Divergence of neighboring magnetic-field lines and fast-particle diffusion in strong magnetohydrodynamic turbulence, with application to thermal conduction in galaxy clusters

Using direct numerical simulations, we calculate the rate of divergence of neighboring magnetic-field lines in different types of strong magnetohydrodynamic turbulence. In the static-magnetic-field approximation, our results imply that tangled magnetic fields in galaxy clusters reduce the electron diffusion coefficient and thermal conductivity by a factor of approximately 5-10, relative to their values in a nonmagnetized plasma.     

A catalogue of X-ray spectra of AGNs: Data presentation and analysis

Summary We present a catalogue of X-ray spectra of AGNs collected from the literature and continuously updated. At the moment the catalogue contains data on 175 AGNs and is complemented by its own data analysis software. Few typical outputs of the data bank are presented as well as the capability of the software implemented. In particular, to illustrate the astrophysical potential of the catalogue, we deal in this paper with the specific problem of intrinsic absorption in active galaxies. By weighting on a number of observations, we obtain accurate estimates ofN _H values for most sources in our sample and are able to improve the study of the relation between this parameter and other AGNs characteristics such as reddening, axial ratio and hardness ratio. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Computational study of the surface properties of aluminum nanoparticles

We calculate the surface energy, surface stress, and lattice contraction of Al nanoparticles using ab initio density functional and empirical computational techniques. Ab initio calculations are carried out using the siesta pseudopotential method combined with the generalized gradient approximation. Empirical calculations are conducted using the embedded atom method. The ab initio density functional approach predicts the surface energies of Al nanoclusters to be in the range of 0.9-2.0 J/m². These values are consistent with the surface energy of bulk aluminum and are close to the surface energies of silver nanoparticles calculated in our previous study. In contrast to our previous results for Ag nanoparticles, we found a significant discrepancy between the theoretical values of surface energy and stress for Al nanoclusters. This result could be explained by a greater degree of surface reconstruction in Al clusters than in Ag clusters.     

Effect of Rapid Thermal Annealing on the Formation of In-N Clusters in Strained InGaNAs

A model for the effect of rapid thermal annealing on the formation of In-N clusters in strained GalnNAs is developed according to thermodynamics. In the model, the lowest annealing temperature influencing the redistribution of atoms is introduced. The average variation of energy for formation per In-N bond is obtained by fitting the experimental values. Using the present model, we calculate the average number of nearest-neighbor In atoms per N atom after annealing. The obtained results are compared with the experiment. The qualitative analysis and quantitative analysis are in good agreement with each other. The model is helpful to explain the essence of the blueshift caused by annealing.     

Quantenchemische abschätzung von sauerstoffadsorptionsenergien an einigen metallen

For a better understanding of adsorption processes at fuel cell electrodes we want to apply quantum-chemical methods. Simple semiempirical models, the Hückel method and the extended Hückel method, were used to calculate MeO bond energies for systems consisting of a few metal atoms and an oxygen atom. The results were compared with adsorption energies of atomic oxygen (from experimental oxygen adsorption energies and oxygen dissociation energy). The best values could be obtained for an extended Hückel calculation on metal clusters with one oxygen atom, using a special modification of Coulomb-integral values from HF-SCF tables.     

An efficient method to identify galaxy clusters by using SuperCOSMOS, 2MASS and WISE data

The survey data of Wide-field Infrared Survey Explorer(WISE)provide an opportunity for the identification of galaxy clusters.We present an efficient method for detecting galaxy clusters by combining the WISE data with SuperCOSMOS and 2MASS data.After performing star-galaxy separation,we calculate the number of companion galaxies around the galaxies with photometric redshifts previously estimated by the SuperCOSMOS,2MASS and WISE data.A scaled richness Rscal 30 is set as a criterion to identify clusters.From a sky area of 275 deg2of the Sloan Digital Sky Survey Stripe 82 region,we identify 302 clusters in the redshift range of 0.1z0.35,247(82%)of which are previously known SDSS clusters.The results suggest that our method is efficient for identifying galaxy clusters by using the all sky data of the SuperCOSMOS,2MASS and WISE.     

Impact of receptor-ligand distance on adhesion cluster stability

Cells in multicellular organisms adhere to the extracellular matrix through two-dimensional clusters spanning a size range from very few to thousands of adhesion bonds. For many common receptor-ligand systems, the ligands are tethered to a surface via polymeric spacers with finite binding range, thus adhesion cluster stability crucially depends on receptor-ligand distance. We introduce a one-step master equation which incorporates the effect of cooperative binding through a finite number of polymeric ligand tethers. We also derive Fokker-Planck and mean field equations as continuum limits of the master equation. Polymers are modeled either as harmonic springs or as worm-like chains. In both cases, we find bistability between bound and unbound states for intermediate values of receptor-ligand distance and calculate the corresponding switching times. For small cluster sizes, stochastic effects destabilize the clusters at large separation, as shown by a detailed analysis of the stochastic potential resulting from the Fokker-Planck equation.     

On the universality of geometrical and transport exponents of rigidity percolation

We develop a three-parameter position-space renormalization group method and investigate the universality of geometrical and transport exponents of rigidity (vector) percolation in two dimensions. To do this, we study site-bond percolation in which sites and bonds are randomly and independently occupied with probabilitiess andb, respectively. The global flow diagram of the renormalization transformation is obtained which shows that thegeometrical exponents of the rigid clusters in both site and bond percolation belong to the same universality class, and possibly that of random (scalar) percolation. However, if we use the same renormalization transformation to calculate the critical exponents of the elastic moduli of the system in bond and site percolation, we find them to be very different (although the corresponding values of the correlation length exponent are the same). This indicates that the critical exponent of the elastic moduli of rigidity percolation may not be universal, which is consistent with some of the recent numerical simulations.     

The angular momentum and parity projected multidimensionally constrained relativistic Hartree–Bogoliubov model

Nuclear deformations are fundamentally important in nuclear physics.We recently developed a multidimensionally constrained relativistic Hartree-Bogoliubov(MDCRHB)model,in which all multipole deformations respecting the V₄ symmetry can be considered self-consistently.In this work we extend this model by incorporating the angular momentum projection and parity projection to restore the rotational and parity symmetries broken in the mean-field level.This projected MDCRHB(p-MDCRHB)model enables us to connect certain nuclear spectra to exotic intrinsic shapes such as triangles or tetrahedrons.We present the details of the method and an exemplary calculation for ¹²C.We develop a triangular moment constraint to generate the triangular configurations consisting of three α clusters arranged as an equilateral triangle.The resulting ¹²C spectra are consistent with that from a triangular rigid rotor for large separations between the α clusters.We also calculate the B(E2)and B(E3)values for low-lying states and find good agreement with the experiments.     

Binding energy of a hydrogen impurity in an f.c.c. lattice

Hydrogen impurities in f.c.c. lattices, Pd metal for example, can occupy two interstitial sites, with tetrahedral or octahedral symmetries. Using the Husimi cactus to represent the f.c.c. lattice and two clusters to describe the two different interstices, we calculate the densities of states and the occupation numbers at the impurity site and on its nearest neighbours. The binding energies are calculated and plotted against the hopping integral between the hydrogen and its neighbours (t_a). We found that the octahedral site is the stable one for values of t_a from 0 to 1.5 eV.     

一种适用于原子团簇的多体展开新方案: 以氮团簇为例

多体展开方法虽然已经广泛地用于估算弱相互作用体系的能量,但是其并不适用于计算共价团簇和金属团簇的能量. 本文提出了一种适用于计算共价体系能量的相互作用多体展开(IMBE)方法. 在相互作用多体展开方法中,体系的能量表示为孤立原子的能量及该原子与其他周围原子间相互作用的和. 首先将该方法应用于计算氮团簇的能量,且多体展开截断至四体项. 结果表明：与传统的多体展开方法相比,相互作用多体展开方法可以显著地降低能量误差. 另外,以密度泛函理论计算结果为参考,相互作用多体展开方法估算能量的误差不依赖于体系的大小和结构,说明相互作用多体展开方法比较适合用于估算共价相互作用大体系的能量.     

Stability and lattice contraction of small platinum particles

The structural stability of Pt clusters of cubooctahedral and icosahedral shapes has been analytically studied by taking an approximation based on the second moment of the density of states to calculate the cohesive energy. We show that the predictions of this model are in agreement with the phenomenological ideas of Ino's model and the known experimental results for the transition from cubooctahedral to icosahedral geometrical structure. The agreement between the calculated values for the contraction of the interatomic distance as a function of decreasing size, and very recent experimental results is excellent.     

Density functional study of the electronic structure and properties of lithium intercalated graphite

Ab initio electronic-structure calculations are performed using density functional theory (DFT) with polarized basis set (LanL2DZ and 6-311G⁺⁺) within the spin polarized generalized gradient approximation for lithium intercalated graphite. Initially different benzene-Li⁺ model clusters are optimized on the basis of their total energy at room temperature. These model clusters are used to calculate the optimized structure of lithium intercalated graphite clusters. The resultant optimized structures are used to calculate dipole moment, ionization potential (IP), electron affinity (EA), binding energy (BE) and vibrational spectra (IR and Raman). For an idea of the band gap of the clusters in the ground state, the HOMO-LUMO gap (ΔE_g) has been calculated. To compare the electron transfer ability of different clusters, chemical potential (μ), hardness (η) and their ratio for different clusters have also been determined.     

Excitonic effects and optical properties of passivated CdSe clusters

We calculate the optical properties of a series of passivated nonstoichiometric CdSe clusters using two first-principles approaches: time-dependent density functional theory within the local-density approximation, and by solving the Bethe-Salpeter equation for optical excitations with the GW approximation for the self-energy. We analyze the character of optical excitations leading to the first low-energy peak in the absorption cross section of these clusters. Within time-dependent density functional theory, we find that the lowest-energy excitation is mostly a single-level to single-level transition. In contrast, many-body methods predict a strong mixture of several different transitions, which is a signature of excitonic effects. The majority of the clusters have a series of dark transitions before the first bright transition. This may explain the long radiative lifetimes observed experimentally.     

Partitioning of a two component particle system and the isotope distribution in nuclear fragmentation

The general problem of partitioning a finite system consisting of two kinds of particles is solved. Average frequencies of clusters are calculated under the assumption that all partitions are equally probable. Some restrictions to this assumption are considered. In the nuclear case where we have protons and neutrons we have utilized a method of generating partitions in order to calculate the isotopic yields of fragmenting nuclei. The results imply that the neutron/proton and dineutron/diproton ratios might be good probes for the excitation energy of an asymmetric fragmenting nuclear system.     

Synchronization in networks of limit cycle oscillators

We investigate the synchronization behaviour of three different networks of nonlinearly coupled oscillators. Each network consists of several clusters of oscillators, and the clusters themselves consist of any number of oscillators. In each cluster the eigenfrequencies scatter around the cluster frequency (mean frequency). The coupling strength varies in each cluster, too. We analyze the synchronized states by means of the center manifold theorem. This enables us to calculate these states explicitly, and to prove their stability. Moreover we are able to determine frequency shifts caused by different coupling mechanisms. In a number of cases we calculate the synchronisation threshold explicitely. Numerical simulations illustrate our analytical results. In one of the three networks we have additionally analyzed a single cluster consisting of infinitely many oscillators, that is an oscillatory field. Again, the center manifold theorem enabled us to calculate the synchronized state explicitly and to prove its stability. Our results concerning the oscillatory field are in contradiction to Ermentrout's analysis [6].     

各向异性扩散DLA的标度性质

本文利用坐标变换的方法讨论了各向异性扩散对DLA集团标度行为的影响。对二维正方形点阵,解析计算了各向异性扩散DLA集团的Hausdorff维数。结果表明,此时生长集困的Hausdorff维数D随各向异性扩散概率p连续变化,其最大值为5/3,最小值为3/2,还将这里所得到的理论结果与Jullien等人的数值结果进行了比较,发现它们符合得很好。最后,还解析讨论并计算了各向异性扩散DLA的广义维数D_q(p)。     

递归方法和团簇磁性的计算

介绍了如何应用递归方法将哈密顿矩阵三对角化,从而求得局域态密度 ,并以计算铑团簇的磁矩为例,说明了递归方法在研究团簇的电子结构方面的应用。     

Cluster analysis and finite-size scaling for Ising spin systems

Based on the connection between the Ising model and a correlated percolation model, we calculate the distribution function for the fraction (c) of lattice sites in percolating clusters in subgraphs with n percolating clusters, f(n)(c), and the distribution function for magnetization (m) in subgraphs with n percolating clusters, p(n)(m). We find that f(n)(c) and p(n)(m) have very good finite-size scaling behavior and that they have universal finite-size scaling functions for the model on square, plane triangular, and honeycomb lattices when aspect ratios of these lattices have the proportions 1:square root[3]/2:square root[3]. The complex structure of the magnetization distribution function p(m) for the system with large aspect ratio could be understood from the independent orientations of two or more percolation clusters in such a system.     

丙酮分子团簇结构的量子化学从头计算研究

采用HF/ 3 2 1G 方法对丙酮分子团簇体系 (CH3 COCH3 ) n(n =2— 5 )进行了全优化 .用密度泛函B3LYP/ 6 31+G 方法计算了各个体系的总能量 .通过计算不同体系的振动频率 ,确定得到的结构是否对应于体系的稳定构型 .对计算结果进行了分析和讨论 ,结果表明 ,环形结构较线形结构更为稳定 . The acetone clusters are studied by ab initio calculations. HF/3-21G * method is used to optimize the configurations for all kinds of systems. B3LYP/6-31+G ** method is adopted to calculate the total energy of the systems. We also calculate the vibration frequencies in order to investigate whether the system correspondes to stable geometries. All results show that ring structure is more stable than linear structure for the acetone clusters of (CH 3COCH 3) n(n=2, 3, 4, 5).