Dynamic Crossover in Metallic Glass Nanoparticles

We report the dynamic crossover behavior in metallic glass nanoparticles(MGNs)with the size reduction based on the extensive molecular dynamics(MD)simulations combined with the activation-relaxation technique(ART).The fragile-to-strong transition of dynamics can be achieved by just modulating the characteristic size of MGNs.It can be attributed to the abnormal fast surface dynamics enhanced by the surface curvature.By determining the potential energy surface,we reveal the hierarchy-to-flat transition of potential energy landscape(PEL)in MGNs,and demonstrate the intrinsic flat potential landscape feature of the MGN with size smaller than a critical size.Our results provide an important piece of the puzzle about the size-modulated potential energy landscape and shed some lights on the unique properties of MGs in nanoscale.     

Exact Calculation of Antiferromagnetic Ising Model on an Inhomogeneous Surface Recursive Lattice to Investigate Thermodynamics and Glass Transition on Surface/Thin Film

An inhomogeneous 2-dimensional recursive lattice formed by planar elements has been designed to investigate the thermodynamics of Ising spin system on the surface/thin film. The lattice is constructed as a hybrid of partial Husimi square lattice representing the bulk and 1D single bonds representing the surface. Exact calculations can be achieved with the recursive property of the lattice. The model has an anti-ferromagnetic interaction to give rise to an ordered phase identified as crystal, and a solution with higher energy to represent the amorphous/metastable phase.Free energy and entropy of the ideal crystal and supercooled liquid state of the model on the surface are calculated by the partial partition function. By analyzing the free energies and entropies of the crystal and supercooled liquid state,we are able to identify the melting and ideal glass transition on the surface. The results show that due to the variation of coordination number, the transition temperatures on the surface decrease significantly compared to the bulk system.Our calculation qualitatively agrees with both experimental and simulation works on the thermodynamics of surfaces and thin films conducted by others. Interactions between particles farther than the nearest neighbor distance are taken into consideration, and their effects are investigated.     

Thermodynamic Comparison and the Ideal Glass Transition of Antiferromagnetic Ising Model on Multi-branched Husimi and Cubic Recursive Lattice with the Identical Coordination Number

Two types of recursive lattices with the identical coordination number but different unit cells (2-D square and 3-D cube) are constructed and the antiferromagnetic Ising model is solved exactly on them to study the stable and metastable states. A multi-branched structure of the 2-D plaquette model, which we introduced in this work, makes it possible to be an analog to the cubic lattice. Two solutions of each model can be found to exhibit the crystallization of liquid, and the ideal glass transition of supercooled liquid respectively. Based on the solutions, the thermodynamics on both lattices, e.g. the free energy, energy density, and entropy of the supercooled liquid, crystal, and liquid state of the model are calculated and compared with each other. Interactions between particles farther away than the nearest neighbor distance and multi-spins interactions are taken into consideration, and their effects on the thermal behavior are examined. The two lattices show comparable properties on the thermodynamics, which proves that both of them are practical to describe the regular 3-D case, especially to locate the ideal glass transition, while the 2-D multi-branched plaquette model is less accurate with the advantage of simpler formulation and less computation time consumption.     

Thermodynamic Comparison and the Ideal Glass Transition of Antiferromagnetic Ising Model on Multi-branched Husimi and Cubic Recursive Lattice with the Identical Coordination Number

Two types of recursive lattices with the identical coordination number but different unit cells(2-D square and 3-D cube) are constructed and the antiferromagnetic Ising model is solved exactly on them to study the stable and metastable states. A multi-branched structure of the 2-D plaquette model, which we introduced in this work, makes it possible to be an analog to the cubic lattice. Two solutions of each model can be found to exhibit the crystallization of liquid, and the ideal glass transition of supercooled liquid respectively. Based on the solutions, the thermodynamics on both lattices, e.g. the free energy, energy density, and entropy of the supercooled liquid, crystal, and liquid state of the model are calculated and compared with each other. Interactions between particles farther away than the nearest neighbor distance and multi-spins interactions are taken into consideration, and their effects on the thermal behavior are examined. The two lattices show comparable properties on the thermodynamics, which proves that both of them are practical to describe the regular 3-D case, especially to locate the ideal glass transition, while the 2-D multi-branched plaquette model is less accurate with the advantage of simpler formulation and less computation time consumption.     

Flat Histogram Monte Carlo Simulations of Triangulated Fixed-Connectivity Surface Models

Using the Wang-Landau flat histogram Monte Carlo (FHMC) simulation technique, we were able to study two types of triangulated spherical surface models in which the two-dimensional extrinsic curvature energy is assumed in the Hamiltonian. The Gaussian bond potential is also included in the Hamiltonian of the first model, but it is replaced by a hard-wall potential in the second model. The results presented in this paper are in good agreement with the results previously reported by our group. The transition of surface fluctuations and collapsing transition were studied using the canonical Metropolis Monte Carlo simulation technique and were found to be of the first-order. The results obtained in this paper also show that the FHMC technique can be successfully applied to triangulated surface models. It is non-trivial whether the technique is applicable or not to surface models because the simulations are performed on relatively large surfaces.     

New state of a 2D electron crystal above liquid helium

A transition to a new, presumably unstable, state of a two-dimensional (2D) electron crystal above liquid helium has been discovered at temperatures well below the melting point. The transition is manifested as an abrupt increase in the active component of the inverse conductivity of the crystal with a decrease in the potential pressing of the electrons to the surface. The state can be destroyed by the in-plane electric field of a sufficiently high amplitude. The new state is supposed to be a 2D electron glass.     

Interacting ghost dark energy in Brans-Dicke theory

We investigate the QCD ghost model of dark energy in the framework of Brans-Dicke cosmology. First, we study the non-interacting ghost dark energy in a flat Brans-Dicke theory. In this case we obtain the equation of state and the deceleration parameters and a differential equation governing the evolution of ghost energy density. Interestingly enough, we find that the equation of state parameter of the non-interacting ghost dark energy can cross the phantom line (wD=−1) provided the parameters of the model are chosen suitably. Then, we generalize the study to the interacting ghost dark energy in both flat and non-flat Brans-Dicke framework and find out that the transition of wD to phantom regime can be more easily achieved for than when resort to the Einstein field equations is made.     

非晶材料玻璃转变过程中记忆效应的热力学

低温下处于非平衡态的非晶材料升温到玻璃转变以上,要先后发生弛豫和回复最终达到平衡过冷液态,其中弛豫过程中释放的能量在回复过程中以等量的方式获取,表现出明显记忆行为.本文基于氧化物、金属与小分子等多种非晶形成体系,全面探讨了在围绕玻璃转变的一个冷却加热循环过程中的焓弛豫特征,建立了弛豫谱,发现弛豫焓在数值上与熔化焓密切相关.基于弛豫焓与非晶材料动力学Fragility之间的关联,展示了非晶体系在动力学极限(m=175)条件下的玻璃转变热力学基本特征,与热力学二级相变进行了对比.研究深化了对非晶弛豫与玻璃转变热力学的理解.     

Flat Information Geometries in Black Hole Thermodynamics

The Hessian of either the entropy or the energy function can be regarded as a metric on a Gibbs surface. For two parameter families of asymptotically flat black holes in arbitrary dimension one or the other of these metrics are flat, and the state space is a flat wedge. The mathematical reason for this is traced back to the scale invariance of the Einstein–Maxwell equations. The picture of state space that we obtain makes some properties such as the occurence of divergent specific heats transparent.Supported by VR.     

Atomic mechanism of vitrification process in simple monatomic nanoparticles *

Glass formation in simple monatomic nanoparticles has been studied by molecular dynamics simulations in spherical model with a free surface. Models have been obtained by cooling from the melt toward glassy state. Atomic mechanism of glass formation was monitored via spatio-temporal arrangement of solid-like and liquid-like atoms in nanoparticles. We use Lindemann freezing-like criterion for identification of solid-like atoms which occur randomly in supercooled region. Their number grows intensively with decreasing temperature and they form clusters. Subsequently, single percolation solid-like cluster occurs at temperature above the glass transition. Glass transition occurs when atoms aggregated into this single percolation cluster are in majority in the system to form relatively rigid glassy state. Solid-like domain is forming in the center of nanoparticles and grows outward to the surface. We found temperature dependence of potential energy, mean-squared displacement (MSD) of atoms, diffusion constant, incoherent intermediate scattering function, radial distribution function (RDF), local bond-pair orders detected by Honeycutt-Andersen analysis, radial density profile and radial atomic displacement distributions in nanoparticles. We found that liquid-like atoms in models obtained below glass transition have a tendency to concentrate in the surface layer of nanoparticles. However, they do not form a purely liquid-like surface layer coated nanoparticles.     

Morphological instability induced by the interaction of a particle with a solid-liquid interface

Motivated by recent experimental findings, we investigate the possible occurrence and characteristics of quasicrystalline order in two-dimensional mixtures of point dipoles with two sorts of dipole moments. Despite the fact that the dipolar interaction potential does not exhibit an intrinsic length scale and cannot be tuned a priori to support the formation of quasicrystalline order, we find that configurations with long-range quasicrystallinity yield minima in the potential energy surface of the many particle system. These configurations emanate from an ideal or perturbed ideal decoration of a binary tiling by steepest descent relaxation. Ground state energy calculations of alternative ordered states and parallel tempering Monte-Carlo simulations reveal that the quasicrystalline configurations do not correspond to a thermodynamically stable state. On the other hand, steepest descent relaxations and conventional Monte-Carlo simulations suggest that they are rather robust against fluctuations. Local quasicrystalline order in the disordered equilibrium states can be strong.Received: 15 September 2004, Published online: 26 November 2004PACS: 61.44.Br Quasicrystals - 75.50.Kj Amorphous and quasicrystalline magnetic materials - 82.70.Dd Colloids     

Geometric approach to the dynamic glass transition

We numerically study the potential energy landscape of a fragile glassy system and find that the dynamic crossover corresponding to the glass transition is actually the effect of an underlying geometric transition caused by the vanishing of the instability index of saddle points of the potential energy. Furthermore, we show that the potential energy barriers connecting local glassy minima increase with decreasing energy of the minima, and we relate this behavior to the fragility of the system. Finally, we analyze the real space structure of activated processes by studying the distribution of particle displacements for local minima connected by simple saddles.     

Tailoring the structure of water at a metal surface: a structural analysis of the water bilayer formed on an alloy template

Recent studies show that structures based on the traditional "icelike" water bilayer are not stable on flat transition metal surfaces and, instead, more complex wetting layers are formed. Here we show that an ordered bilayer can be formed on a SnPt(111) alloy template and determine the structure of the water layer by low energy electron diffraction. Close agreement is found between experiment and the structure calculated by density functional theory. Corrugation of the alloy surface allows only alternate water molecules to chemisorb, stabilizing the H-down water bilayer by reducing the metal-hydrogen repulsion compared to a flat surface.     

金属玻璃流变的扩展弹性模型

玻璃-液体转变现象,简称玻璃转变,被诺贝尔物理学奖获得者安德森教授评为最深奥与重要的凝聚态物理问题之一.金属玻璃作为典型的非晶态物质,具有与液体相似的无序原子结构,因此又称为冻结了的液态金属,是研究玻璃转变问题的理想模型材料.当加热至玻璃转变温度,或者加载到力学屈服点附近时,金属玻璃将会发生流动.由于热或应力导致的流动现象对金属玻璃的应用具有重要意义.本文简要回顾了金属玻璃流变现象,综述了流变扩展弹性模型的研究进展和未来发展趋势.     

Investigation of transition States in bulk and freestanding film polymer glasses

We have performed transition state searches on the potential energy landscape for bulk and freestanding film polymer glasses and identified connected minima. An analysis of the displacements between minima shows that the sites that undergo the greatest displacement are highly localized in space for both the bulk and the thin-film systems studied. In the case of the thin film, the clusters originate at the surface and penetrate into the center of the film thereby coupling the relaxation in the center of the film to the mobile surface layer. Furthermore, the energy barriers between minima are lower in the thin film than in the bulk system. These findings can rationalize the experimentally observed depression of the glass transition temperature in freestanding polymer films.     

Kinetic isotope studies of the gas-phase reaction (H,D,Mu)+HBr→(H,D,Mu)H+Br

Reaction rate constants and kinetic isotope effects for the gas-phase reaction A + HBrAH+Br, where A can be H, D, or Mu, have been studied using variational transition state theory with semiclassical tunneling on two semiempirical potential energy surfaces and a newab initio surface. The rate constants and kinetic isotope effects are compared to experimental results. This comparison is used to test the potential energy surfaces, to determine which regions of the potential energy surface control the reaction rates and the kinetic isotope effects, and to learn whether or not any of the potential energy surfaces is accurate for a wide enough range of features to predict both the reaction rate constants and the kinetic isotope effects for this chemical system.     

Dynamical Casimir–Polder atom-surface interaction

We have calculated dynamical Casimir–Polder force between a moving ground state atom and a flat polarizable surface. The velocity of an atom can be close to the velocity of light. The material properties are taken into account using a single oscillator model of the atomic dynamic polarizability and the Drude dielectric function of a metal substrate. The limit cases of nonrelativistic velocities and an ideal metal substrate are also considered. We have found specific dependence of the calculated forces on the velocity (energy), distance and material properties.     

Critical-point symmetry in a finite system

At a critical point of a second-order phase transition the intrinsic energy surface is flat and there is no stable minimum value of the deformation. However, for a finite system, we show that there is an effective deformation which can describe the dynamics at the critical point. This effective deformation is determined by minimizing the energy surface after projection onto the appropriate symmetries. We derive analytic expressions for energies and quadrupole rates which provide good estimates for these observables at the critical point.     

质子辐照硼硅酸盐玻璃盖片的物理效应分析

作为航天器电源系统的重要组成部分,太阳电池需要更高的转换效率和可靠性以及更长的使用寿命。通过在太阳电池表面覆盖抗辐照玻璃盖片,可以增强太阳电池对粒子辐射的防护,延长太阳电池的服役寿命,使航天器获得可靠的能源供应。硼硅酸盐玻璃就是一种理想的太阳电池玻璃盖片材料。采用蒙特卡罗方法,结合SRIM软件模拟研究质子辐照硼硅酸盐玻璃的损伤物理机理。基于粒子与物质相互作用的理论以及基本公式,通过分析不同入射能量的质子在硼硅酸盐玻璃中的阻止本领、电离能损、位移能损、空位的产生情况,对辐照损伤的物理机制进行研究。结果表明:能量为30~120 keV的质子辐照损伤主要发生在硼硅酸盐玻璃表面;质子沉积、空位分布等均为Bragg峰型分布;电离能损是能量损失的主要部分,随入射能量的增加而增大,导致电子的电离和激发;位移能损在玻璃内部随能量降低而增大,导致硼、氧和硅等空位缺陷的产生;电离效应和缺陷的产生是硼硅酸盐玻璃色心形成的重要原因。