Formation of two-dimensional crystal-like structures from inclusions in smectic C films

The structures formed by inclusions in smectic C (SmC) free-standing films are investigated using polarized light microscopy. The domains confined in these two-dimensional (2D) systems induce distortion of the inplane orientational order, which governs the elastic interaction between the inclusions. The balance between long-range quadrupolar attraction and short-range repulsion gives rise to a nontrivial collective behavior of domains. Various 2D structures are created as a function of the concentration and size of inclusions. We observe the formation of chains and then a 2D square lattice when the concentration of domains increases. Further increase in the domain size leads to the transition from square to hexagonal close-packed structure.     

Observation of a liquid phase with an orthorhombic orientational order

Using surface x-ray diffraction, we have determined the structure of liquid Bi monolayers on Cu(111) for a range of coverages. By combining diffuse scattering data from the liquid with information from the substrate scattering, the ordering properties of Bi have been fully determined. Even though the substrate is hexagonal, we find that the liquid does not show hexatic order but has an orthorhombic orientational order that occurs in three domains. Simultaneously, Bi has partial solidlike properties, even well above its melting point.     

Hall crystal states at nu = 2 and moderate landau level mixing

The structure and orientational ordering of N2O molecules physisorbed on graphite (0001) is investigated applying x-ray, neutron, and low-energy electron diffraction techniques. Combining the results of the three techniques, we find that N2O forms a highly ordered, hexagonal, commensurate (sqrt[21]xsqrt[21])R10.89 degrees phase. The unit cell contains seven molecules which are arranged in a seven-sublattice pinwheel structure, unexpected for linear molecules on a hexagonal lattice. Potential energy calculations corroborate these results.     

The quasicrystal-crystal interface between icosahedral Al-Pd-Mn and deposited Co: Evidence for the affinity of the quasicrystal structure to the CsCl structure

We have evaporated Co on the pentagonal surface of an icosahedral Al-Pd-Mn quasicrystal kept at room temperature. For submonolayer Co deposits, five AlCo domains are formed exposing their (1 1 0) faces parallel to the surface and rotated by 72° with respect to each other. The orientational relationship between these domains and the substrate is determined by the optimum matching of the average quasicrystal structure and the CsCl structure with a lattice constant of about 2.88 Å. For further deposition, body-centred cubic Co domains grow epitaxially on the AlCo domains.     

Relaxation contribution to shear moduli of the low-temperature phase of solid C60

The relaxation contribution of the molecular reorientation in an elastic field of an acoustic wave to the effective shear moduli is calculated in the framework of the phenomenological two-level model of orientational states in the low-temperature phase of solid C₆₀. The polarity of the rotation axis of C₆₀ molecules and the possible existence of orientational domains in the structure of the low-temperature phase are taken into consideration. The estimates obtained are compared with the available experimental data.     

冷却过程中Pd923 团簇的结构转变研究

本文利用分子动力学模拟方法, 研究了钯团簇在不同冷却条件下形成晶体及非晶的过程. 利用平均原子体积、双体分布函数、键对分析和键序参数方法研究了微观局域结构随温度的变化关系. 研究发现：在50 K/ps冷却过程中,液态Pd923团簇在1000 K发生玻璃化转变,在100 K下形成非晶结构; 而在0.1 K/ps冷却过程中,液态Pd923团簇发生结晶,并最终形成六角密排（hcp）晶体结构。     

冷却过程中Pd_(923)团簇的结构转变研究

本文利用分子动力学模拟方法,研究了钯团簇在不同冷却条件下形成晶体及非晶的过程.利用平均原子体积、双体分布函数、键对分析和键序参数方法研究了微观局域结构随温度的变化关系.研究发现:在50 K/ps冷却过程中,液态Pd923团簇在1000 K发生玻璃化转变,在100 K下形成非晶结构;而在0.1K/ps冷却过程中,液态Pd923团簇发生结晶,并最终形成六角密排(hcp)晶体结构.     

Effect of short and long range forces on the structure of water. II. Orientational ordering and the dielectric constant

The effects of short and long range interactions on the structure of water, both spatial and orientational, has been studied in detail by computing the full pair correlation function, site-site correlation functions, 2-dimensional site-site correlation functions in the (r _OO, r _OH) and (r _OO, r _HH) planes, dipole-dipole correlation function, radial Kirkwood g factor, and the dielectric constant. Two model potentials, the T1P4P and ST2, and their short range versions have been considered at ambient and elevated temperatures and under supercritical conditions. The Ewald summation under different conditions has been used to investigate also their effect on results. An analysis of the results shows that although all site-site correlation functions for the short and long range systems are similar, the orientational ordering in systems of different range may be considerably different, this evidence being provided mainly by the dipole-dipole correlation function and the radial Kirkwood factor. The orientational ordering is only short range in long range systems, whereas in short range systems the hydrogen bonding gives rise to a damped long range regular pattern of alignment. Nonetheless, the resulting dielectric constants for the short and long range systems coincide within the combined error bars. All findings are more pronounced at low temperatures but otherwise they are only marginally temperature and density dependent.     

颗粒尺寸分散度对颗粒体系力学和几何结构特性的影响

利用颗粒离散元方法, 研究了由2048个光滑颗粒组成的体系在各向同性压缩条件下, 颗粒尺寸分散度s对颗粒体系力学和几何结构特性的影响. 结果表明: 配位数、剪切模量、静态结构因子以及取向序关联函数等都随分散度的变化而变化, 而力的累积分布不受分散度的影响. 其中, 单分散体系(亦即s=0)的静态结构因子在低波数区域(亦即低k)遵从幂律标度S(k)∝0.2k^-4/3, 各分散度下取向序关联函数峰值符合e指数变化规律g₆(r)∝ae^-r/ξ₆, 其中序关联长度ξ₆随分散度s增大而减小. 关键词： 颗粒物质 尺寸分散度 结构因子 取向序关联     

Mesoscopic and macroscopic dipole clusters: Structure and phase transitions

A two dimensional (2D) classical system of dipole particles confined by a quadratic potential is studied. This system can be used as a model for rare electrons in semiconductor structures near a metal electrode, indirect excitons in coupled quantum dots etc. For clusters of N ≤ 80 particles ground state configurations and appropriate eigenfrequencies and eigenvectors for the normal modes are found. Monte-Carlo and molecular dynamic methods are used to study the order-disorder transition (the “melting” of clusters). In mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower temperatures an intershell orientational disordering of pairs of shells takes place; at higher temperatures the intershell diffusion sets in and the shell structure disappears. In “macroscopic” clusters (N > 37) an orientational “melting” of only the outer shell is possible. The most stable clusters (having both maximal lowest nonzero eigenfrequencies and maximal temperatures of total melting) are those of completed crystal shells which are concentric groups of nodes of 2D hexagonal lattice with a number of nodes placed in the center of them. The picture of disordering in clusters is compared with that in an infinite 2D dipole system. The study of the radial diffusion constant, the structure factor, the local minima distribution and other quantities shows that the melting temperature is a nonmonotonic function of the number of particles in the system. The dynamical equilibrium between “solid-like” and “orientationally disordered” forms of clusters is considered.     

Structure and electrocompression of chloride adlayers on Cu(111)

The specific adsorption of chloride on Cu(111) from acidic aqueous electrolyte (pH 3) was investigated by in-situ surface X-ray diffraction, revealing an incommensurate hexagonal rotated adlayer. The structure and its potential dependence suggest a strong adsorbate–adsorbate interaction as compared to other halide adlayers on (111)-oriented metal electrode surfaces. The orientational epitaxy can be rationalized by the model suggested by Grey and Bohr (Europhys. Lett. 18, 171, 1992), which is based on symmetry considerations.     

X-ray study of the order—disorder transition in high-stage alkali metal intercalated graphite single crystals

Graphite intercalation compounds display a variety of structural properties because of their composite nature (graphite + intercalate) and their layered arrangement. Alkali metal intercalated graphite compounds undergo order-disorder phase transitions when the temperature is varied in the range 300–10 K. The disordered state shows true two-dimensional character, whereas three-dimensional coupling takes place on ordering. Results of single-crystal X-ray diffractometric and photographic studies of stage-2 KC₂₄ single crystals are presented. The positional and orientational correlations of the modulated liquid phase have been studied from 300 K down to the temperature transition T_u = 123.5° K. At the transition, the hexagonal incommensurate solid structure of the alkali metal is modulated by the graphite potential. This transition is discussed in terms of the relaxed-close packed structure model (Dicenzo, 1982). At low temperature a second transition takes place at T_L ≈ 95 K. It is found to correspond to the breaking of the 2D hexagonal symmetry of the K layer.     

Orientational order in CO and N2 monolayers on graphite studied by X-ray diffraction

X-ray diffraction has been used to study the structure and orientational phase transitions of CO and N₂ adsorbed on graphite (Papyex). Both form orientationally ordered 2√3 × 2√3 R30° commensurate phases on graphite at low temperatures (10 K). The in-plane herringbone structure of N₂ has been confirmed but CO has more orientational disorder than N₂, which may be associated either with tilting, random static or systematic, of the molecules away from the surface and/or with orientational order of shorter range than the centre of mass order. In the first case the average tilt would have to be about 26° and in the second case the orientational correlation length would have to be 200 Å compared with 450 Å for the translational order. The orientational phase transition is sharp for N₂, occurring over the range 27–30 K, in agreement with previous work. For CO the transition is broad and starts at lower temperatures. This and the structural data indicate that a point quadrupolar interaction is not a suitable model for comparing the properties of N₂ and CO layers. The orientational phase transition in the incommensurate phase of N₂ is found to be broad and occurs below 20 K. For CO it is sharper than for the commensurate phase and occurs at a higher temperature. The lattice parameter changes by 0.75% across the orientational phase transition. For both N₂ and CO there is evidence of translational disorder in the commensurate phases but it cannot be interpreted quantitatively.     

Microphase separation in polymer solutions containing surfactants

A microphase separation in solutions containing a polymer and a mixture of two solvents, one of which consists of amphiphilic molecules (surfactant), is considered theoretically in the weak-segregation regime. A surfactant molecule is described as a dimer consisting of hydrophobic and polar parts. The energy gain due to the orientation of surfactant molecules can lead to the appearance of non-homogeneities in the solution, where density fluctuations cause the orientational ordering of surfactant molecules. The difference in the interaction energies of hydrophobic and polar groups of a surfactant with solvent is considered as a main reason for orienting surfactant molecules. The free energy is calculated for various morphologies (lamellar, cylindrical hexagonal, spherical particles arranged at different cubic lattices). The phase diagrams are presented. With worsening the solvent quality, the transitions from disordered to a macro-separated state at low polymer and surfactant concentrations or to a body-centered-cubic, then hexagonal, and then lamellar structure at high polymer and surfactant concentrations are predicted. The amphiphilicity degree of surfactant molecules should exceed a certain critical value to make a microstructure formation possible. The period of the lamellar microstructure decreases with increasing the surfactant and polymer concentrations.     

Spectral features of a nematic liquid crystal consisting of biaxial molecules with internal rotation

Expressions for the components of the optical permittivity tensor of a nematic liquid crystal consisting of biaxial π-conjugated molecules with a conformational degree of freedom (internal rotation) are obtained. A relationship between the intensity and dichroism of absorption bands in isotropic and nematic phases and the parameters of conformational, orientational, and mixed conformational and orientational orders of the molecules is established. A distribution function for the molecules is obtained that takes into account the mutual correlation of their conformational and orientational degrees of freedom. The effect of this correlation on the above-noted order parameters, the dichroism of the absorption bands, and the dependence of the measured oscillator strengths of molecular transitions on the character and degree of orientational order of biaxial molecules is studied. On the basis of comparison with experimental data, the relative role of different terms in the distribution function that are responsible for the correlation between the conformational and orientational degrees of freedom of the molecules is ascertained.     

Crystalline phases of alkyl-thiol monolayers on liquid mercury

The structure of octadecanethiol monolyers on liquid Hg surfaces, measured with subangstrom resolution, evolves with increasing coverage from a laterally disordered phase of surface-parallel molecules to ordered rotator phases of surface-normal molecules. For the latter, an abrupt transition is found at 19 A(2)/molecule from a rectangular packing of molecules tilted by 27 degrees in the nearest-neighbor direction to a hexagonal unit cell of untilted molecules. The unit cell of the tilted phase is centered for the chains and noncentered for the headgroups. The thiol headgroups associate in pairs with a single Hg atom, and the bonds form long-range orientational order. The different order of thiols on Au(111) and on Hg highlights the subphase's role in determining the overlayer's structure.     

Surface structure of an ultrathin alumina film on Ni3Al(111): a dynamic scanning force microscopy study

The surface structure of an ultrathin alumina film on a Ni3Al(111) substrate has been studied by dynamic scanning force microscopy. The alumina film exhibits a hexagonal superstructure with a lattice parameter of 4.14 nm and a (1/sqrt[3] x 1/sqrt[3])R30 degrees substructure. Two domains rotated by 24 degrees are present. The film is terminated by a hexagonal lattice of oxygen ions with a lattice parameter of 0.293 nm, which is rotated by 30 degrees with respect to the substrate lattice. The nodes of the 4.14 nm superstructure and the 2.39 nm substructure are pinned on points of the substrate lattice, where the surface atomic lattice is almost commensurable. The oxygen lattice is perfectly hexagonal close to these nodes and disordered in the surrounding regions.     

Specific features of spontaneous reorientation of the magnetic moment in a single-crystal thin plate of the iron garnet Tb0.2Y2.8Fe5O12

The Faraday effect, light scattering, and changes in the domain structure of a single-crystal thin plate of the iron garnet Tb_0.2Y_2.8Fe₅O₁₂ cut parallel to the (110) crystallographic plane have been experimentally investigated in the temperature range of the spontaneous magnetic orientational phase transition. It has been established that the transformation of the domain structure of the studied sample due to the orientational phase transition is accompanied by a significant temperature hysteresis. A model of the spontaneous reorientation of the magnetic moment vector of the crystal has been proposed by analyzing the results of visual observations of the domain structure and the temperature dependences of the Faraday effect and light scattering. In the proposed model, preference is given to the spin wave mechanism of nucleation of domains of the equilibrium magnetic phases.     

Translational transitions at domain boundaries in octanethiol monolayers on Au(1 1 1)

Systematic studies of domain boundaries of self-assembled monolayers of octanethiols on (1 1 1) oriented gold surfaces have been performed by ultrahigh vacuum scanning tunnelling microscopy (STM). The monolayers consist of domains that exhibit the c(4×2) superstructure of the hexagonal structure of alkanethiols on gold. By high-resolution images domain boundaries are displayed with molecular resolution. These are used to deduce the exact relation between the adsorption sites of the molecules belonging to different domains on the gold substrate. A translational transition is shown here in detail and it is demonstrated that hexagonal close-packed (hcp), and face-centred cubic (fcc) triple hollow sites are occupied similarly by octanethiols.     

取向玻璃体系中分子之间取向关联的模型化及其模拟与分析

提出了一个描述取向玻璃系统中分子间取向关联的有效键模型.基于该模型对该系统中分子之间取向关联的拓扑结构进行了计算机模拟.结果表明当成键概率较小时,成键分子大部分以分子串的形式出现,并基于概率分析对此现象进行了解释.深入地,用分子串模型对模拟结果进行了分析,并且基于分析结果讨论了分子串模型的合理性和可行性. 关键词： 玻璃化转变 取向玻璃 拓扑结构 分子串模型