Monte-Carlo simulations of rotating clusters

A new scheme for estimating densities of states at non zero angular momentum is proposed, using the Monte-Carlo (MC) and multiple histogram methods. It is based on a rigorous expression of the classical density of states for a rotating system. Two features appear: the centrifugal energy ( angular momentum and I the instantaneous inertia tensor in the center of mass reference frame) is added to the potential energy and the configurational densities of states is weighted by . Comparing the MC results for the 13-atom Lennard-Jones cluster and a calculation based on molecular dynamics (MD) shows that this weight is important if the rotation induces a structural change at a finite temperature. The MC algorithm proves to be much more efficient than MD, even at finite . Received: 27 January 1998 / Received in final form: 15 June 1998 / Accepted: 18 June 1998     

Ising-like agent-based technology diffusion model: Adoption patterns vs. seeding strategies

The well-known Ising model used in statistical physics was adapted to a social dynamics context to simulate the adoption of a technological innovation. The model explicitly combines (a) an individual’s perception of the advantages of an innovation and (b) social influence from members of the decision-maker’s social network. The micro-level adoption dynamics are embedded into an agent-based model that allows exploration of macro-level patterns of technology diffusion throughout systems with different configurations (number and distributions of early adopters, social network topologies). In the present work we carry out many numerical simulations. We find that when the gap between the individual’s perception of the options is high, the adoption speed increases if the dispersion of early adopters grows. Another test was based on changing the network topology by means of stochastic connections to a common opinion reference (hub), which resulted in an increment in the adoption speed. Finally, we performed a simulation of competition between options for both regular and small world networks.     

Monte-Carlo simulations of a flux tube model

We present a re-analysis of the decaybs in the Minimal Supersymmetric Standard Model with radiatively induced breaking of the gauge groupSU(2)×U(1). We extend this analysis to regions of the supersymmetric parameter space wider than those previously studied. Results are explicitly presented form _t =150 GeV. We discuss the consequences of a measurement of this decay at the Standard Model level for future searches of charged Higgs and charginos. In spite of the strong sensitivity of this decay to the effects of supersymmetry, we find that, at the moment, no lower limit on the mass of these particles can be obtained. The large chargino contribution to the branching ratio forbs obtained at large values of tan is effectively reduced by pushing the lightest eigenvalue of the up-squark mass matrix closer to the electroweak scale, i.e. by increasing the degree of degeneracy among the up-squarks.Supported by the Bundesministerium für Forschung und Technologie, 05 5 HH 91P(8), Bonn, Germany     

Quantum Ising-like spin-j model with multipole-multipole interactions: Second order mean-field approach

A quantum Ising-like spin-j model, characterized by arbitrary multipole-multipole interactions as well as external fields, is proposed and analyzed. A thorough discussion of the algebraic properties of the dynamical variables (which belong to SU(2j+1)) is performed, whereby by the application of an improved version of the mean-field theory (which has the usual mean-field theory as its zeroeth order approximation) an explicit expression for the free energy is derived.     

Driven translocation dynamics of polynucleotides through a nanopore: off-lattice Monte-Carlo simulations

The driven translocation dynamics of a polynucleotide chain through a nanopore is studied using off-lattice Monte-Carlo simulations, which plays an important role in the nanopore sequencing of polynucleotides. We report a detailed study on the dependence of translocation dynamics on the chain length and the local geometry near the nanopore. In particular, we find that the length dependence of the infection time of the chain could exhibit very different behaviors for different geometries.     

Melting and evaporation transitions in small Al clusters: canonical Monte-Carlo simulations

A dimer of bound atoms cannot melt, only dissociate. Bulk metals show a well defined first order transition between their solid and liquid phases. The appearance of the melting transition is explored for increasing clusters sizes via the signatures in the specific heat and the root mean square of the bond lengths B (Berry parameter) by means of Monte-Carlo simulations of Al clusters modelled by Gupta potentials. Clear signatures of a melting transition appear for N6 atoms. Closed-shell effects are shown for clusters with up to 56 atoms. The melting transition is compared in detail with the dissociation transition, which induces a second and possibly much larger local maximum in the specific heat at higher temperatures. Larger clusters are shown to fragment into dimers and trimers, which in turn dissociate at higher temperatures.     

Numerical simulations of confocal Raman spectroscopic depth profiles of materials: a photon scattering approach

We present a simple model that uses a novel photon scattering approach to predict the depth profile response obtained when confocal Raman spectroscopy is applied both to silicon and to a number of related polymeric materials of varying optical clarity. This paper first provides an overview of the models proposed to date to demonstrate the evolution in understanding of the confocal Raman response of semi‐transparent materials, based upon geometrical optics. A new model is then described that is based upon the twin notions of a permanent extended Raman illuminated volume and the degree of extinction of the incident and Raman scattered photons from the whole of the illuminated volume as it is gradually moved further into, or defocused above, the sample. The model's predictions are compared with empirical data from previous studies of a range of semi‐crystalline polymers with different scattering properties and, by means of contrast, with that of a silicon sample. We show that, despite its inherent simplicity, the physics this model utilizes is able successfully to predict the form of the depth profile for each material, something that has not been achieved by any model previously proposed, and that the parameters used in the model scale with independent physical measurements. Finally the model is used to account for the fact that useful Raman spectra can be obtained when the laser is focused as much as 40 µm above the sample surface. Copyright © 2007 John Wiley & Sons, Ltd.     

Efficient method of finding scaling exponents from finite-size Monte-Carlo simulations

Monte-Carlo simulations are routinely used for estimating the scaling exponents of complex systems. However, due to finite-size effects, determining the exponent values is often difficult and not reliable. Here, we present a novel technique of dealing with the problem of finite-size scaling. This new method allows not only to decrease the uncertainties of the scaling exponents, but makes it also possible to determine the exponents of the asymptotic corrections to the scaling laws. The efficiency of the technique is demonstrated by finding the scaling exponent of uncorrelated percolation cluster hulls.     

Surface topography problem and argon adsorption on crystalline faces: Monte-Carlo and lattice-gas model simulations

The lattice-gas model in the quasi-chemical approximation (QCA) was used for adsorption isotherms and heats of adsorption calculations. The theory considers the surface topography, taking into account the atomic surface structure and the occupancy correlation of different adsorption sites. A comparison between Monte-Carlo and QCA simulations of the adsorption isotherms for argon atoms on three faces (1 1 0), (1 0 0) and (1 0 1) of rutile shows that both techniques give rather similar results with the advantage of QCA calculations being performed in a fraction of the time necessary for the Monte-Carlo simulation.     

Monte-Carlo法模拟二维Ising模型 ——Metropolis、Swendsen-Wang与Wolff算法的对比

Ising模型是一种应用广泛的磁自旋相互作用模型,其二维情况严格求解极为复杂,实际应用中通常利用Wolff算法进行模拟.Wolff算法目前被认为是最好的聚类翻转Monte-Carlo算法.Metropolis和Swendsen-Wang算法同Wolff算法类似,理论上也适用于Ising模型的模拟,却未有文章将三者系统对比来说明Wolff算法的优越性,本科课程对于Monte-Carlo算法的介绍也较少.本文分别利用三种算法模拟了二维Ising模型,介绍了其算法原理、参数选择及实现方式,分析对比了三种算法的模拟效果和适用范围,从而总结说明在二维Ising模型的模拟中Wolff算法效果更好的原因.     

低能Pt原子团簇沉积过程的分子动力学模拟

利用分子动力学模拟系统研究了低能Pt38,Pt141和Pt266原子团簇与Pt(001)表面的相互作用过程,详细分析了初始原子平均动能为0.1,1.0和10eV的原子团簇的沉积演化过程及其对基体表面形貌的影响.研究表明,初始原子平均动能是描述低能原子团簇的重要参量.当团簇的平均原子动能较低时,团簇对基体表层原子点阵损伤较小,基本属于沉积团簇;随着入射团簇的原子平均动能的增加,团簇对表层原子点阵结构的破坏能力增强,当团簇的原子平均动能增加到10eV时,团簇已经显现出注入特征.低能原子团簇对基体表面形貌的影响 关键词： 分子动力学模拟 低能原子团簇 载能沉积     

Theoretical study of solvent-mediated Ising-like systems: One-dimensional version

We theoretically study physical properties of solutes placed in a straight line and at regular intervals. The solute is a rigid-body and has an arrow-like shape, which changes its direction up (↑$\uparrow$) or down (↓$\downarrow$). If the rigid solutes are immersed in a continuum solvent, nothing happens in the system (it is an obvious fact). However, the property of the directions differs in a granular solvent (e.g., hard-sphere solvent). Depending on the distance between the nearest-neighbor solutes, the directional correlation between them is periodically changed as follows: “parallel-tendency (↑↑$\uparrow \uparrow$)” ↔$\leftrightarrow$ “random” ↔$\leftrightarrow$ “antiparallel-tendency (↑↓$\uparrow \downarrow$)”. Studying a newly created nanosystem, it is able to discover interesting properties hiding in the nanosystem. We believe that such an approach contributes to the development of nanotechnology.     

Si60团簇的结构及其与Si(111)面间碰撞的分子动力学模拟

利用紧束缚分子动力学退火方法模拟研究了Si60团簇的稳定结构和基态能量,结果表明Si60团簇为具有T对称性的截顶二十面体的富勒烯结构,平均键长为0236nm,直径为0933nm,原子结合能为4.45eVatom,JahnTeller效应对Si60团簇的结构有很大影响.通过对Si60分子和Si(111)面碰撞机理的粒子数、体积和能量不变分子动力学模拟,发现Si60分子吸附在Si(111)面所需要的垂直入射动能为40eV,Si60分子远不如C60分子稳定 关键词： 紧束缚 JahnTeller效应 碰撞     

Ultrasonically assisted turning of aviation materials: simulations and experimental study

Ultrasonically assisted turning of modern aviation materials is conducted with ultrasonic vibration (frequency f approximately 20 kHz, amplitude a approximately 15 microm) superimposed on the cutting tool movement. An autoresonant control system is used to maintain the stable nonlinear resonant mode of vibration throughout the cutting process. Experimental comparison of roughness and roundness for workpieces machined conventionally and with the superimposed ultrasonic vibration, results of high-speed filming of the turning process and nanoindentation analyses of the microstructure of the machined material are presented. The suggested finite-element model provides numerical comparison between conventional and ultrasonic turning of Inconel 718 in terms of stress/strain state, cutting forces and contact conditions at the workpiece/tool interface.     

A self-consistent cluster theory of amorphous materials

A cluster method for the electronic structure of amorphous materials is presented with an application to amorphous diamond. Eight-atom cluster model gives a pseudogap in the density of states.     

Water peak suppression: time-frequency vs time-scale approach

Wavelets are the most popular time-scale analysis tool. A well-known application of wavelets in nuclear magnetic resonance spectroscopy is water peak extraction/suppression. However, spectroscopists are more familiar with frequency than scale. So, from a spectroscopist point of view, a time-scale analysis tool (i.e., wavelets) is not natural and a time-frequency approach would be much more satisfactory. We explain a time-frequency solution to this problem based on Gabor analysis. As the two formalisms are closely linked together we continuously emphasize their similarities and differences. In particular we show that, here, the Gabor method is as efficient as the wavelet approach, and we give some examples. Those remarks also apply to other NMR problems solved previously with the continuous wavelet transform, such as quantification or dynamical phase correction.     

Interactions of like-charged rods at low temperatures: Analytical theory vs. simulations

We investigate a system consisting of two like-charged infinitely long rods and neutralizing counterions at low temperatures, using both analytic theory and simulations. With some reasonable approximations we can analytically solve for several ground-state structures of the model, starting with states where all counterions are lined up in the gap between the rods, over planar configurations, where the counterions are divided up into a fraction which resides between the rods, and counterions which are located on the outer surfaces, up to configurations which cover the full rod surfaces. Using parallel tempering simulations, we are able to study the system over a wide range of temperatures. At low temperatures we find good agreement with our T = 0 results. At higher temperatures, the strong coupling (SC) theory delivers qualitatively better results. We furthermore demonstrate that for the SC theory and our ground-state approximations to yield quantitative agreement, three parameters are required to be large, the strong-coupling parameter Ξ, the Rouzina-Bloomfield parameter, and the ratio of the average distance of the counterions to the radius of the rods. In the case of the latter ratio being small, our T = 0 results show better agreement with the simulation data at very low temperatures.