for surgery. Composition and structure of the wire, and its martensitic and physical-chemical properties were studied using SEM, TEM, optic microscopy, profilometry, mechanical tests, etc. The TiNi alloy was found to consist of B2 and B19' and secondary-phase particles of Ti2Ni, TiNi3 and Ti3Ni4. Its matrix was slightly enriched in Ni (50.3 at. % of Ni). Homogeneous grain structure was revealed (an average grain size of 19±0.3 μm) with equal quantities of grain boundaries of special and general types. Surface oxide layer provide improved biocompatibility and promote adhesion of protein molecules. Overall, the obtained TiNi wire was concluded to exhibit martensitic, physical and mechanical properties suitable for its use as an implant material. The wire was then used for manufacturing compression clips with the shape memory effect and applied in surgery. The medical experiment that involved 46 children demonstrated that the use of such clips in children with double-barreled enterostomies permitted to improve the results of surgical treatment.

In the framework of the concept of time correlation functions, we develop a self-consistent relaxation theory of the transverse collective particle dynamics in liquids. The theory agrees with well-known results in both the short-wave (free-particle dynamics) and the long-wave (hydrodynamic) limits. We obtain a general expression for the spectral density $$C_{ \mathrm{T} }(k,\omega)$$ of the transverse particle current realized in a range of wave numbers $$k$$ . In the domain of microscopic spatial scales comparable to the action range of effective forces of interparticle interaction, the theory reproduces a transition from a regime with typical equilibrium liquid dynamics to a regime with collective particle dynamics where properties similar to solid-state properties appear: effective shear stiffness and transverse (shear) acoustic waves. In the framework of the corresponding approximations, we obtain expressions for the spectral density of transverse particle current for all characteristic regimes in equilibrium collective dynamics. We obtain expressions for the dispersion law for transverse (shear) acoustic waves and also relations for the kinematic shear viscosity $$\nu$$ , the transverse speed of sound $$v^{( \mathrm{T} )}$$ , and the corresponding sound damping coefficient $$\Gamma^{( \mathrm{T} )}$$ . We compare the theoretical results with the results of atomistic dynamics simulations of liquid lithium near the melting point.

В рамках концепции временны́х корреляционных функций развивается самосогласованная релаксационная теория поперечной коллективной динамики частиц в жидкостях. Теория согласуется с хорошо известными результатами как для коротковолнового предела (динамика свободно движущейся частицы), так и для длинноволнового (гидродинамического) предела. Получено общее выражение для спектральной плотности $C_\mathrm{T}(k,\omega)$ поперечного потока частиц, реализуемое в диапазоне волновых чисел $k$. В области микроскопических пространственных масштабов, сопоставимых с масштабом действия эффективных сил межчастичного взаимодействия, теория воспроизводит переход от режима с типичной равновесной жидкостной динамикой к режиму с коллективной динамикой частиц, в которой проявляются свойства, подобные свойствам твердых тел: эффективная сдвиговая жесткость, поперечные (сдвиговые) акустические волны. В рамках соответствующих приближений получены выражения для спектральной плотности поперечного потока частиц для всех характерных режимов в равновесной коллективной динамике. Получено выражение для закона дисперсии поперечных (сдвиговых) акустических волн, а также соотношения для кинематической сдвиговой вязкости $\nu$, скорости поперечного звука $v^{(\mathrm{T})}$ и соответствующего коэффициента затухания звука $\Gamma^{(\mathrm{T})}$. Теоретические результаты сопоставляются с результатами моделирования атомарной динамики жидкого лития вблизи точки плавления.

This study presents the approach to analyzing the evolution of an arbitrary complex system whose behavior is characterized by a set of different time-dependent factors. The key requirement for these factors is only that they must contain an information about the system; it does not matter at all what the nature (physical, biological, social, economic, etc.) of a complex system is. Within the framework of the presented theoretical approach, the problem of searching for non-linear regression models that express the relationship between these factors for a complex system under study is solved. It will be shown that this problem can be solved using the methodology of \emph{genetic (evolutionary)} algorithms. The resulting regression models make it possible to predict the most probable evolution of the considered system, as well as to determine the significance of some factors and, thereby, to formulate some recommendations to drive by this system. It will be shown that the presented theoretical approach can be used to analyze the data (information) characterizing the educational process in the discipline "Physics" in the secondary school, and to develop the strategies for improving academic performance in this discipline., Comment: 8 pages, 3 figures, 1 table

The elastic properties of the Zr50Cu40Ag10 metallic alloy, such as the bulk modulus B, the shear modulus G, the Young’s modulus E and the Poisson’s ratio σ, are investigated by molecular dynamics simulation in the temperature range T=250–2000 K and at an external pressure of p=1.0 bar. It is shown that the liquid–glass transition is accompanied by a considerable increase in the shear modulus G and the Young’s modulus E (by more than 50%). The temperature dependence of the Poisson’s ratio exhibits a sharp fall from typical values for metals of approximately 0.32–0.33 to low values (close to zero), which are characteristic for brittle bulk metallic glasses. Non-monotonic temperature dependence of the longitudinal and transverse sound velocity near the liquid-glass transition is also observed. The glass forming ability of the alloy is evaluated in terms of the fragility index m. Its value is m≈64 for the Zr50Cu40Ag10 metallic glass, which is in a good agreement with the experimental data for the Zr-based metallic glasses.

Titanium nickelide (nitinol) is of great applied interest in various industries due to unique combination of its physical and mechanical characteristics. In the present work, we consider the possibility of obtaining nitinol with mesoporous structure by rapidly cooling the molten sample to room temperature. Based on molecular dynamics simulation data, it was shown that the rapid cooling of the nitinol melt leads to formation of a porous structure. It was shown that the inner pore wall is formed mainly by titanium atoms, which provide biocompatibility of nitinol. It was found that the porosity of nitinol weakly depends on the cooling rate, while the porosity increases linearly with decreasing melt density.

Amorphous metallic foams are prospective materials due to unique combination of their mechanical and energy-absorption properties. In the present work, atomistic dynamics simulations are performed under isobaric conditions with the pressure $p = 1.0$ atm in order to study how cooling with extremely high rates ($5\times10^{13}$--$5\times10^{14}$ K/s) affects the formation of pores in amorphous titanium nickelide. For equilibrium liquid phase, vaporization temperature $T_{b}$ and the equation of states in the form of $\rho(T)$ are determined. It is found that the porosity of this amorphous solid does not depend on cooling at such high rates, whereas the pore morphology depends on the magnitude of the cooling rate. The obtained results will be in demand in study of mechanical properties of amorphous metallic foams with a nanoporous structure., Comment: 3 pages, 2 figures

Binary Ni$_{62}$Nb$_{38}$ alloy belongs to the unique class of binary off-eutectic systems, which are able to form a bulk glassy state [L. Xia et al., J. Appl. Phys. 99 (2006) 026103]. In the present work, the ($p$, $T$) phase diagram of Ni$_{62}$Nb$_{38}$ alloy was first determined for a wide thermodynamic range with temperatures from $300$\,K to $6000$\,K and with pressures from $1$\,atm to $1.2\times10^7$\,atm. For this thermodynamic range, elements of the phase diagram such as the liquid-crystal coexistence line and the glass transition line are defined. Our results reveal good agreement between the simulation results and the known experimental values of the liquidus temperature and the glass transition temperature for the isobar $p=1$\,atm. The phase diagram is detailed for pressures greater than $1\times10^{7}$\,atm. For the first time, the phase separation conditions at which the liquid Nb and crystalline Ni phases coexist in the system were determined., Comment: 12 pages, 4 figures, 1 table

In this study, we present the original method for reconstructing the potential of interparticle interaction from statistically averaged structural data, namely, the radial distribution function of particles in many-particle system. This method belongs to a family of machine learning methods and is implemented through the differential evolution algorithm. As demonstrated for the case of the Lennard-Jones liquid taken as an example, there is no one-to-one correspondence between structure and potential of interparticle interaction of a many-particle disordered system at a certain thermodynamic state. Namely, a whole family of the Mie potentials determined by two parameters $p_{ 1 }$ and $p_{ 2 }$ related to each other according to a certain rule can reproduce properly a unique structure of the Lennard-Jones liquid at a given thermodynamic state. It is noteworthy that this family of the potentials quite correctly reproduces for the Lennard-Jones liquid the transport properties (in particular, the self-diffusion coefficient) over a temperature range as well as the dynamic structure factor, which is one of the key characteristics of the collective dynamics of particles., Comment: 29 pages, 7 figures

Control of the crystallization process at the microscopic level makes it possible to generate the nanocrystalline samples with the desired structural and morphological properties, that is of great importance for modern industry. In the present work, we study the influence of supercooling on the structure and morphology of the crystalline nuclei arising and growing within a liquid metallic film. The cluster analysis allows us to compute the diffraction patterns and to evaluate the morphological characteristics (the linear sizes of the homogeneous part and the thickness of the surface layer) of the crystalline nuclei emergent in the system at different levels of supercooling. We find that the liquid metallic film at the temperatures corresponded to low supercooling levels crystallizes into a monocrystal, whereas a polycrystalline structure forms at deep supercooling levels. We find that the temperature dependence of critical size of the crystalline nuclei contains two distinguishable regimes with the crossover temperature $T/T_{g}\approx1.15$ ($T_{g}$ is the glass transition temperature), which appears due to the specific geometry of the system., Comment: 12 pages, 7 figures

Kinetic rate factors of crystallization have a direct effect on formation and growth of an ordered solid phase in supercooled liquids and glasses. Using crystallizing Lennard-Jones liquid as an example, in the present work we perform a \textit{direct} quantitative estimation of values of the key crystallization kinetic rate factors -- the rate $g^{+}$ of particle attachments to a crystalline nucleus and the rate $g^{-}$ of particle detachments from a nucleus. We propose a numerical approach, according to which a statistical treatment of the results of molecular dynamics simulations was performed without using any model functions and/or fitting parameters. This approach allows one to accurately estimate the critical nucleus size $n_{c}$. We find that for the growing nuclei, whose sizes are larger than the critical size $n_{c}$, the dependence of these kinetic rate factors on the nucleus size $n$ follows a power law. In the case of the subnucleation regime, when the nuclei are smaller than $n_{c}$, the $n$-dependence of the quantity $g^{+}$ is strongly determined by the inherent microscopic properties of a system and this dependence cannot be described in the framework of any universal law (for example, a power law). It has been established that the dependence of the growth rate of a crystalline nucleus on its size goes into the stationary regime at the sizes $n>3n_{c}$ particles., 18 pages, 5 figures, 1 table

The porous titanium nickelide is very popular in various industries due to unique combination of physical and mechanical properties such as shape memory effect, high corrosion resistance, and biocompatibility. The non-equilibrium molecular dynamics simulation was applied to study the influence of porosity degree on mechanical properties of porous amorphous titanium nickelide at uniaxial tension, uniaxial compression, and uniform shear. We have found that the porous amorphous alloy is characterized by a relatively large value of Young's modulus in comparison to its crystalline analogue. It has been found that the system with a percolated network of pores exhibits improved elastic characteristics associated with resistance to tensile and shear. The system contained isolated spherical pores is more resistant to compression and less resistant to tensile and shear. These results can be applied to develop and improve the methods for making amorphous metal foams., Comment: 23 pages, 7 figures, 2 tables

The paper presents the results of molecular dynamics study of the viscosity of nickel-containing binary metal melts for a wide range of temperatures, including the region of the equilibrium liquid phase and supercooled melt. It is shown that the temperature dependencies of the viscosity of binary metal melts are described by the Kelton's quasi-universal model. Based on the analysis of the viscosity coefficient of the binary melt composition within the framework of the Rosenfeld's scale transfor\-mations, it has been established that to correctly describe the viscosity of binary/multi\-component metal melts within the framework of entropy models, it is necessary to use a more complex representation of the excess entropy $S_{ex}$ than in the approximation of pair correlation entropy $S_2$., 7 pages, 3 figures

Collective excitations in liquids are important for understanding liquid dynamical and thermodynamic properties. Gapped momentum states (GMS) are a notable feature of liquid dynamics predicted to operate in the transverse sector of collective excitations. Here, we combine inelastic neutron scattering experiments, theory and molecular dynamics modelling to study collective excitations and GMS in liquid Ga in a wide range of temperature and $k$-points. We find that all three lines of enquiry agree for the longitudinal sector of liquid dynamics. In the transverse sector, the experiments agree with theory, modelling as well as earlier X-ray experiments at larger $k$, whereas theory and modelling agree in a wide range of temperature and $k$-points. We observe the emergence and development of the $k$-gap in the transverse sector which increases with temperature and inverse of relaxation time as predicted theoretically., 11 pages, 8 figures Reference to theoretical equations used in version 1 (arXiv:2005.00470v1) is corrected

The paper discusses the features of supercooled water thin film of width $d=3.97$~nm contained by the perfect graphene layers and crystallizing under external stationary electric field. It was found that the electric field applied perpendicular to graphene layers impedes structural ordering, while the electric field applied in lateral direction contributes to formation of the cubic ice ($Ic$) phase, which is thermodynamically less stable compared to the hexagonal ice ($Ih$) phase. It is shown that the growth of the $Ic$ crystalline phase occurs without formation of intermediate crystalline phases. It was found that the crystallization rate depends strongly on the magnitude of the applied electric field. In particular, the processes of full electrocrystallization of the system do not appear over simulation time scale ($\sim 40$~ns) if the electric field of the magnitude less than $0.07~\rm{V/\AA}$ is applied., Comment: 3 pages, 4 figures

Polyvalent metal melts (gallium, tin, bismuth, etc.) have microscopic structural features, which are detected by neutron and X-ray diffraction and which are absent in simple liquids. Based on neutron and X-ray diffraction data and results of \textit{ab initio} molecular dynamics simulations for liquid gallium, we examine the structure of this liquid metal at atomistic level. Time-resolved cluster analysis allows one to reveal that the short-range structural order in liquid gallium is determined by a range of the correlation lengths. This analysis performed over set of independent samples corresponding to equilibrium liquid phase discloses that there are no stable crystalline domains as well as molecule-like Ga$_2$ dimers typical for crystal phases of gallium. Structure of liquid gallium can be reproduced by the simplified model of the close-packed system of soft quasi-spheres. The results can be applied to analyze the fine structure of other polyvalent liquid metals., 10 pages, 7 figures, supplementary information

Crystallization kinetics has features that are universal and independent of the type of crystallized system. The possibility of using scaling relations to describe the temperature dependences of the surface self-diffusion coefficient $D_s$, which is one of the key characteristics of crystallization kinetics, has been demonstrated in application to various crystallized molecular glasses. It has been shown that the surface self-diffusion coefficient $D_s$ as a function of the dimensionless temperature is reproduced by a power law and is universally scaled for all considered systems. The analysis of experimental data has revealed a correlation between the crystallization kinetic characteristics, index of fragility, and criterion of the glass-forming ability of a liquid. It has been shown that this correlation can be obtained within the generalized Einstein-Stokes relation., 6 pages, 3 figures

Features of the crystallization kinetics define directly the rate characteristics: the crystal nucleation rate, the crystal growth rate and the so-called kinetic rate factor known also as the attachment rate (of particles to the surface of a crystalline nucleus). We show that the kinetic rate factor as function of the reduced temperature follows a unified scaled power law. This scenario is confirmed by our simulation results for model atomistic systems (crystallizing volumetric liquids and liquid thin film) and by available experimental data for crystallizing polymers. We find that the exponent of this unified scaling law is associated with a measure of the glass-forming ability of a system. The results of the present study extend the idea of a unified description of the rate characteristics of the crystal nucleation and growth kinetics by means of the scaling relations., Comment: 4 pages, 3 figures

A key feature of the crystallization of supercooled water confined in an applied static electric field is that the structural order here is determined not only by usual thermodynamic and kinematic factors (degree of supercooling, difference between chemical potentials for a liquid and a crystal, and viscosity) but also by the strength and direction of the applied electric field, size of a system (size effects), and the geometry of bounding surfaces. In this work, the electrocrystallization of supercooled water confined between ideally flat parallel graphene sheets at a temperature of $T=268$~K has been considered in this work. It has been established that structural order is determined by two characteristic modes. The initial mode correlates with the orientation of dipolar water molecules by the applied electric field. The subsequent mode is characterized by the relaxation of a metastable system to a crystalline phase. The uniform electric field applied perpendicularly to the graphene sheets suppresses structural ordering, whereas the field applied in the lateral direction promotes cubic ice $I_c$., Comment: 5 pages, 4 figures

Analytical equation of state for pure alkali metals (lithium, sodium, potassium, rubidium and cesium) in equilibrium gas phase is proposed. This equation has a simple form, generalizes the equation of state for a perfect gas and is universal for all alkali elements. It correctly reproduces the experimental data for the equilibrium gas phase over a wide range of pressures (up to $\sim 10^5$~Pa) and temperatures (up to $\sim 3 \cdot 10^3$~K). On the basis of this equation of state, expressions for thermal and caloric coefficients as well as for \textit{other physical characteristics} are obtained. The results of this study confirm feasibility of the principle of corresponding states in relation to the group of alkali elements., Comment: 18 pages, 2 figures, 1 table

The process of homogeneous crystal nucleation has been considered in a model liquid, where the interparticle interaction is described by a short-range spherical oscillatory potential. Mechanisms of initiating structural ordering in the liquid at various supercooling levels, including those corresponding to an amorphous state, have been determined. The sizes and shapes of formed crystal grains have been estimated statistically. The results indicate that the mechanisms of nucleation occurs throughout the entire considered temperature range. The crystallization of the system at low supercooling levels occurs through a mononuclear scenario. A high concentration of crystal nuclei formed at high supercooling levels (i.e., at temperatures comparable to and below the glass transition temperature $T_g$) creates the semblance of the presence of branched structures, which is sometimes erroneously interpreted as a signature of phase separation. The temperature dependence of the maximum concentration of crystal grains demonstrates two regimes the transition between which occurs at a temperature comparable to the glass transition temperature $T_g$., 4 figures

It is generally accepted that the complicated character of the interparticle interaction in liquid metals is reproduced most correctly by many-particle potentials of the EAM-type (embedded atom model) interparticle interaction. It is shown that in the case of liquid lithium near the melting temperature (Tm = 453.65 K), the spherical pseudopotential provides a better agreement with experimental data on elastic and inelastic X-ray scattering as compared to the known EAM potentials. The calculations of the dynamic structural factor and spectral densities of the longitudinal and transverse atomic currents lead to the conclusion that although the pseudopotential and EAM potentials generate a certain qualitative correspondence in the features of collective dynamics, the interparticle interaction of the spherical type reproduces correctly the general form of the dynamic structure factor in a certain wavenumber range, as well as the dispersion relation for collective excitations.

Study of condensed matter in certain extreme conditions allows one to better understand the mechanisms of microscopic structural transformations and to develop materials with completely new mechanical properties. In the present work, we study the structure and crystallization kinetics of bulk metallic glass (BMG) Ni 62 Nb 38 . We show that this BMG is rapidly crystallized under the combined effect of shear deformation and ultrahigh pressure. A threshold pressure required to initiate the formation of a stable crystalline phase is revealed. Shear deformation and pressure lead to the phase separation: two high-density inhomogeneous crystal structures are formed. We find that the crystallization of this BMG occurs in two stages due to the significant difference in the growth rates of Ni and Nb crystalline phases. The results of the present study make a significant contribution to understanding the crystallization and amorphization features of Ni-based BMG’s.

In the last few decades, theoretical and experimental studies of glass-forming liquids have revealed presence of universal regularities in the viscosity-temperature data. In the present work, we propose a viscosity model for scaling description of experimental viscosity data. A feature of this model is presence of only two adjustable parameters and high accuracy of experimental data approximation by this model for a wide temperature range. The basis of the scaling description is an original temperature scale. Within this scaling description we obtain the transformed Angell plot, in which the area separating "fragile" and "strong" glass-formers emerges. The proposed scaling procedure make it possible to reconsider belonging some liquids to the type of "fragile" glass-formers. The obtained results form basis for development of a generalized scaling description of crystallization kinetics in supercooled liquids and glasses., 18 pages, 4 figures, 1 tables

The assumption proposed in [U. Balucani et al., Phys. Rev. B 47, 3011 (1993)] that the space and time dependences of the characteristics of the microscopic structure and dynamics for the group of liquid alkali metals are reduced to a common general form through scaling transformations has been discussed. It has been found that such description is possible when scale units are (i) the effective size of a particle corresponding, in particular, to the experimentally measured position of the main peak in the static structure factor, (ii) the characteristic time scale of the thermal mean free path of the particle, and (iii) the parameters of the “liquid”–“crystal” phase separation (in particular, the melting temperature). This conclusion follows directly from the comparative analysis of experimental data on X-ray diffraction, as well as on the inelastic neutron and X-ray scattering data. This work develops the ideas proposed in [A. V. Mokshin et al., J. Chem. Phys. 121, 7341 (2004)].

An approach to determining the nucleation barrier and interfacial free energy (surface tension) based on molecular dynamics simulations of structural transformations, in particular, the formation of new phase nuclei, is reported. The approach is based on the thermodynamic integration method, wherein key elements are trajectories characterizing the potential energy change, which are obtained from independent numerical experiments. An important feature of the approach is its applicability to both equilibrium and nonequilibrium systems, as well as the possibility of determining the above thermodynamic characteristics for small new-phase nuclei, with a significant curvature of the surface. For example, we present the temperature dependencies of the surface tension of water droplet nuclei for water vapor nucleation and of the nucleation barrier to crystal nucleation in two model glassy systems, which are computed within the framework of the proposed approach. The calculated values of the surface tension coefficient of water droplet nuclei are compared with the available experimental data.

A method has been proposed for determining interfacial free energy from the data of molecular dynamics simulation. The method is based on the thermodynamic integration procedure and is distinguished by applicability to both planar interfaces and those characterized by a high curvature. The workability of the method has been demonstrated by the example of determining the surface tension for critical nuclei of water droplets upon condensation of water vapor. The calculation has been performed at temperatures of 273–373 K and a pressure of 1 atm, thus making it possible to determine the temperature dependence of the surface tension for water droplets and compare the results obtained with experimental data and the simulation results for a “planar” vapor–liquid interface.

Crystal growth and crystal coalescence processes in supercooled systems strongly depend on the concentration of crystallization centers. We perform atomistic dynamics simulations of the crystallization process in the ultrathin metallic film at different supercooling levels corresponding to supercooled liquid and amorphous solid states. Scaled relations are applied to identify the characteristic regimes in the time-dependent crystalline nuclei concentration: steady-state nucleation regime, saturation regime and coalescence regime. We show that the crystal growth at the saturation regime appears due to mixing nucleation and coalescence processes. We find that the crystallite coalescence realizes mainly through the mechanism of restructurization/absorption of crystal nuclei, whereas the mechanism of oriented attachment is manifested only at low levels of supercooling., 5 pages, 3 figures

Physical properties of an interacting system are governed by collective excitations, but their nature at extreme supercritical conditions is unknown. Here, we present direct evidence for propagating solid-like longitudinal phonon-like excitations with wavelengths extending to interatomic separations deep in the supercritical state at temperatures up to 3,300 times the critical temperature. We observe that the crossover of dispersion curves develops at k points reducing with temperature. We interpret this effect as the crossover from the collective phonon to the collisional mean-free path regime of particle dynamics and find that the crossover points are close to both the inverse of the shortest available wavelength in the system and to the particle mean free path inferred from experiments and theory. Notably, both the shortest wavelength and mean free path scale with temperature with the same power law, lending further support to our findings.

The concept of time correlation functions is a very convenient theoretical tool in describing relaxation processes in multiparticle systems because, on one hand, correlation functions are directly related to experimentally measured quantities (for example, intensities in spectroscopic studies and kinetic coefficients via the Kubo-Green relation) and, on the other hand, the concept is also applicable beyond the equilibrium case. We show that the formalism of memory functions and the method of recurrence relations allow formulating a self-consistent approach for describing relaxation processes in classical multiparticle systems without needing a priori approximations of time correlation functions by model dependencies and with the satisfaction of sum rules and other physical conditions guaranteed. We also demonstrate that the approach can be used to treat the simplest relaxation scenarios and to develop microscopic theories of transport phenomena in liquids, the propagation of density fluctuations in equilibrium simple liquids, and structure relaxation in supercooled liquids. This approach generalizes the mode-coupling approximation in the G\"{o}tze-Leutheusser realization and the Yulmetyev-Shurygin correlation approximations., Comment: 38 pages, 4 figures

International audience; The structural and dynamic properties of the three-component Zr47Cu46Al7ă system are subjected to a molecular dynamics simulation in theă temperature range T = 250-3000 K at a pressure p = 1.0 bar. Theă temperature dependences of the Wendt-Abraham parameter and theă translation order parameter are used to determine the glass transitionă temperature in the Zr47Cu46Al7 system, which is found to be T (c) ae 750ă K. It is found that the bulk amorphous Zr47Cu46Al7 alloy containsă localized regions with an ordered atomic structures. Cluster analysis ofă configuration simulation data reveals the existence of quasi-icosahedrală clusters in amorphous metallic Zr-Cu-Al alloys. The spectral densitiesă of time radial distribution functions of the longitudinal (CI integrală (L)(k, omega)) and transverse (CI integral (T) (k, omega)) fluxes areă calculated in a wide wavenumber range in order to study the mechanismsă of formation of atomic collective excitations in the Zr47Cu46Al7 system.ă It was found that a linear combination of three Gaussian functions isă sufficient to reproduce the (CI integral (L) (k, omega)) spectra,ă whereas at least four Gaussian contributions are necessary to exactlyă describe the (CI integral (T) (k, omega)) spectra of the supercooledă melt and the amorphous metallic alloy. It is shown that the collectiveă atomic excitations in the equilibrium melt at T = 3000 K and in theă amorphous metallic alloy at T = 250 K are characterized by twoă dispersion acoustic-like branches related with longitudinal andă transverse polarizations.

The atomic dynamics of the binary Al100–x Cu x system is simulated at a temperature T = 973 K, a pressure p = 1.0 bar, and various copper concentrations x. These conditions (temperature, pressure) make it possible to cover the equilibrium liquid Al100–x Cu x phase at copper concentrations 0 ≤ x ≤ 40% and the supercooled melt in the concentration range 40% ≤ x ≤ 100%. The calculated spectral densities of the time correlation functions of the longitudinal $${\tilde C_L}$$ (k, ω) and transverse $${\tilde C_T}$$ (k, ω) currents in the Al100–x Cu x melt at a temperature T = 973 K reveal propagating collective excitations of longitudinal and transverse polarizations in a wide wavenumber range. It is shown that the maximum sound velocity in the v L (x) concentration dependence takes place for the equilibrium melt at an atomic copper concentration x = 10 ± 5%, whereas the supercooled Al100–x Cu x melt saturated with copper atoms (x ≥ 40%) is characterized by the minimum sound velocity. In the case of the supercooled melt, the concentration dependence of the kinematic viscosity ν(x) is found to be interpolated by a linear dependence, and a deviation from the linear dependence is observed in the case of equilibrium melt at x < 40%. An insignificant shoulder in the ν(x) dependence is observed at low copper concentrations (x < 20%), and it is supported by the experimental data. This shoulder is caused by the specific features in the concentration dependence of the density ρ(x).

The coefficient of the electron–ion energy exchange in liquid aluminum is calculated within the framework of Ziman approach for electron kinetic coefficients. Calculations are made to study dependence of the electron–ion heat transfer coefficient on the electron and ion temperatures.

The shear viscosity of liquid cobalt at the pressure $p=1.5$~bar and at the temperatures corresponding to equilibrium liquid and supercooled liquid states is measured experimentally and evaluated by means of molecular dynamics simulations. Further, the shear viscosity is also calculated within the microscopic theoretical model. Comparison of our experimental, simulation and theoretical results with other available data allows one to examine the issue about the correct temperature dependence of the shear viscosity of liquid cobalt. It is found a strong correlation between the viscosity and the configuration entropy of liquid cobalt over the considered temperature range, which can be taken into account by the Rosenfeld's model., 10 pages, 3 figures

The dynamic structure factor is the quantity, which can be measured by means of Brillouin light-scattering as well as by means of inelastic scattering of neutrons and X-rays. The spectral (or frequency) moments of the dynamic structure factor define directly the sum rules of the scattering law. The theoretical scheme formulated in this study allows one to describe the dynamics of local density fluctuations in simple liquids and to obtain the expression of the dynamic structure factor in terms of the spectral moments. The theory satisfies all the sum rules, and the obtained expression for the dynamic structure factor yields correct extrapolations into the hydrodynamic limit as well as into the free-particle dynamics limit. We discuss correspondence of this theory with the generalized hydrodynamics and with the viscoelastic models, which are commonly used to analyze the data of inelastic neutron and X-ray scattering in liquids. In particular, we reveal that the postulated condition of the viscoelastic model for the memory function can be directly obtained within the presented theory. The dynamic structure factor of liquid lithium is computed on the basis of the presented theory, and various features of the scattering spectra are evaluated. It is found that the theoretical results are in agreement with inelastic X-ray scattering data., Comment: 34 pages, 7 figures

The results of experimental measurements, molecular dynamics simulation, and theoretical calculations of the viscosity of a cobalt melt in a temperature range of $1400-2000$~K at a pressure $p=1.5$~bar corresponding to an overcooled melt at temperatures of $1400-1768$~K and an equilibrium melt with temperatures from the range $1768-2000$~K are presented. Theoretical expressions for the spectral density of the time-dependent correlation function of the stress tensor $\tilde{S}(\omega)$ and kinematic viscosity $\nu$ determined from the frequency and thermodynamic parameters of the system are obtained. The temperature dependences of the kinematic viscosity for the cobalt melt are determined experimentally by the torsional oscillation method; numerically, based on molecular simulation data with the EAM potential via subsequent analysis of the time correlation functions of the transverse current in the framework of generalized hydrodynamics; and by the integral Kubo-Green relation; they were also determined theoretically with the Zwanzig-Mori memory functions formalism using a self-consistent approach. Good agreement was found between the results of theoretical calculations for the temperature dependence of the kinematic viscosity of the cobalt melt using experimental data and the molecular dynamics simulation results. From an analysis of the temperature dependence of the viscosity, we obtain an activation energy of $E=(5.38\pm0.02)\times10^{-20}$~J., Comment: 16 pages, 4 figures

The features of the microscopic structure, as well as one-particle and collective dynamics of liquid gallium in the temperature range from T = 313 to 1273 K, are studied on the p = 1.0 atm isobar. Detailed analysis of the data on diffraction of neutrons and X-rays, as well as the results of atomic dynamics simulation, lead to some conclusions about the structure. In particular, for preset conditions, gallium is in the equilibrium liquid phase showing no features of any stable local crystalline clusters. The pronounced asymmetry of the principle peak of the static structure factor and the characteristic “shoulder” in its right-hand part appearing at temperatures close to the melting point, which are clearly observed in the diffraction data, are due to the fact that the arrangement of the nearest neighbors of an arbitrary atom in the system is estimated statistically from the range of correlation length values and not by a single value as in the case of simple liquids. Compactly located dimers with a very short bond make a significant contribution to the statistics of nearest neighbors. The temperature dependence of the self-diffusion coefficient calculated from atomic dynamics simulation agrees well with the results obtained from experimental spectra of the incoherent scattering function. Interpolation of the temperature dependence of the self-diffusion coefficient on a logarithmic scale reveals two linear regions with a transition temperature of about 600 K. The spectra of the dynamic structure factor and spectral densities of the local current calculated by simulating the atomic dynamics indicate the existence of acoustic vibrations with longitudinal and transverse polarizations in liquid gallium, which is confirmed by experimental data on inelastic scattering of neutrons and X-rays. It is found that the vibrational density of states is completely reproduced by the generalized Debye model, which makes it possible to decompose the total vibrational motion into individual contributions associated with the formation of acoustic waves with longitudinal and transverse polarizations. Comparison of the heights of the low-frequency component and of the high-frequency peak in the spectral density of vibrational states also indicates a temperature of T ≈ 600 K, at which the diffusion type of one-particle dynamics changes to the vibrational type upon a decrease in temperature. It is demonstrated that the modified Einstein–Stokes relation can be derived using the generalized Debye model.

This paper presents the results of investigation of the propagation mechanism of collective excitations in amorphous C60x /C70(1 − x) fullerene mixtures (with equimolar concentration x = 0.50), which were obtained using molecular dynamics simulation. The critical glass-transition temperature of the system T c = 1548 K was determined from the change in the behavior of the Wendt-Abraham parameter. Spectral densities of the time correlation functions of the longitudinal $$\tilde C_L (k,\omega )$$ and transverse $$\tilde C_T (k,\omega )$$ currents for a wide region of wave numbers at temperatures below the glass-transition temperature were calculated. It was found that the dynamics of density fluctuations in amorphous C60x /C70(1 − x) fullerene mixtures is characterized by two dispersion acoustic-like branches of the longitudinal and transverse polarizations. The influence of poly-dispersity and form factor of the molecule of the mixture component on the microscopic dynamics of the density fluctuation in multicomponent systems was established.

Analysis of three-particle correlations is performed on the basis of simulation data of atomic dynamics in liquid and amorphous aluminium. A three-particle correlation function is introduced to characterize the relative positions of various three particles -- the so-called triplets. Various configurations of triplets are found by calculation of pair and three-particle correlation functions. It was found that in the case of liquid aluminium with temperatures $1000\,$K, $1500\,$K, and $2000\,$K the three-particle correlations are more pronounced within the spatial scales, comparable with a size of the second coordination sphere. In the case of amorphous aluminium with temperatures $50\,$K, $100\,$K, and $150\,$K these correlations in the mutual arrangement of three particles are manifested up to spatial scales, which are comparable with a size of the third coordination sphere. Temporal evolution of three-particle correlations is analyzed by using a time-dependent three-particle correlation function, for which an integro-differential equation of type of the generalized Langevin equation is output with help of projection operators technique. A solution of this equation by means of mode-coupling theory is compared with our simulation results. It was found that this solution correctly reproduces the behavior of the time-dependent three-particle correlation functions for liquid and amorphous aluminium., Comment: Physica A (2017)