Structural Crossover Research Articles

Structure of Ag Clusters Grown on Fs‐Defect Sites of an MgO(1 0 0) Surface

The structure of AgN clusters (N=1-4, 6, 8, 10), both in the gas phase and grown on the MgO(1 0 0) surface containing Fs-defects, has been investigated by a density functional basin-hopping (DF-BH) approach. In analogy with what observed in the case of gold clusters, it is found that the presence of the defect implies a double frustration and a cylindrical invariance of the metal-surface interaction, causing small Ag clusters growing around the Fs defect to be highly fluxional. Nevertheless, two different structural crossovers are found to be induced by the metal-defect interaction for the adsorbed clusters such that: 1) planar structures prevail for N<or=4 (as in the gas phase); 2) noncrystalline (fivefold symmetric) structures, which are the lowest energy ones in the gas phase for medium sized AgN clusters (N>or=7), prevail for N=6 and N=8; 3) distorted face-centered cubic (fcc) structures grown pseudomorphically on the defected surface prevail for N=10. The transition from fivefold to fcc motifs is rationalized in terms of the double-frustration effect, which increases the bond strain of the noncrystalline structures. Detrapping energies from the defect were also calculated; the lowest energy pathway corresponds to the detachment of a dimer.

Experimental Observation of Structural Crossover in Binary Mixtures of Colloidal Hard Spheres

Using confocal microscopy, we investigate the structure of binary mixtures of colloidal hard spheres with size ratio q=0.61. As a function of the packing fraction of the two particle species, we observe a marked change of the dominant wavelength in the pair-correlation function. This behavior is in excellent agreement with a recently predicted structural crossover in such mixtures. In addition, the repercussions of structural crossover on the real-space structure of a binary fluid are analyzed. We suggest a relation between crossover and the lateral extension of networks containing only equally-sized particles that are connected by nearest-neighbor bonds. This is supported by Monte Carlo simulations which are performed at different packing fractions and size ratios.

Gas-phase electronic spectrum of the C14ring

The electronic spectrum of a cyclic C(14) in the visible range has been detected in the gas phase by a mass selective resonant two-color two-photon ionization technique coupled to a laser ablation source. Absorption is localized in the 19 000 to 20 000 cm(-1) region and appears as a dozen of 3-7 cm(-1) narrow peaks belonging to one or two close-lying electronic states. Bands have structures which for the narrowest ones is likely to be the rotational profile contour. The spectrum is attributed to a cyclic form of C(14) based on time-dependent density-functional calculations and reactivity with H(2). The spectral pattern differs from that previously seen in the larger C(4n+2) member rings, C(18) and C(22), indicating some sort of a structural crossover.

Observation of Vortex Pinning in Bose-Einstein Condensates

We report the observation of vortex pinning in rotating gaseous Bose-Einstein condensates. Vortices are pinned to columnar pinning sites created by a corotating optical lattice superimposed on the rotating Bose-Einstein condensates. We study the effects of two types of optical lattice: triangular and square. In both geometries we see an orientation locking between the vortex and the optical lattices. At sufficient intensity the square optical lattice induces a structural crossover in the vortex lattice.

Homogeneous and inhomogeneous hard-sphere mixtures: manifestations of structural crossover

Various manifestations of structural crossover in the properties of a binary mixture of hard-spheres are studied. For homogeneous mixtures that are sufficiently asymmetric, there is a crossover line in the phase diagram such that for thermodynamic states that lie on one side, the decay of the three bulk pair correlation functions is oscillatory with a common wavelength approximately equal to the diameter of the smaller spheres, and for states on the other side, the common wavelength is approximately the diameter of the bigger spheres. Using density functional theory it is shown that structural crossover manifests itself in the intermediate range decay of (i) the one-body density profiles of a hard-sphere mixture adsorbed at planar walls, (ii) the effective (depletion) potential between two big hard-spheres immersed in the same binary mixture, and (iii) the solvation force, or excess pressure, of the same mixture confined between two planar hard walls. The structural crossover line is determined exactly for a one-dimensional binary mixture of hard-rods and evidence is presented, based on density functional theory calculations and Monte-Carlo simulations, for structural crossover in homogeneous binary mixtures of hard-disks in two dimensions. By considering a multicomponent mixture of hard-spheres, with an appropriate bimodal distribution of diameters, it is argued that structural crossover should still occur in the presence of polydispersity and that these results could be relevant to colloidal mixtures where correlation functions can be measured using real-space techniques.

Structural Search Spaces and Genetic Operators

In a previous paper (Rowe et al., 2002), aspects of the theory of genetic algorithms were generalised to the case where the search space, omega, had an arbitrary group action defined on it. Conditions under which genetic operators respect certain subsets of omega were identified, leading to a generalisation of the term schema. In this paper, search space groups with more detailed structure are examined. We define the class of structural crossover operators that respect certain schemata in these groups, which leads to a generalised schema theorem. Recent results concerning the Fourier (or Walsh) transform are generalised. In particular, it is shown that the matrix group representing omega can be simultaneously diagonalised if and only if omega is Abelian. Some results concerning structural crossover and mutation are given for this case.

Decay of correlation functions in hard-sphere mixtures: structural crossover.

We investigate the decay of pair correlation functions in a homogeneous (bulk) binary mixture of hard spheres. At a given state point the asymptotic decay r-->infinity of all three correlation functions is governed by a common exponential decay length and a common wavelength of oscillations. Provided the mixture is sufficiently asymmetric, size ratios q less than or approximately 0.7, we find that the common wavelength reflects either the size of the small or that of the big spheres. By analyzing the (complex) poles of the partial structure factors we find a sharp structural crossover line in the phase diagram. On one side of this line the common wavelength is approximately the diameter of the smaller sized spheres whereas on the other side it is approximately the diameter of the bigger ones; the wavelength of the longest ranged oscillations changes discontinuously at the structural crossover line. Using density functional theory and Monte Carlo simulations we show that structural crossover also manifests itself in the intermediate range behavior of the pair correlation functions and we comment on the relevance of this observation for real (colloidal) mixtures. In highly asymmetric mixtures, q< or =0.1, where there is metastable fluid-fluid transition, we find a Fisher-Widom line with two branches. This line separates a region of the phase diagram where the decay of pair correlations is oscillatory from one in which it is monotonic.

Optimal Placement of MR Dampers for Structural Control Using Identification Crossover Genetic Algorithm

In order to study the optimal placement of magnetorheological (MR) fluid dampers for structural control, an improved genetic algorithm, i.e. identification crossover genetic algorithm (ICGA), is presented in this paper. The method avoids the constraint violations caused by simple crossover and basic mutation operation. The ICGA produces an identification code. The crossover based on the identification code as well as mutation based on bit-by-bit basis guarantees the fulfillment of constraints. The analytical results by ICGA and general genetic algorithm are compared. It is concluded that the converging speed by ICGA is faster than the converging speed by general genetic algorithm and the control effect by optimal placement is satisfactory.

Structural crossover in dense irreversibly aggregating particulate systems.

Cluster-cluster aggregation has been simulated by off-lattice Monte Carlo methods for diffusion-limited (DLCA), ballistic-limited, and reaction-limited cluster aggregation classes. We find that as the system evolves and becomes dense, the largest cluster develops a hybrid structure with mass fractal dimension D(f) approximately 2.6 over large length scales, while at smaller length scales, the early time dilute-limit fractal structure is frozen in. The largest cluster is thus an aggregate of smaller aggregates with a different fractal dimension, and we call it a "superaggregate." The crossover length separating the two morphologies, which we call the critical radius of gyration, can be calculated based on a simple theory that assumes a monodisperse cluster size distribution. This agrees well with simulation results for DLCA. However, for other classes we find that the increasing polydispersity in cluster size pushes the simulated crossover length radius of gyration to values systematically larger than the predicted value.

Thermo-Kinetic Anomalies across Rigidity Threshold in Ge&lt;SUB&gt;x&lt;/SUB&gt;Se&lt;SUB&gt;1-x&lt;/SUB&gt;

We have investigated the glass-transition kinetics of nine GexSe1−x glasses by differential scanning calorimetry. Relation between the drive (heating-rate q) and response (heatflow shift ΔH at Tg) is seen to be strictly linear only for GeSe4, known to signify the bulk-rigidity threshold for this series. From an Arrhenius analysis the activation energies for glassy relaxation are estimated, and point to the existence of different thermokinetic phases below and above the threshold composition. Series behaviour of the kinetic activation is conciled to a concurring one seen in the size of cooperatively diffusing regions. The anomalies are attributed to structural crossovers with Ge doping; first from the parent uniform Se-chains to that of backbones out-branching at corner-shared Ge(Se1⁄2)4 tetrahedral clusters, and subsequently interconnecting by edge-shared configurations to realize a random pearl-necklace 3-D covalent network.

Complex structural phase transition in a defect-populated two-dimensional system

A complex phase transition in Sn/Ge(111) and similar systems can be decomposed into two intertwined phase transitions: a structural symmetry lowering $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})(3\ifmmode\times\else\texttimes\fi{}3)$ transition and a disorder-order transition in the defect distribution. We present two phenomenological models that describe these transitions and their interrelation. These models allow us to understand the formation of domains and domain walls at low temperatures, defect-induced density waves above the structural transition temperature, and ordering of the defects caused by lattice-mediated defect-defect interactions. The models predict a destruction of the pure structural transition when impurities are introduced into the system, a shift in the structural crossover temperature with impurity density, and a dependence of the $(3\ifmmode\times\else\texttimes\fi{}3)$ lattice structure on the specific defect alignment.

3D Monte Carlo Simulations of Diffusion Limited Cluster Aggregation up to the Sol-Gel Transition: Structure and Kinetics

Three-dimensional (3D) Monte Carlo simulations of diffusion limited cluster aggregation at different concentrations (φ) show a crossover from a flocculation regime at short times to a percolation regime close to the gel time (tg). Contrary to suggestions in the literature tg is independent of the system size (L) for large L. The structural and temporal crossovers between flocculation and percolation take place at characteristic values of the cluster mass (mc) and the time (tc) which depend on φ. After normalisation by these characteristic values the crossovers are independent of φ except for very small clusters and at short times. The concentration dependence of mc and tc indicates that the crossover takes place at a given cumulated volume fraction of the clusters independent of φ. At low concentrations the φ-dependence of tg is determined by the cluster growth in the flocculation regime.

Relation of structural and elastic crossover length investigated by low-frequency Raman scattering inGexSe1−xglasses

Relation of structural and elastic crossover length investigated by low-frequency Raman scattering in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ge</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Se</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi>−</mml:mi><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>glasses

Molecular dynamics simulation of Li-alkaline earth-F3 compounds

Abstract We have performed first principle molecular dynamics simulations on LiXF3 compounds where X is Be, Mg, Ca, Sr or Ba. Short range pair potentials were determined using the Gordon-Kim approach. The Be and Mg compounds are predicted to have distorted perovskite structures at low temperature, while the rest are antiperovskites. The role of size effect in this structural crossover will be discussed. We also present prediction of phase transitions and the variation of physical properties as function of temperature for these materials.