Phase transitions and dynamic entropy in small two-dimensional systems: Experiment and numerical simulation

The results of experimental and numerical analysis are presented for phase transitions in strongly nonequilibrium small systems of strongly interacting Brownian particles. The dynamic entropy method is applied to analysis of the state of these systems. Experiments are carried out with kinetic heating of the structures of micron-size particles in a laboratory rf discharge plasma. Three phase states of these small systems are observed: crystalline, liquid, and transient. The mechanism of phase transitions in cluster structures of strongly interacting particles is described.     

Solid structures of benzene at high pressures: molecular dynamics study

The structure of the experimentally-observed phase II of solid benzene at high pressures has been investigated through structural phase transitions in molecular dynamics simulations. Several structures are obtained by the pressure-induced phase transitions from the low-pressure phase I. Among them, the most possible structure is a high-pressure phase III with stacking faults, which is more stable than two structures previously proposed for the phase II by molecular packing analysis at high pressures. Furthermore, the experimental powder X-ray diffraction pattern for the phase II can be explained by the structure.     

一种二元整形元件激光直写方法的实验研究

为了实现具有复杂位相结构的二元光束整形元件,提出了采用方光点的激光直写系统来逐点光刻浮雕位相结构的方法,通过双远心投影缩微光路获得5-20 μm方点,光点尺寸对应于最小位相单元,从而获得了位相结构的高质量直写.分析了高斯光点和方光点位相单元结构对二元整形元件衍射效率的影响,方形两台阶二元整形元件的+1级衍射效率达到31%,给出了实验结果.     

Fractal aggregates evolution of methyl red in liquid crystal

The spontaneous formation of dendritic aggregates is observed in a two-dimensional confined layered system consisting of a film composed of liquid crystal, dye and solvent cast above a polymer substrate. The observed aggregates are promoted by phase separation processes induced by dye diffusion and solvent evaporation. The growth properties of the aggregates are studied through the temporal evolution of their topological properties (surface, perimeter, fractal dimension). The fractal dimension of the completely formed structures, when they are coexistent with different types of structures, is consistent with theoretical and experimental values obtained for Diffusion-Limited Aggregates. Under different experimental conditions (temperature and local dye concentration) the structure forms without interactions with other kinds of structures, and its equilibrium fractal dimension is smaller. The fractal dimension is thus not a universal property of the observed structures, but rather depends on the experimental conditions.     

循环流化床内颗粒团流动的研究

循环流化床颗粒相流动具有多尺度效应:单颗粒运动的微尺度、颗粒团运动的介尺度和固相运动的宏尺度。颗粒相流动参数受单颗粒运动和颗粒聚团运动的制约,同时影响气相流动。基于气体分子运动论和颗粒动理学,建立相平均稠密气固两相流流动模型。介尺度模型考虑颗粒团与单颗粒之间、颗粒团与气相之间的动量和能量的传递和耗散。模拟计算颗粒容积份额、速度等参数与实测值相吻合。     

Unveiling new magnetic phases of undoped and doped manganites

Novel ground-state spin structures in undoped and lightly doped manganites are investigated based on the orbital-degenerate double-exchange model, via mean-field and numerical techniques. In undoped manganites, a new antiferromagnetic (AFM) state, called the E-type phase, is found adjacent in parameter space to the A-type AFM phase. Its structure is in agreement with recent experimental results. This insulating E-AFM state is also competing with a ferromagnetic metallic phase as well. For doped layered manganites, the phase diagram includes another new AFM phase of the CxE1-x type. Experimental signatures of the new phases are discussed.     

Formation of liquidlike and crystalline structures in dusty plasmas

Under certain conditions in a dusty plasma, which is a low-temperature plasma with dust grains, the strong interaction between grains can give rise to gas-liquid-solid-state phase transitions. A study is made of ordered (liquidlike and crystalline) grain structures in various kinds of dusty plasmas: a thermal plasma at atmospheric pressure, a plasma of a dc glow discharge, and a UV radiation-driven plasma. The results of experimental observations of ordered dust structures are reported, and the characteristic features of the dust structures and the conditions for their appearance are discussed.     

Guided wave dispersion curves for a bar with an arbitrary cross-section,a rod and rail example

Theoretical and experimental issues of acquiring dispersion curves for bars of arbitrary cross-section are discussed. Since a guided wave can propagate over long distances in a structure, guided waves have great potential for being applied to the rapid non-destructive evaluation of large structures such as rails in the railroad industry. Such fundamental data as phase velocity, group velocity, and wave structure for each guided wave mode is presented for structures with complicated cross-sectional geometries as rail. Phase velocity and group velocity dispersion curves are obtained for bars with an arbitrary cross-section using a semi-analytical finite element method. Since a large number of propagating modes with close phase velocities exist, dispersion curves consisting of only dominant modes are obtained by calculating the displacement at a received point for each mode. These theoretical dispersion curves agree in characteristic parts with the experimental dispersion curves obtained by a two-dimensional Fourier transform technique.     

Selectivity and temperature dependence of phase and phase transition in diblock copolymer solution

In order to study the effects of solvent selectivity and temperature on phase behavior and transition of diblock copolymer solution, self-consistent field theory is modified to incorporate the short-range interaction and non-local effects. Inhomogeneous free-energy density is shown to be dependent on solvent selectivity, temperature and copolymer concentration. Enthalpic quantity and entropic contributions are crucial to phase diagrams of diblock copolymer solution. Three selective strengths of solvent --weak, moderate and strong-- are chosen for comparison. For a weakly selective solvent, theoretical and experimental results illustrate the same variation tendency in the phase boundary of the order-disorder transition for a symmetric diblock of polystyrene and polyisoprene. Self-consistent field equations can be used to calculate the exact FCC-BCC structural phase transition temperatures in moderately and strongly selective solvents. Detailed comparison with the experimental phase diagrams including lamellar, cylindrical and spherical structures is presented.     

Ferroelectric mesophase with randomized interlayer structure

We present the experimental and theoretical evidence that a new Sm X phase made of asymmetric bent-shaped molecules has a layered, nontilted, optically uniaxial and polarly ordered structure with random direction of the layer polarization. The randomness results from the sign degeneracy of the difference in polarization directions in neighboring layers, although the magnitude of the phase difference is constant. Lifting the degeneracy by external fields two additional structures of the Sm X phase are possible.     

Two-dimensional ‘hat-scratch’ periodic structures induced by a single femtosecond laser pulse on silica glass

Two-dimensional ‘hat-scratch’ structures are fabricated on silica glass by the interference of three non-coplanar beams originating from a single femtosecond laser pulse. The scanning electron microscope (SEM) characterizations show that the as-formed structures are composed of hat holes and scratch marks. The experimental results indicate that the structures are dependent on the intensity of laser beam. The formation of the two-dimensional ‘hat-scratch’ structures is mainly due to the combined laser ablation effects including ionization, shock wave, plasma expansion, and phase explosion.     

A method for measuring the phase of the reflection coefficient in the visible range of the spectrum

A method for measuring the phase of the reflection coefficient in the optical wavelength range is proposed. The method is simple in experimental implementation and is based on measuring the energyreflection coefficients of a sample in two media with different refractive indices. Analytical and numerical estimates show that the measurement accuracy of the phase is on the order of 1°. The possibilities of using the results of the phase measurement in practice for a more complete characterization of materials and structures under investigation are considered.     

Hubert model for modulated states in systems with competing exchange interactions

Within a continuum model for magnetic systems with competing interactions introduced by Hubert the classification of all possible modulated structures for axial easy-plane systems has been carried out, and the phase portraits for these solutions are constructed. Analytic solutions for the structure of isolated domain walls between helicoids of different chirality are presented. The equilibrium structures of the modulated phases in an applied magnetic field and the critical field for transitions from helical to fan phase are in qualitative agreement with results obtained within the corresponding discretized model and various experimental data.     

First principles calculations of structural phase transformation in CaTe at high pressure

First principles calculations of the total energy of CaTe as a function of unit cell volume have been carried out for the NaCl, MnP and CsCl structures on the basis of density functional theory (DFT). All these calculations are performed with the CRYSTAL06 program package. The sequence of high-pressure phases for CaTe transforms from NaCl phase to an intermediate state with a mixture of NaCl and MnP phases and then to the CsCl phase is obtained, which is in good agreement with the previous experimental results. Several structural properties (equilibrium lattice constant, bulk modulus, etc.) of NaCl structure have been calculated, which are also in agreement with the previous experimental results.     

金属氢研究新进展

简要介绍了金属氢的研究意义和应用前景 ,详细评述了有关的高压实验方法和最近的研究成果及进展 ,特别是固体氢的相图、结构和相变 .近十多年来 ,随着超高压技术的发展 ,已能在金刚石对顶砧 (DAC)上产生30 0GPa的静态压力 ,并可进行高压原位实验研究 .对固体氢进行了高压拉曼、同步辐射X射线、光反射和吸收、同步辐射红外光谱等一系列高压物性和相变研究 .从而确定了固体氢的三个相 ,并提出了可能的相结构 .     

MAPbCl3钙钛矿中甲胺阳离子运动的固体2H NMR研究

本文利用固体²H核磁共振（²H NMR）技术探究了甲胺铅氯（MAPbCl₃）钙钛矿正交相、四方相和立方相晶体结构内甲胺阳离子（MA）的运动特征.结合²H NMR实验数据和拟合数据，本文分析和建立了不同相结构中MA阳离子的运动模型，揭示了不同相结构中MA阳离子的运动特征：正交相中MA并不处于完全静止状态，而是在围绕C-N键快速内旋转的同时局部晃动；随着温度升高，MA在钙钛矿晶格中呈现出多种不同的运动模式，MA阳离子运动在不同温度下的变化过程与该材料的相转变过程并不一致.该研究有助于加深对MAPbCl₃相结构转变分子机制的理解.     

Ab initio calculation of the phase stability,mechanical properties and electronic structure of ZrCr2 Laves phase compounds

Ab initio calculations have been used to investigate the phase stability, mechanical properties and electronic structure of ZrCr₂ Laves phase compounds, based on the method of augmented plane waves plus local orbitals with the generalized gradient approximation. The calculated lattice constants for the C15, C36 and C14 structures are in good agreement with experimental values. The calculation of heats of formation showed that C15 is a ground-state phase, whereas C36 is an intermediate phase and C14 the high-temperature phase. The elastic constants and elastic moduli for the C15 structure were calculated systematically and compared with experiments and previous theoretical calculations. The intrinsic and extrinsic stacking fault energies are found to be 112 and 98?mJ?m^?2, respectively. The equilibrium separations between Schockley are also predicted using the calculated elastic moduli and stacking fault energies. Finally, the calculated electronic structures of these Laves phases are discussed based on these results.     

Pressure induced magnetic and semiconductor–metal phase transitions in Cr_2MoO_6

We investigate magnetic ordering and electronic structures of Cr_2MoO_6under hydrostatic pressure. To overcome the band gap problem, the modified Becke and Johnson exchange potential is used to investigate the electronic structures of Cr_2MoO_6. The insulating nature at the experimental crystal structure is produced, with a band gap of 1.04 eV, and the magnetic moment of the Cr atom is 2.50 μB, compared to an experimental value of about 2.47 μB. The calculated results show that an antiferromagnetic inter-bilayer coupling–ferromagnetic intra-bilayer coupling to a ferromagnetic inter-bilayer coupling–antiferromagnetic intra-bilayer coupling phase transition is produced with the pressure increasing. The magnetic phase transition is simultaneously accompanied by a semiconductor–metal phase transition. The magnetic phase transition can be explained by the Mo–O hybridization strength, and ferromagnetic coupling between two Cr atoms can be understood by empty Mo-d bands perturbing the nearest O-p orbital.     

三元Ⅲ─V半导体合金的基态有序结构

利用了自旋１／２二维Ｉｓｉｎｇ模型及其“不等式方法”,确定了三元Ⅲ─Ｖ半导体会金（００１）面外延生长中的二维平面基态有序结构和基态相图,并通过层状叠加得到了合金中可能出现的长程有序结构,较好地说明了实验发现的（００１）衬底外延生长中的长程有序现象．     

Elastic and thermodynamic properties of vanadium nitride under pressure and the effect of metallic bonding on its hardness

By the particle-swarm optimization method, it is predicted that tetragonal P42mc, 141md, and orthorhombic Amm2 phases of vanadium nitride （VN） are energetically more stable than NaCl-type structure at 0 K. The enthalpies of the predicted three new VN phases, along with WC, NaC1, AsNi, CsCl type structures, are calculated each as a function of pressure. It is found that VN exhibits the WC-to-CsCl type phase transition at 256 GPa. For the considered seven crystal- lographic VN phases, the structures, elastic constants, bulk moduli, shear moduli, and Debye temperatures are investigated. Our calculated equilibrium structural parameters are in very good agreement with the available experimental results and the previous theoretical results for the NaC1 phase. The Debye temperatures of VN predicted three novel phases, which are all higher than those of the remaining structures. The elastic constants, thermodynamic properties, and elastic anisotropies of VN under pressure are obtained and the mechanical stabilities are analyzed in detail based on the mechanical stability criteria. Moreover, the effect of metallic bonding on the hardness of VN is also investigated, which shows that VNs in P42mc, 141md, and Amm2 phases are potential superhard phases. Further investigation on the experimental level is highly recommended to confirm our calculations presented in this paper.