The method of local fields: A bridge between molecular modelling and dielectric theory

At the nanoscale, the charge distribution in a cluster of several atoms or molecules can be calculated ab initio, i.e. without free parameters. Molecular modelling is limited to a relatively small number of atoms compared to macroscopic materials with myriads of atoms. On the other hand, dielectric and ferroelectric properties of macroscopic matter are described by classical theory using mean-field approximations, e. g. the formula of Clausius–Mossotti for dielectrics and the Landau–Ginzburg–Devonshire theory or rather the molecular field theory by Weiss for ferroelectrics. In the context of multiscale simulations we present a microscopic model for dielectrics and ferroelectrics consisting of discrete atoms and / or dipoles. Parameters calculated from molecular modelling can be used here as input to our simulations in order to calculate bigger systems now. All electrostatic interactions are considered and the electrodes are taken into account using the method of images. Based on thermally activated processes, permanent dipoles fluctuate in double well potentials according to the Boltzmann statistics. Neutral atoms are modelled by induced dipoles having dipole moments proportional to the locally prevailing field. The numerical calculations are based on deterministic local field computations and on weighted probabilistic dynamic Monte Carlo steps.     

Local fields near the surface of a crystalline dielectric

We calculate the variation of the local electric field with depth near the surface of a crystalline dielectric, for the cases of induced atomic dipoles oriented perpendicular and parallel to the surface. The crystalline dielectric is modelled by a cubic lattice of polarizable atoms, with the surface in the (001) plane. Our calculations show that the departure of the local field from its bulk value is confined almost entirely to the outermost plane, and even there is small (at the most a few percent).     

Demixing and field-induced population inversion in a mixture of neutral and dipolar-hard spheres confined in a slit pore

We study the influence of an external field on demixing and its interplay with the field-induced population inversion for a binary mixture of neutral and dipolar non-additive hard-spheres confined in a slit pore. Demixing lines are determined by Gibbs Ensemble Monte Carlo simulations and population inversion paths by grand-canonical/canonical simulations of the pore/bulk equilibrium. Besides the demixing line in the bulk, results are given for two different pore widths and in parallel and normal fields. Similar to the effect of geometrical confinement, a normal field is found to favour the mixed state so that the population inversion does not interfere with demixing. A parallel field leads to more complex scenarios.     

Electrostatic interactions of charges and dipoles near a polarizable membrane

A variational method is used to determine the polarization charge density induced on the surfaces of a dielectric slab (a coarse-grained model of a membrane) by electric point charges and extended dipoles. The results are used to obtain analytic expressions for the polarization energy and the resulting forces acting on and between pairs of charges positioned in various geometries relative to the membrane. The polarization effects are shown to be substantial and often non-trivial on the nanometre scale, compared to direct Coulomb interactions.     

细胞骨架微管中水的电偶极集体辐射

微管是细胞骨架中的重要组成部分和功能组件，其中充满了液体水.采用量子场论方法，对细胞骨架微管中水分子的电偶极子与电磁辐射场的相互作用进行了探讨,研究水分子系统的电偶极子集体电磁辐射的特性.此外，还讨论了微管中水分子系统与其周围热库的耦合. 关键词： 微管 水 电偶极子 电磁辐射 集体辐射 热库     

The multiple time step r-RESPA procedure and polarizable potentials based on induced dipole moments

In the present study, we present an accelerating scheme based on the reversible multiple time step r-RESPA method to be used in molecular dynamics simulations with polarizable potentials based on induced dipole moments. Even if the induced dipoles are estimated with an iterative self-consistent procedure, this scheme significantly reduces the CPU time needed to perform a molecular dynamics simulation, up to a factor 2, as compared to the Car–Parrinello method where additional dynamical variables are introduced for the treatment of the induced dipoles. The tests show that stable and reliable molecular dynamics trajectories can be generated with that scheme, and that the physical properties derived from the trajectories are equivalent to those computed with the classical all atom iterative approach and the Car–Parrinello one.     

Equilibrium Sampling From Nonequilibrium Dynamics

We present some applications of an Interacting Particle System (IPS) methodology to the field of Molecular Dynamics. This IPS method allows several simulations of a nonequilibrium random process to keep closer to equilibrium at each time, thanks to a selection mechanism based on the relative virtual work induced on the system. It is therefore an efficient improvement of usual nonequilibrium simulations, which can be used to compute canonical averages, free energy differences, and typical transitions paths. AMS: 65C05 65C35 80A10     

Pair approximation for polarization interaction: efficient method for Monte Carlo simulations of polarizable fluids

A new method is presented for the Monte Carlo simulations of polarizable models with induced dipole moments. This method updates induced dipole moments on all molecules when a single molecule is moved, without evaluating all pair interactions. Thus, depending on the number of molecules, it is 10–20 times faster than Monte Carlo simulations with full iteration. The efficiency makes it a powerful tool for the study of phase equilibria of polarizable models in grand canonical and Gibbs ensembles.     

Microscopic theory of some surface effects in a dense electron gas

We consider interacting electrons confined to a slab of finite thickness or a half space in the random phase approximation. The electronic charge is assumed to be neutralized by a homogeneous positive background. A linear response function is introduced from which it is possible to calculate the induced density variation of the electron gas, caused by an external field. The linear response function also determines the Helm-holtz free energy and, by a sum rule, the density of the undisturbed system. The interaction of a classical point charge with the electrons of a half-space is discussed. In addition, a formula for the surface energy is given.     

Characteristics analysis of hybrid plasmonic waveguide for low-loss lightwave guiding

It has been experimentally demonstrated that a low-loss guided hybrid mode is supported if a metal strip is embedded in a low index polymer layer surrounded by two high index slabs. In this paper, further numerical analyses on the guided hybrid modes are reported to fully elucidate the characteristics of the hybrid plasmonic waveguide. For a one-dimensional slab structure with a metal film of infinite width, simulation results exhibit that low-loss guided hybrid modes are associated with surface plasmon modes and dual dielectric slab modes. The optical properties of the guided modes are improved by increasing the field intensity which is confined into lossless dielectric layers by decreasing the metal film thickness and increasing the refractive index and thickness of the high-index slabs. The finite element method is used to investigate the lateral mode confinement of the optical guided modes by the corresponding metal strip. By reducing the metal film width, the guided modes are confined in the plane transverse to the direction of propagation and the characteristics are significantly improved. The hybrid plasmonic waveguide can be exploited for long-range propagation-based application such as optical interconnection.     

A simulation study of microwave field effects on a 3D orthorhombic lattice of rotating dipoles: short-range potential energy variation

Variation of the short-range potential energy of interaction of nearest dipoles in a three-dimensional (3D) orthorhombic lattice exposed to microwave electric fields is studied by means of the Langevin dynamics simulations. The global increase of the mean potential energy is typical for all the frequencies and intensities at lower temperatures, whereas separate potential energy peaks or peak chains are observed at intermediate temperatures. A simple statistical model proposed to account for the temperature dependence of the field intensity for potential energy peaks suggests the concerted collective rotation of the dipoles. The temperature dependence of the peak frequency is explained using a combination of the one-dimensional Kramers and the resonant activation theories applied to the field-driven collective rotation, with the nearly degenerate angular coordinates of the dipoles being used as a single effective coordinate.     

Fast computation of many-particle hydrodynamic and electrostatic interactions in a confined geometry

An O(N) method is presented for calculation of hydrodynamic or electrostatic interactions between N point particles in a confined geometry. This approach splits point forces or sources into a local contribution for which rapidly decaying free-space analytical solutions to the Stokes or Poisson equations are used, and a global contribution whose effect is determined numerically using a fast iterative method. The scheme is applied to Brownian dynamics simulations of flowing confined polymer solutions, and the effects of concentration on hydrodynamically induced migration phenomena are illustrated.     

Reflection and transmission of an Airy beam in a dielectric slab

The reflection and transmission of a finite-power Airy beam incident on a dielectric slab are investigated by an analytical method. Based on the plane-wave angular spectrum expansion and Fresnel approximation, the analytical expressions of the reflected field, internal field as well as transmitted field in each region are obtained. Through numerical simulations, the intensity distributions of the incident beam, reflected beam, internal beam as well as transmitted beam are presented at oblique incidence. Besides, we also compare the intensity distributions of the geometrical-optics beam field, the first order beam mode field and the actual beam field, which indicates that the contribution of each order beam mode field to the actual beam field is related to the refractive index of the dielectric slab. Meanwhile, the reflection characteristics of the Airy beams in the special cases of Brewster incidence and total reflection are investigated. Finally, the effects of the optical thickness and refractive index of the dielectric slab on the peak intensity distributions and beam shifts of the reflected and transmitted beams are also discussed in detail. The analytical and numerical results will be useful to analyze the propagation dynamics of Airy beam in the dielectric slab and provide some theoretical supports to the design of optical film.     

An adjustable linear Halbach array

The linear Halbach array is a well-known planar magnetic structure capable, in the idealized case, of generating a one-sided magnetic field. We show that such a field can be created from an array of uniformly magnetized rods, and rotating these rods in an alternating fashion can smoothly transfer the resultant magnetic field through the plane of the device. We examine an idealized model composed of infinite line dipoles and carry out computational simulations on a realizable device using a magnetic boundary element method. Such an arrangement can be used for an efficient latching device, or to produce a highly tunable field in the space above the device.     

Calculation of electrostatic fields in ionic crystals based upon the Ewald method

Formulas for the electrostatic potential, field strength and field gradient due to monopoles, dipoles and quadrupoles are given, based upon the Ewald summation method. A convenient choice of the convergence parameter in the Ewald method is demonstrated in some actual calculations. A method to calculate self-consistent electrostatic fields due to self induced dipoles is evaluated. Monopole and self-consistent fields are calculated in Pb₂O₃, βMn₂O₃, FeTiO₃, MnF₃ and Pb₃O₄. The influence of the anion parameter on the electrostatic field is investigated for idealized CdCl₂ and TiO₂. Results of Madelung and induced dipole polarization energy for a series of arbitrary compounds are given. The effect of spontaneous polarization in polar crystals is discussed.     

金属纳米锥表面吸附分子分布的数值模拟

在均匀外电场中,将吸附在由过渡金属基底生长的纳米锥颗粒表面的CO分子等效为偶极子,在考虑偶极子与局域电场,偶极子之间色散力及偶极子与锥表面原子之间三种相互作用的情况下,给出各相互作用能的数学模型,并用Monte Carlo方法进行数值模拟,得到纳米锥颗粒表面吸附CO分子的空间分布构型.结果表明,在这些相互作用下,纳米锥表面吸附CO分子产生局部凝聚,且随着纳米锥角的变小,吸附在锥顶部的分子更加密集,导致吸附分子间相互作用更强,为解释纳米结构表面吸附体系的异常红外效应提供依据.     

Local-field effect near the surface of dipolar lattices

The local-field correction near the surface of a dipolar crystal has been studied by explicitly computing the non-retarded electric field produced by a finite array of classical point dipoles induced by a time-varying electric field. Crystals of simple cubic, f.c.c. and b.c.c. structures are considered, and results presented for the self-consitent dipole moment and the local electric field at a lattice point. The utility of the method and its possible applications are discussed.     

Local Observation of Field Polarity Dependent Flux Pinning by Magnetic Dipoles

A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.     

Chaotic Dynamics of Test Particle in the Gravitational Field with Magnetic Dipoles

We investigate the dynamics of the test particle in the gravitational field with magnetic dipoles in thispaper. At first we study the gravitational potential by numerical simulations. We find, for appropriate parameters, thatthere are two different cases in the potential curve, one of which is the one-well case with a stable critical point, and theother is the three-well case with three stable critical points and two unstable ones. As a consequence, the chaotic motionwill rise. By performing the evolution of the orbits of the test particle in the phase space, we find that the orbits of thetest particle randomly oscillate without any periods, even sensitively depending on the initial conditions and parameters.chaotic motion of the test particle in the field with magnetic dipoles becomes even obvious as the value of the magneticdipoles increases.