Brownian dynamics simulation of the crystallization dynamics of charged colloidal particles

Crystal formation process of charged colloidal particles is investigated using Brownian dynamics (BD) simulations. The particles are assumed to interact with the pair-additive repulsive Yukawa potential. The time evolution of crystallization process and the crystal structure during the simulation are characterized by means of the radial distribution functions (RDF) and mean square displacement (MSD). The simulations show that when the interaction is featured with long-range, particles can spontaneously assemble into body-centered-cubic (BCC) arrays at relatively low particle number density. When the interaction is short-ranged, with increasing the number density particles become trapped into a stagnant disordered configuration before the crystallization could be actualized. The simulations further show that as long as the trapped configurations are bypassed, the face-centered-cubic (FCC) structures can be achieved and are actually more stable than BCC structures. (C) 2010 Elsevier Inc. All rights reserved.

Higher Moments of Net Proton Multiplicity Distributions at RHIC

200 GeV corresponding to baryon chemical potentials (mu(B)) between 200 and 20 MeV. Our measurements of the products kappa sigma(2) and S sigma, which can be related to theoretical calculations sensitive to baryon number susceptibilities and long-range correlations, are constant as functions of collision centrality. We compare these products with results from lattice QCD and various models without a critical point and study the root s(NN) dependence of kappa sigma(2). From the measurements at the three beam energies, we find no evidence for a critical point in the QCD phase diagram for mu(B) below 200 MeV.

重离子耗散反应中的长程关联

本论文介绍了重离子耗散反应产物激发函数中能量结构研究的概况及相关的理论模型，描述了重离子耗散反应激发函数测量实验的基本技术和数据分析方法，并着重分析了19F+27Al耗散反应产物激发函数之间的关联特性。重离子耗散反应产物激发函数的涨落结构研究，是分析重离子耗散反应中各物理量随时间的演化过程的有效方法。本论文报道了实验室系110.25―118.75MeV 19F+27Al耗散反应产物激发函数测量的实验结果，通过对激发函数的涨落结构进行关联函数分析，结合有关理论模型，探讨了耗散反应产物激发函数的能量结构的物理本质。 19F+27Al耗散反应中，不同角度测量到的类弹产物的激发函数之间存在强烈的关联，当出射产物在质心系180±对称时，这种关联有一个极大值。以“时间―功率谱”方法分析数据，运用态叠加原理和宇称守恒在弹靶核组成的中间双核系统的演化过程中，可以合理地解释观测到的实验现象，阐述耗散反应中长程角度关联的物理本质。 19F+27Al耗散反应中，同一角度测量到的不同类弹产物的激发函数之间也存在着强烈的关联。这种关联随着耗散程度的加深而减弱，它与耗散反应的“弛豫”过程是一致的。耗散反应中的这种关联起源于弹靶核组成的中间双核系统中不同角动量之间的干涉以及中间双核系统极其缓慢的退相干过程

二维无关联振子系统的量子化及能谱中长程关联的理论分析

量子经典对应是人们一直关心的基本问题．早期的WKB量子论及其推广EBK理论分别给出了一维及多维可积系统周期轨道的作用量量子化条件，但是，这些理论都没有明确的给出周期轨道与量子能级之间的对应关系．另一方面，近年来，人们在数值计算中发现量子能谱中存在着与周期轨道有紧密联系的长程关联，但是目前对长程关联的研究大多局限于数值计算，其背后的动力学原因有待进一步的探讨。应用二维无关联振子系统具有的标度不变性，对量子态密度进行Fourier变换，得到二维无关联振子系统的回归函数．另一方面，在有理环面上积分Hamiltonian运动方程，数值计算给出二维无关联四次振子系统的半经典回归谱。对二维无关联四次振子系统的量子回归谱和经典回归谱进行比较，量子和经典回归谱中的峰(显示了能级之间存在着长程关联)的位置大致一定验证了Berry-Tabor求迹公式的有效性。从可积系统的Be：rry-Tabor公式出发，导出了二维可积系统周期轨道作用量量子化条件，考虑有理环面上周期轨道必须满足的周期性条件，找到了量子能级与经典周期轨道之间的对应关系．这一对应关系表明，二维无关联振子系统的一组能级与一组周期轨道之间存在着一一对应关系。这组能级对应的周期轨道具有相同的拓扑，但每条周期轨道对应的系统能量等于它所对应的量子能级。进一步的，我们用二维无关联振子系统的量子经典对应关系去分析量子能谱中的长程关联。分析表明，当二维无关联振子系统回归函数中的作用量取某一系统能量下某一周期轨道的作用量的值时，那些与这一周期轨道拓扑相同的周期轨道对应的量子能级对回归函数的贡献相干。这些具有相同拓扑的周期轨道对应的量子能级间存在着长程关联。

Collisional quantum interference effect on rotational energy transfer in an atom-diatom system

The theoretical model of collisional quantum interference (CQI) in intramolecular rotational energy transfer is described in an atom-diatom system, based on the first Born approximation of time-dependent perturbation theory and considering a long-range interaction potential. The relation between differential and integral interference angles is obtained. For the CO A(1)Pi (v = 0)/e(3)Sigma (-)(v = 1)-He collision system, the calculated integral interference angles are consistent with the experimental values. The physical significance of interference angle and the essential factors it depends on as well as the influence of the short-range interaction on CQI are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

特大洪灾后对中国生态环境问题的思考

分析了洪灾之后中国经济发展面临的主要环境问题和退化症结 ,根据 2 0 10年远景目标的要求 ,提出了“提高环保认识 ,开展生态建设 ,控制环境污染”的综合整治对策 ,为中国制定生态环境规划提供基本思路。

Effects of pH on the Interactions and Conformation of Bovine Serum Albumin: Comparison between Chemical Force Microscopy and Small-Angle Neutron Scattering

The conformation of bovine serum albumin (BSA), as well as its interactions with negatively charged mica surfaces in saline solutions of different pH values, have been studied by small-angle neutron scattering (SANS) and chemical force microscopy (CFM), respectively. A new approach to extract the contribution of elementary interactions from the statistically averaged force-extension curves through self-consistent fitting was proposed and used to understand the effects of pH on the interactions and conformation of BSA in saline solutions. When pH increases, the SANS results reveal that the sizes of BSA molecules increase slightly, while the statistical analysis of the CFM results shows that the averaged pull-off force for the elongation monotonously decreases. The decrease of pull-off force with the increase of pH results from the decrease in the strength of hydrogen bonding and the number of interaction pairs, as well as the slight increase of the strength of van der Waals interaction. When pH approaches the isoelectric point (pI) of BSA, results from both SANS and CFM suggest a loss of long-range interactions in BSA molecules. Our results also suggest that the force-extension curve is mainly contributed by the van der Waals interaction. The combination of SANS and CFM provides new insight to understand the interactions and conformation of BSA molecules

Microphase Separation of Mixed Polymer Brushes: Dependence of the Morphology on Grafting Density, Composition, Chain-Length Asymmetry, Solvent Quality, and Selectivity

Microphase separation of binary mixed A/B polymer brushes exposed to different solvents is studied using Single-Chain-in-Mean-Field simulations. Effects of solvent quality and selectivity, grafting density, composition, and chain-length asymmetry are systematically investigated, and diagrams of morphologies in various solvents are constructed as a function of grafting density and composition or chain-length asymmetry. The structure of the microphase segregated morphologies lacks long-range periodic order, and it is analyzed quantitatively Using Minkowski measures.

Particle Size-Dependent Charge Ordering and Magnetic Properties in Pr0.55Ca0.45MnO3

A series of Pr0.55Ca0.45MnO3 compounds with average particle size ranging from 2000 to 30 nm have been synthesized by the sol-gel method and their charge ordering (CO) and magnetic properties are investigated. It is observed that with particle size decreasing, the CO transition is gradually suppressed and finally disappears upon particle size down to 35 nm, while the ferromagnetism (FM) emerges and exhibits a nonmonotonous variation with a maximum at 45 nm samples. The FM components in all samples never reach long-range ordering but rather only show short-range clusters. A new explanation considering the coupling between lattice, charge, and spin in the system is raised to understand the suppression of the CO state, Both the competition between the CO/AFM and FM states and the core-shell model are employed to explain the variation of the FM phase. These results may provide a deeper insight into the physics of particle size effect on the charge ordering manganite.

Structure and magnetic properties of new compounds CdYFeWO7

The solid solutions of CdYFeWO7, which are cubic pyrochlores of the type A(2)B(2)O(7), have been prepared and their structures were determined using Ab initio method. Rietveld refinement of the powder XRD data showed that CdYFeWO7 adopted cubic (Fd-3m) structure, while oxides crystallized in a defect-pyrochlore structure where both O (48f) and O'(8b) sites were partially occupied, and the frustrated cations sublattice precluded long range ordering of Fe/W in the pyrochlore structure. Charge distribution analysis also suggested incomplete occupation of different oxygen sites, thus the compound was non-stoichiometric, with the formula CdYFeW0.982O6.94, Magnetic measurements were carried out to find that Fe ions were in the high spin trivalent state. Curie Weiss paramagnetism down to similar to 5 K and the characteristic superposition between FC and ZFC suggested spin liquid rather than spin glass state.

Formation of ordered microphase-separated pattern during spin coating of ABC triblock copolymer

In this paper, the authors have systematically studied the microphase separation and crystallization during spin coating of an ABC triblock copolymer, polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-b-P2VP-b-PEO). The microphase separation of PS-b-P2VP-b-PEO and the crystallization of PEO blocks can be modulated by the types of the solvent and the substrate, the spinning speed, and the copolymer concentration. Ordered microphase-separated pattern, where PEO and P2VP blocks adsorbed to the substrate and PS blocks protrusions formed hexagonal dots above the P2VP domains, can only be obtained when PS-b-P2VP-b-PEO is dissolved in N,N-dimethylformamide and the films are spin coated onto the polar substrate, silicon wafers or mica. The mechanism of the formation of regular pattern by microphase separation is found to be mainly related to the inducement of the substrate (middle block P2VP wetting the polar substrate), the quick vanishment of the solvent during the early stage of the spin coating, and the slow evaporation of the remaining solvent during the subsequent stage. On the other hand, the probability of the crystallization of PEO blocks during spin coating decreases with the reduced film thickness. When the film thickness reaches a certain value (3.0 nm), the extensive crystallization of PEO is effectively prohibited and ordered microphase-separated pattern over large areas can be routinely prepared.

Annealing effects on the surface morphologies of thin PS/PMMA blend films with different film thickness

The surface morphology evolution of three thin polystyrene (PS)/polymethyl methacrylate (PMMA) blend films (<70 nm) on SiOx substrates upon annealing were investigated by atomic force microscopy (AFM) and some interesting phenomena were observed. All the spin-coated PS/PMMA blend films were not in thermodynamic equilibrium. For the 67.1 and the 27.2 nm PS/PMMA blend films, owing to the low mobility of the PMMA-rich phase layer at substrate surfaces and interfacial stabilization caused by long-range van der Waals forces of the substrates, the long-lived metastable surface morphologies (the foam-like and the bicontinuous morphologies) were first observed. For the two-dimensional ultrathin PS/PMMA blend film (16.3 nm), the discrete domains of the PS-rich phases upon the PMMA-rich phase layer formed and the secondary phase separation occurred after a longer annealing time.

Investigation into self-assembled monolayers of a polyether dendron thiol: Chemisorption, kinetics, and patterned surface

Different sizes of Frechet-type dendrons with a thiol group at the focal point were synthesized, well characterized, and used as building blocks for the preparation of self-assembled monolayers (SAMs) on metal surfaces. From the studies of the kinetic process of dendron thiol self-assembling on gold, it is shown that the dendron thiol assembling proceeds with different adsorption rates depending on the assembly time. In contrast to normal alkanethiols forming highly molecular structures on metal surfaces, the SAMs of polyether dendron form patterned surfaces with nanometer-sized features and in long-range order. It is found that the patterned stripes are closely related to the size of the dendron, and the patterned stripes can be improved by thermal annealing.

Phase identification in a series of liquid crystalline TPP polyethers and copolyethers having highly ordered mesophase structures. 6. Structure changes from smectic to columnar phases in a series of copolyethers containing odd and even numbers of methylene units in equal molar composition

A series of liquid crystalline copolyethers has been synthesized from 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)propane and different alpha,omega-dibromoalkanes [coTPP(n/m)]. In this report, coTPPs having n = 5, 7, 9, 11 and m = 12 are studied, which represent copolyethers having both varying odd number and a fixed even number of methylene units. The compositions were fixed at an equal molar ratio (50/50). These coTPPs(nlm) show multiple phase transitions during cooling and heating in differential scanning calorimetry experiments. The undercooling dependence of these transitions is found to be small, indicating that these transitions are close to equilibrium, Although the coTPPs possess a high-temperature nematic (N) phase, the periodicity order along the chain direction is increasingly disturbed when the length of the odd-numbered methylene units decreases from n 11 to 5. in the coTPPs(5/12, 7/12, and 9/12), wide-angle X-ray diffraction experiments at different temperatures show that, shortly after the N phase formation during cooling, the lateral molecular packing improves toward a hexagonal lattice, as evidenced by a gradual narrowing of the scattering halo. This process represents the possible existence of an exotic N phase, which serves as a precursor to the columnar (Phi(H)) phase. A further decrease in temperature leads to a (PH phase having a long-range ordered, two-dimensional hexagonal lattice. In coTPP(11/12), the phase structures are categorized as highly ordered and tilted, smectic and smectic crystal phases, similar to homoTPPs, such as the smectic F (S-F) and smectic crystal G (SCG) phases. An interesting observation is found for coTPP(9/12), wherein a structural change from the high-temperature Phi(H) phase to the low-temperature S-F phase occurs. It can be proven that, upon heating, the well-defined layer structure disappears and the lateral packing remains hexagonal. The overall structural differences in this series of coTPPs between those of the columnar and highly ordered smectic phases are related to the disorders introduced into the layer structure by the dissimilarity of the methylene unit lengths in the comonomers.