Model predictions and experimental characterization of Co-Pt alloy clusters

Model and real cobalt-platinum alloy clusters are compared in terms of structure, composition and segregation. Canonical and semi grand canonical Metropolis Monte Carlo simulations are performed to model these clusters, using embedded atom (EAM) and modified embedded atom (MEAM) potentials. All of them correctly predict the bulk L1₂ Co₃Pt and CoPt₃ structures as well as the L1₀ CoPt phase. However, the lattice parameters, phase stability and the L1₀-fcc order-disorder transition temperature are at variance. Segregation predictions with EAM and MEAM potentials are contradictory. Experimentally, mixed clusters with various compositions were deposited by Low Energy Cluster Beam on amorphous carbon at room temperature. Their size distribution, crystalline structure and composition were examined by Transmission Electron Microscopy (TEM). Clusters with the same size distributions were modelled. Both experiment and modelling show their crystallographic parameters to continuously correspond to the fcc CoPt chemically disordered phase. Diffraction measurements indicate surface segregation of the specie in excess, in agreement with EAM predictions for the Co-rich phase. The consequences on magnetic properties are discussed.     

Molecular dynamics of liquid lead near its melting point

The molecular dynamics of liquid lead is simulated at T = 613 K using the following three models of an interparticle interaction potential: the Dzugutov pair potential and two multiparticle potentials (the “glue” potential and the Gupta potential). One of the purposes of this work is to determine the optimal model potential of the interatomic interaction in liquid lead. The calculated structural static and dynamic characteristics are compared with the experimental data on X-ray and neutron scattering. On the whole, all three model potentials adequately reproduce the experimental data. The calculations using the Dzugutov pair potential are found to reproduce the structural properties and dynamics of liquid lead on the nanoscale best of all. The role of a multiparticle contribution to the glue and Gupta potentials is studied, and its effect on the dynamic properties of liquid lead in nanoregions is revealed. In particular, the neglect of this contribution is shown to noticeably decrease the acoustic-mode frequency.     

Ripples on the surface of a two-component fluid

The dispersion relation of ripplons on the surface of a two-component liquid has been derived using the hydrodynamic mean-field model, in terms of the interaction pair potentials of the constituent particles. The modes for planar and spherical geometries have been derived explicitly. The formalism is applied to ripples on the surfaces of liquids in which the interparticle interaction is not Coulombic, as also on the surfaces of jellium and on electron-hole droplets.     

Melting curves of FCC-metals by cell-theory

The cell model is developed to account for many body interaction effects, as occurring in the embedded atom method (EAM) potentials, and crystal lattice features to determine the free energy of FCC metals. The well known smearing approximation, which is generally used in conjunction with pair potentials, is also developed for EAM potentials. The free energy so obtained is used to determine melting curves of FCC metals. For this purpose, the liquid phase free energy is calculated using the corrected rigid spheres model. The advantage of our scheme, which is verified with data on Lennard-Jones solids, is that both free energies are based on the same interaction potential. Results match well with the available experimental/theoretical data for Al and Cu. A good agreement is also found for the transition metals, Ni and Pt, for which molecular dynamics as well as theories like Lindemann's law do not give accurate results. It is also found that smearing approximation, which neglects interactions beyond nearest neighbors, also provides good estimates for free energy if many body effects are accounted with EAM potential.     

Pseudopotential in metals and alloys

In this communication, the pseudopotential investigation of, the various properties of non-transition metals and alloys, is discussed. Various one parametric model pseudopotentials, derived from well known spherical functions s_l(x), are employed in the calculations. Many recurrence relations of the s_l(x) function have been described. The effects of exchange and correlation on conduction electrons are also considered separately by using different dielectric screenings in various properties. The ion-ion interaction, force constants, phonon spectrum, temperature coefficient of Knight shift and electronic transport coefficients of certain metals and alloys are evaluated. The results are compared with available experimental values. Generally good agreement is achieved. The screening charge density of certain metals in low and high density region are also determined.     

Systematically too low values of the cranking model collective inertia parameters

Deformed Nilsson and Woods-Saxon potentials were employed for generating single particle states used henceforth for calculating the inertia tensor (cranking model and monopole pairing) and the collective energy surfaces (Strutinsky method). The deformation was parametrized in terms of quadrupole and hexadecapole degrees of freedom. The classical energy expression obtained from the inertia tensor and energy surfaces was quantized and the resulting stationary Schrödinger equation was solved using the approximate method. The secondI ^π=0 ₂ ⁺ collective level energies were calculated for the Rare Earth and Actinide nuclei and the results compared with the experimental data. The vibrational level energies agree with the experimental ones much better for spherical nuclei for both single particle potentials; the discrepancies for deformed nuclei overestimate the experimental results by roughly a factor of two. It is argued that coupling of the axially symmetric quadrupole degrees of freedom to non-axial and hexadecapole ones does not affect the conclusions about systematically too low mass parameter values. The alternative explanation of the systematic deviations from the 0 ₂ ⁺ level energies could be a systematically too high stiffness of the energy surfaces obtained with the Strutinsky method.     

自由能方法与零压下Al的熔化温度

利用自由能方法的分子动力学模拟,计算了零压下Al的熔化温度.在计算液相自由能的过程中,采用勒纳-琼斯(LJ)液体作为参考系统,同时将计算结果与Mei和Davenport等人的计算结果进行了比较,计算结果表明：1)选用LJ参考系统使液相自由能的计算时间节省一半,并且不影响熔化温度的计算结果;2)采用不同的埋入原子势(EAM)的分子动力学模拟计算得到的熔化温度与实验值都存在偏差,而就金属Al而言,采用Cai等人的EAM势的熔化温度的计算结果比Mei和Davenport及Morris等人采用的势模型的结果略有改 关键词： 熔化温度 自由能方法 分子动力学模拟     

Application of a structure factor-potential relationship to the electrical resistivity of the liquid alkali metals

In the Ziman formulation for the electrical properties of liquid metals, the resistivity depends on an average of the product of the structure factor and the pseudopotential. Ascarelli, Harrison and Paskin have derived a relationship for small wave vector between the structure factor and the pseudopotential for liquid metals such as the alkali metals. This formulation has been used over the entire range of wave vector (k = 0 to 2k _F). The resistivities of Na, K, Rb and Cs calculated with no adjustable parameters are within 25% of the observed values, while Li is underestimated by about a factor of five. The temperature dependencies of all but Li (which is anomalously non-linear) are in similar agreement with experiments made at constant volume.     

Change in the Crystallization Features of Supercooled Liquid Metal with an Increase in the Supercooling Level

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 mechanism 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.     

High frequency cut-offs in superconductivity theory

The meaning of a high frequency cut-off for the Coulomb interaction at ±iω_max with ω_max ≈ E_f is investigated. It is not difficult to justify such a cut-off for nearly-free-electron metals. The real frequency Coulomb pseudopotential corresponding to the usual imaginary axis (μ¹, iω_c) pseudopotential is rigorously derived. It is markedly different from any assumed in previous work.     

Unified study and transport properties of simple metals

The resistivity, thermoelectric power at the melting points and temperature variation of resistivity of the liquid alkali metals are studied with local Heine-Abarenkov and the modified Ashcroft pseudopotentials. The parameters of the model potentials are obtained from a unified study of the static and dynamic properties of the metals. The effect of different dielectric function and structure factor on the transport properties is discussed. It is shown that the simple modified Ashcroft pseudopotential reproduces the result fairly well except for Li and Cs.     

A unified description of MCI3 systems with a polarizable ion simulation model

Computer simulations of a range of ionic systems of stoichiometry MX₃ using a polarizable, formal charge ionic interaction model are described. The objective of the present work is to describe the optimization of the interaction potentials in the light of new structural information which has become available from neutron scattering studies of the liquid. As well as substantially improving the agreement with experiment for LaC1₃, TbC1₃, YC1₃ and A1C1₃, simulation results are presented for the first time for ScC1₃, which is shown to exhibit a fascinating crosslinked network structure. The optimization of potentials for the crystal structures is also considered. It is shown that the C1^? ion must be considered as of smaller size in the crystal than in the liquid in order to successfully reproduce the properties of both phases.     

Neutron refractive index of liquid mixtures in the critical region

Size effects in neutron optics of mesoscale liquid mixtures in the critical region are studied. It is shown that the Schrödinger equation for the neutron wave function can be presented as the corresponding electrodynamical wave equation with the Fermi pseudopotential for a binary liquid mixture taken into account. Formulae for the neutron refractive index of bulk and confined liquid mixtures are derived and discussed. As an example, numerical calculations for a mixture of ethane and carbon dioxide C₂H₆-CO₂ are presented, which illustrates a good agreement between experimental and theoretical data.     

Analysis of semi-empirical interatomic potentials appropriate for simulation of crystalline and liquid Al and Cu

We investigate the application of embedded atom method (EAM) interatomic potentials in the study of crystallization kinetics from deeply undercooled melts, focusing on the fcc metals Al and Cu. For this application, it is important that the EAM potential accurately reproduces melting properties and liquid structure, in addition to the crystalline properties most commonly fit in its development. To test the accuracy of previously published EAM potentials and to guide the development of new potential in this work, first-principles calculations have been performed and new experimental measurements of the Al and Cu liquid structure factors have been undertaken by X-ray diffraction. We demonstrate that the previously published EAM potentials predict a liquid structure that is too strongly ordered relative to measured diffraction data. We develop new EAM potentials for Al and Cu to improve the agreement with the first-principles and measured liquid diffraction data. Furthermore, we calculate liquid-phase diffusivities and find that this quantity correlates well with the liquid structure. Finally, we perform molecular dynamics simulations of crystal nucleation from the melt during quenching at constant cooling rate. We find that EAM potentials, which predict the same zero-temperature crystal properties but different liquid structures, can lead to quite different crystallization kinetics. More interestingly, we find that two potentials predicting very similar equilibrium solid and liquid properties can still produce very different crystallization kinetics under far-from-equilibrium conditions characteristic of the rapid quenching simulations employed here.     

Size and shape-dependent melting mechanism of Pd nanoparticles

Molecular dynamics simulation was employed to understand the thermodynamic behavior of cuboctahedron (cub) and icosahedron (ico) nanoparticles with 2–20 number of full shells. The original embedded atom method (EAM) was compared to the more recent highly optimized version as inter-atomic potential. The thermal stability of clusters were probed using potential energy and specific heat capacity as well as structure analysis by radial distribution function (G(r)) and common neighbor analysis (CNA), simultaneously, to make a comprehensive picture of the solid-state and melting transitions. The result shows ico is the only stable shape of small clusters (Pd₅₅–Pd₃₀₉ using original EAM and Pd₅₅ using optimized version) those are melting uniformly due to their small diameter. An exception is cub Pd₃₀₉ modeled via optimized EAM that transforms to ico at elevated temperatures. A similar cub to ico transition was predicted by original EAM for Pd₉₂₃–Pd₂₀₇₅ clusters, while for the larger clusters both cub and ico are stable up to the melting point. As detected by \(G(r)\) and CNA, moderate and large cub clusters were showing surface melting by nucleation of the liquid phase at (100) planes and growth of liquid phase at the surface before inward growth. While diagonal (one corner to another) melting was dominating over ico clusters owing to their partitioned structure, which retarded the growth of the liquid phase. The large ico clusters, using optimized EAM, presented a combination of surface and diagonal melting due to the simultaneous diagonal melting started from different corners. Finally, the melting temperature as well as latent heat of fusion were calculated and compared with the available models and previous studies, which showed, unlike the present result, the models failed to predict size-dependent motif cross-over.     

Discrete breathers and nonlinear normal modes in monoatomic chains

We consider a relation between discrete breathers (DBs) and nonlinear normal modes in some nonlinear monoatomic chains. The dependence of the breathers’ stability on the strength of interparticle interaction in the K₄ chain (the chain with uniform on-site and intersite potentials of the fourth order) is investigated. A general method for constructing DBs which provides the pair synchronization between the individual particles’ vibrations is discussed. Many-frequency breathers as DBs of a new type and quasibreathers [introduced in Physical Review E 74 (2006) 036608] are analyzed.     

Thermodynamic and physical properties of FeAl and Fe3Al: an atomistic study by EAM simulation

With this work we present a newly developed potential for the Fe–Al system, which is based on the analytical embedded atom method (EAM) with long range atomic interactions. The potential yields for the two most relevant phases B2-FeAl and D0₃-Fe₃Al lattice constants, elastic constants, as well as bulk and point defect formation enthalpies, which are in good agreement with experimental and other theoretical data. In addition, the phonon dispersions for B2-FeAl and D0₃-Fe₃Al show a good agreement with available experiments. The calculated lattice constants and formation enthalpy for disordered Fe–Al alloys are in good agreement with experimental data or other theoretical calculations. This indicates that the present EAM potentials of Fe–Al system is suitable for atomistic simulations of structural and kinetic properties for the Fe–Al system.     

Microstructural analysis of liquid and amorphous SiO2 nanoparticles

Microstructural properties of liquid and amorphous SiO₂ nanoparticles have been investigated via molecular dynamics (MD) simulations with the interatomic potentials that have weak Coulomb interaction and Morse-type short-range interaction under non-periodic boundary conditions. Structural properties of spherical nanoparticles with different sizes of 2, 4 and 6 nm obtained at 3500 K have been studied through partial radial distribution functions (PRDFs), coordination number and bond-angle distributions, and compared with those observed in the bulk. The core and surface structures of liquid SiO₂ nanoparticles have been studied in detail. We found significant size effects on structure of nanoparticles. Calculations also show that if the size is larger than 4 nm, liquid SiO₂ nanoparticles at the temperature of 3500 K have a lightly distorted tetrahedral network structure with the mean coordination number Z_Si-O≈4.0 and Z_O-Si≈2.0 like those observed in the bulk. Moreover, temperature dependence of structural defects and SiO_x stoichiometry in nanoparticles on cooling from the melt has been found and presented.