The effect of 3-body forces and anharmonicity on the phonon spectrum of solid argon

The phonon spectra of solidified argon have been computed by a phenomenological rigid-atom-model. This model, which takes the constituent atoms as rigid-hard spheres, assumes that the potential energy of the solid is the sum of central and non-central interactions, and derives the same from the Buckingham-Corner potential function together with the Axilrod-Teller interaction term. The zero-point quantum and anharmonic effects, have been included. The effect of many-body forces as well as anharmonicity on the frequency spectrum and the lattice heat capacities of the solid is seen to be appreciable. The agreement between theoretical and the experimental results is not very satisfactory.     

Lattice heat capacities of solid argon

An anharmonic rigid-atom model has been used to study the heat capacities of solid argon. The model derives the interatomic forces by BuckinghamCorner potential and takes account of all neighbour interaction. The contribution of the cubic and quartic terms of the potential energy expansion to the heat capacities have been accounted for through Helmholtz-free energy by perturbation theory. It is concluded that the model is most suitable among the ones designed for the class of solids to which argon belongs.     

Debye-Waller factor in rare gas-solids

The Debye-Waller exponents of solidified krypton at various temperatures have been computed by a phenomenological ‘rigid-atom-model’. The model, which takes rare gas atoms as rigid-hard spheres and derives their central and non-central interactions through Buckingham-Corner and Axilrod-Teller potentials, includes zero-point vibration through potential parameters by a self-consistent method and accounts for the cubic and quartic potential terms as perturbation to the harmonic Hamiltonian. The results show a fairly good agreement with the data obtained from the Mössbauer fractions of ⁸³Kr-9.3 keV transition. Possibilities of further improvement of results have been discussed.     

Dispersion of phonons in molecular solids by an anharmonic rigid-atom model

Phonon dispersion relations for molecular (rare-gas) solids have been computed by an anharmonic central-force rigid-atom model. The model derives the intermolecular interaction from a two-piece four-parameter pair potential, includes zero-point vibrations through potential parameters by a self-consistent method, and accounts for the contribution of the cubic and quartic anhamonic potential terms as perturbation to the harmonic Hamiltonian. The results show reasonably good agreement with the latest observed data. The merits and demerits, with possibilities of improvement, of the model have been discussed in full.     

Amplitudes of thermal vibrations in molecular solids

The mean-square nuclear displacements 〈U²〉 _T have been evaluated for the molecular (rare-gas) solids by a lattice dynamical rigid-atom Model. The model derives the inter-molecular interaction from a “two-piece four parameter” pair-potential, includes the contribution of zero-point vibration through potential parameters by a self-consistent method, and accounts for the cubic and quartic anharmonic potential terms as perturbations to the harmonic Hamiltonian. The effect of many body interactions has been included on the basis of Axilrod-Teller approximation. The effect of including three-body forces as well as anharmonicity is found to decrease the values of 〈U²〉_T at all temperatures for all the solids. However, the ratio of the root mean-square nuclear displacements at melting to the nearest-neighbour distance, i.e. the Lindemann parameter (δ) is approximately the same for all the solids under study, which shows that the Lindemann parameter is structure as well as interaction dependent. The results are consistent with the other conventional calculations.     

Many-body effects in molecular solids

The effects of many-body interactions in the molecular (rare-gas) solids have been investigated, on the basis of Axilrod-Teller approximation, by a rigid-atom model. It is found that the 3-body interaction is the most dominant of all and the rest may be safely ignored. The discrepancy seen in the phonon dispersion curves is expected to be removed by the inclusion of appropriate anharmonic effects.     

Low-temperature magnetization dynamics of magnetic molecular solids in a swept field

The swept-field experiments on magnetic molecular solids such as Fe₈ are studied using Monte Carlo simulations, and a kinetic equation developed to understand collective magnetization phenomena in such solids, where the collective aspects arise from dipole–dipole interactions between different molecules. Because of these interactions, the classic Landau–Zener–Stückelberg theory proves inadequate, as does another widely used model constructed by Kayanuma. It is found that the simulations provide a quantitatively accurate account of the experiments. The kinetic equation provides a similarly accurate account except at very low sweep velocities, where it fails modestly. This failure is attributed to the neglect of short-range correlations between the dipolar magnetic fields seen by the molecular spins. The simulations and the kinetic equation both provide a good understanding of the distribution of these dipolar fields, although analytic expressions for the final magnetization remain elusive.     

Inelastic collisions of medium energy atomic elements in solids

The stopping of medium atomic elements in solids has been calculated in the framework of classical approximation, taking into account elastic and inelastic collisions. The calculations are based on the 1) revised equations for the law of energy and impulse conservation, which were earlier used to describe inelastic collisions of atomic particles, 2) theory of quasi-small angle scattering, 3) power potential of LNS, and 4) limitation of the maximum distance of atoms approach determined by the interatomic distance in solids. Analytical equations have been obtained to calculate 1) a differential cross-section of elastic scattering in the presence of inelastic collisions, 2) energy transferred, 3) cross-sections of elastic and inelastic stopping, and penetration depth. Implantation into solids was found to be of threshold character.     

Defects induced melting in alkali halides

In the present paper we study the pressure dependence of melting of NaCl and CsCl crystals. A formulation has been presented for the pressure dependence of melting temperature on the basis of the vacancy model using the expression for the pressure dependence of the volume of Schottky defects from the Roy-Roy equation of state. Values of pressure derivatives of melting temperature have been calculated at elevated pressures to determine the rate of change of melting temperature with increase in pressures using the data of vacancy formation energy and effective volume of Schottky defects. The vacancy model revised in the present study takes into account the variation of bulk modulus with pressure, whereas in the Ksiazek and Gorecki model, it was treated constant. Results for pressure derivative of melting temperature are calculated for the solids under study. The melting curves have also been obtained and found to compare well with results based on molecular dynamics simulation and experimental data reported in recent literature.     

Static and thermophysical properties of chalcogenide crystals with NaCl structure

In the present communication an analysis of interionic potentials in fourteen chalcogenide crystals has been performed. This interionic potential has been used to predict the values of cohesive energy, isothermal bulk modulus and the pressure derivatives of bulk modulus in the solids under study. The many body interaction (MBI) effects have been taken into account within the framework of Hafemeister Zarht potential. Instead of using BM potential the Hafemeister-Zarht (HZ) type short range overlap potential has been considered between nearest as well as between next nearest neighbour ions. The short-range interactions, effective up to second neighbours are treated by considering the hardness parameter as an ionic property. The hardness parameter ρ_ij is evaluated using the data on overlap integrals. The results achieved in the present study are generally in good agreement with available experimental data. Values of cohesive energy, bulk modulus and its pressure derivatives calculated by previous investigators have also been shown for the sake of comparison.     

Carbon nanotubes: opportunities and challenges

Carbon nanotubes are graphene sheets rolled-up into cylinders with diameters as small as one nanometer. Extensive work carried out worldwide in recent years has revealed the intriguing electrical and mechanical properties of these novel molecular scale wires. It is now well established that carbon nanotubes are ideal model systems for studying the physics in one-dimensional solids and have significant potential as building blocks for various practical nanoscale devices. Nanotubes have been shown to be useful for miniaturized electronic, mechanical, electromechanical, chemical and scanning probe devices and materials for macroscopic composites. Progress in nanotube growth has facilitated the fundamental study and applications of nanotubes. Gaining control over challenging nanotube growth issues is critical to the future advancement of nanotube science and technology, and is being actively pursued by researchers.     

尾流效应对快速双原子分子离子在固体中电荷态及分子轴取向的影响

研究了快速双原子分子离子在固体中穿行时,尾流效应对各离子电荷态以及库仑爆炸过程的影响.借助于线性介电响应理论和局域介电函数,离子之间的动力学相互作用势可以表示成对称的屏蔽库仑势和非对称的尾势.通过对分子离子上所有束缚电子的总能量进行变分和求解单个离子的运动方程,自洽地确定出分子离子中每个离子的电荷态.数值结果表明,由于尾流效应的影响,在初始穿行阶段,分子离子中导航离子的电荷数随穿行深度的增加而单调递增,而尾随离子的电荷数则随穿行深度的增加而振荡.但当穿行深度很大时,两个离子的电荷数都趋于具有相同速度的孤立离子的电荷数.此外,还发现分子轴的取向朝入射速度方向偏转     

Role of the Coulomb interaction in the flow and the azimuthal distribution of kaons from heavy ion reactions

Coulomb final-state interaction of positive charged kaons in heavy ion reactions and its impact on the kaon transverse flow and the kaon azimuthal distribution are investigated within the framework of QMD (quantum molecular dynamics) model. The Coulomb interaction is found to tend to draw the flow of kaons away from that of nucleons and lead to a more isotropic azimuthal distribution of kaons in the target rapidity region. The recent FOPI data have been analyzed by taking into account both the Coulomb interaction and a kaon in-medium potential of the strong interaction. Although the effect of the kaon Coulomb potential on the kaon flow and azimuthal distribution is much smaller than that of the strong potential, it is found to be visible, and therefore, should be taken into account if one wants to extract unambiguous information about the kaon strong potential in nuclear matter from the kaon flow and azimuthal distribution data.     

Molecular dynamics simulations of laser-induced damage of nanostructures and solids

A theoretical approach to treat laser induced femtosecond structural changes in covalently bonded nanostructures and solids is described. Our approach consists in molecular dynamic simulations performed on the basis of time-dependent, many-body potential energy surfaces derived from tight-binding Hamiltonians. The shape and spectral composition of the laser pulse is explicitly taking into account in a non-perturbative way. We show a few examples of the application of this approach to describe the laser damage and healing of defects in carbon nanotubes with different chiralities and the ultrafast nonequilibrium melting of bulk germanium, initiated by the laser-induced softening and destabilization of transversal acoustic phonon modes.     

Extension of the effective solid-fluid Steele potential for Mie force fields

Molecular simulation of fluid systems in the presence of surfaces require computationally expensive calculations due to the large number of solid–fluid pair interactions involved. Representing the explicit solid as a continuous wall with an effective potential can significantly reduce the computational time and allows exploring larger temporal and spatial scales. The well-known (10-4-3) Steele potential is one such analytic expression that faithfully represents the effective solid–fluid interactions for homonuclear crystalline solids with hexagonal lattice symmetry. However, this and most of the effective potentials found in the literature have been developed for fluids and solids interacting exclusively through Lennard-Jones potentials. In this work, we extend the Steele model to obtain the effective wall–fluid potentials for Mie force fields. We perform molecular dynamics simulations of coarse-grained fluids modelled via the SAFT force field approach in the presence of explicit and implicit surfaces to compare structural and dynamic properties in both representations. Also, we study the adsorption of ethane into slit-like pores with explicit and implicit surfaces via grand canonical Monte Carlo simulations. We explore the validity and the improvement in the simulation performance as well as the limitations of the proposed expression.     

固态高分子体系多尺度分子运动的核磁共振研究

固体核磁共振技术是研究固态高分子材料中结构和分子动力学的一种非常重要和有效的手段. 该技术的一个重要特点是可以通过合理的实验方法,实现对研究体系中从低频(Hz)到中频(kHz)乃至高频(MHz)范围内分子运动的观测. 因此,固体核磁共振技术非常适合研究高分子体系中各类不同尺度分子运动. 该文首先简要介绍核磁共振研究分子运动的基本原理和方法,以及固态高分子体系的结构和分子动力学特点,然后结合固态高分子体系中的一些例子对核磁共振在固态高分子多尺度分子运动方面的一些研究成果展开讨论.     

Solid echo in the slow-motion region. Effects of the finite pulse widths

The effects of nonzero radio-frequency pulse widths on the echo signals in solids with molecular motions have been investigated. It has been shown that in the slow-motion region a time position and an amplitude of the echo signal depend not only on the width of the pulse, but also on the shape of potential wells and the correlation time, describing the molecular motion. A comparison of the developed theory with experimental results obtained for polycrystalline NH4Cl demonstrates a good agreement between them.     

Auger electron spectroscopy - a local probe for solid surfaces

The Auger electron transition in solids is discussed under the aspect of a local excitation due to the strongly localized primary hole in an inner atomic core level. In first approximation the solid is represented by a cluster model, consisting of the excited atom and its neighbors. Using this simple model it is possible to describe the Auger electron energies, intensities and line shapes of transitions in solids in a satisfactory way. Only for the angular dependent Auger emission, characteristic long-range crystalline order has to be taken into account. It is the aim of this introductory review to point out that Auger spectra bear more information about the solid surface and particularly on its chemical bonds as has yet been exploited by surface spectroscopists.     

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.     

Low-energy double plasmon satellites in the X-ray spectra of metals

Bohm and Pines' Hamiltonian and their dispersion relation have been extended up to second order to account for double plasmon oscillations in the X-ray emission spectra of solids. The effect of interaction between electrons and plasmons has also been taken into account in the dispersion relation. An expression for the relative intensity of low-energy double plasmon satellites has been derived. Fairly good agreement is obtained between values of the relative intensity calculated using this expression and values observed experimentally by Jenkins and Chung.