Showing papers on "Cluster (physics) published in 1975"

Molecular dynamics study of the structure and thermodynamic properties of argon microclusters

Abstract: Microclusters of 2–100 argon atoms were studied using molecular dynamics. The microclusters were ordered solids at low temperatures and energies and disordered liquids at high temperatures and energies. The melting transition occurred considerably below the bulk melting temperature. Radial density functions, interference functions, diffusion coefficients, and surface energies were calculated for both the solid and liquid phases. The surface energies of the microclusters could be expressed as a function of the form Ai2/3+Bi1/3 where i is the number of atoms in the cluster. For 0 °K solid clusters A and B have the values 0.26×10−12 and −0.12×10−12 erg, respectively; for liquid clusters at 40 °K, A and B have the values 0.17×10−12 and −0.045×10−12 erg, respectively. Clusters containing 7 or more atoms melted with the occurence of a first‐order‐like transition. This transition was studied further through Monte Carlo calculations. A possible model for the first‐order‐like transition is proposed.

New type of cluster theory for molecular fluids: Interaction site cluster expansion

Abstract: A new cluster series, called the interaction site cluster expansion, is derived for classical molecular fluids. For a general class of molecular models, the series provides an exact formula for the equilibrium pair correlations between interaction sites on different molecules. In the models considered, each molecule contains m of these interaction sites. The total intermolecular potential between two molecules is the sum of m2 site–site potentials. A site–site potential depends on the separation between the sites only. However, because the sites are not necessarily located at the centers of molecules, the total molecular pair interaction can depend strongly on molecular orientations. Even though the interaction site cluster expansion is exact, the cluster integrals in the series involve the translational coordinates of the interaction sites only. The molecular orientational coordinates are removed by a transformation which introduces intramolecular correlation functions into the series. Thus, the new clus...

Calculations of defect clustering in Fe1-xO

Abstract: Lattice energy calculations based on the Born model are used to examine the possible structures of defect clusters in non-stoichiometric FeO. The basic cluster is identified as a complex of four cation vacancies and one tetrahedral Fe3+ ion (or 4:1 cluster) which is formed as a result of a large Coulomb term favouring the occupation of the tetrahedral site when all nearest-neighbour cations are vacant. The aggregation of these 4:1 clusters occurs by vacancy-sharing, and the calculations suggest that the most stable small aggregates are formed by edge- rather than corner-sharing (6:2 or 8:3 clusters). If more extended clusters are formed they are likely to involve corner-sharing, but the binding energy for a cluster closely related to the Fe3O4 structure is larger than that calculated for the Koch-Cohen cluster.

Physical cluster mechanics: Statistical thermodynamics and nucleation theory for monatomic systems

Abstract: We extend previous computations of mechanical stability of atomic microclusters to the realm of statistical thermodynamics, obtaining thermodynamic functions for small, solid-like Van der Waals clusters of less than some 100 atoms possessing non-crystalline structures of ‘polytetrahedral’ type. These are shown to be almost invariably at a thermodynamic advantage over alternative lattice structures of the same number of atoms, at least for the Lennard-Jones potential in the harmonic-oscillator/rigid-rotor approximation. The dependence of thermodynamic functions upon cluster size appears to be essentially monotonic in the number of internal degrees of freedom; although there are certain exceptional structures, particularly with icosahedral symmetry, there proves to be little evidence for the occurrence of ‘magic numbers’ for stability at any temperature and within the size-range considered. Particular attention is given to the heat capacity of model systems in relation to their vibrational spectra....

On the energy distribution of sputtered clusters

Abstract: Our previous model on dimer sputtering is extended to the formation of larger clusters. The general formalism for the calculation of the momentum distribution function of these clusters is given. A simple analytical expression is obtained for the distribution function of any cluster in the high energy limit. Comparison of this expression with experimental data on the energy and angle distribution of Wk + clusters at high energies shows good agreement. The experimental results on K2 and KI sputtering with low energies are recompared with the theory. For the ionic crystal KI, evidence is obtained that sputtering of non adjacent K and I atoms from the lattice gives an important contribution to dimer formation. The role of the binding energy Eb , of the particles with respect to the surface is discussed.

Thermodynamic properties of small aggregates of rare-gas atoms

Abstract: The present work reports on the equilibrium thermodynamic properties of small clusters of xenon, krypton, and argon atoms, determined from a biased random-walk Monte Carlo procedure. Cluster sizes ranged from 3 to 13 atoms. Each cluster was found to have an abrupt liquid-gas phase transition at a temperature much less than for the bulk material. An abrupt solid-liquid transition is observed for thirteen- and eleven-particle clusters. For cluster sizes smaller than 11, a gradual transition from solid to liquid occurred over a fairly broad range of temperatures. Distribution of number of bond lengths as a function of bond length was calculated for several systems at various temperatures. The effects of box boundary conditions are discussed. Results show the importance of a correct description of boundary conditions. A surprising result is the slow rate at which system properties approach bulk behavior as cluster size is increased.

An Alternative Definition for "Neighborhood of a Point"

Abstract: The concept of "neighborhood of a point" has been used in various programs for analyzing spatial dot patterns. The common definition based on k-nearest neighbors does not reflect the intuitive cluster associations of certain points in simple patterns. An alternative definition is proposed to reflect such associations more satisfactorily. Its applications includes cluster analysis and descriptive measures for dot patterns.

Ionized-cluster deposited on a substrate and method of depositing ionized cluster on a substrate

Abstract: The present ion source called "Vaporized-Metal Cluster Ion Source" is adapted to produce ionized vapor aggregate (ionized cluster) instead of atomic or molecular ions in conventional ion sources. Clusters consisting of 10 2 -10 3 atoms are formed by the adiabatic expansion due to the ejection into a high vacuum region through a nozzle of a heated crucible and ionized by electron bombardment. By "Ionized-Cluster Beam Deposition" using the ion source, fine-quality deposited films of many kinds of materials can be obtained on metal, semiconductor and insulator substrate with strong adhesion and with a fairly high deposition rate.

Theory of binary alloys including short-range order properties

Abstract: A new method for studying the electronic structure of binary alloys within the tight-binding framework is presented and developed in detail. The method involves treating part of the system exactly as a cluster of atoms and simulating the rest of the environment by connecting to each atom at the surface of the cluster a Bethe lattice of the same coordination number. The method includes from its inception the idea of short-range correlation effects of a nearest-neighbor order parameter. Clusters of different size have been studied and analyzed in detail. An illustrative example is presented to show the striking differences which appear in various concentration sequences. These sequences are selected to be (i) random, (ii) with a tendency to segregation, and (iii) with a tendency to form binary compounds. The results emphasize the need to include short-range correlation effects when dealing with alloys. The method also provides a clear identification of localized states. In addition, it gives very naturally the position of band edges and energy gaps. The method, when averaged over all alloys with equal concentration and all short-range order parameters, reproduces in a satisfactory way the results of the single-site coherent-potential approximation.

Vegetation clustering by means of isodata: Revision by multiple discriminant analysis

Abstract: An iterative, nonhierarchical, divisive, polythetic method of clustering phytosociological data is described and an efficient means of revising the results is presented. The procedures are applied to two sets of forest vegetation in western Washington (U.S.A.) and found to produce ecologically interpretable results. The clustering technique, ISODATA, produces clusters in n-species space on the basis of distance between cluster means. The iterations are truncated much sooner than is recommended by its originators, but the provisional clusters are analyzed by stepwise multiple discriminant analysis. This produces an effective and efficient analysis of the individual contribution of species to the classification and arranges the samples in canonical space, the axes of which are mutually orthogonal. This last procedure results in the clusters displayed in a reduced dimensional space and establishes the relationship between stands. The combination of procedures appears superior to several with which it was compared.

Positron annihilation in low-temperature rare gases. I. Helium

Abstract: The lifetime spectra of positrons annihilating in helium gas were obtained over the temperature range 4.6 to 300$sup 0$K and the density range 0.016 to 0.030 g/cm$sup 3$. The spectra are decomposed into slow- positron and ortho-positronium annihilation components which are presented and discussed separately. At low temperatures, the density and temperature dependences of the data are found to depart markedly from the usual behavior observed at room temperature. Simple quantitative treatments are given of the cluster model for the slow-positron data and of the cavity model for the ortho-positronium results. (auth)

A variational approach to disordered systems

Abstract: An exact variational principle involving the density-of-eigenstates function is derived. This variational principle differs from previously published conjectures, and has been applied to two different types of band-tailing problems in order to demonstrate its usefulness. A closely related variational ansatz concerning the complex self-energy of a random medium is then formulated, and is shown, under the appropriate circumstances, to lead to the CPA approximation as well as to cluster generalizations of this approximation. It is shown that such cluster generalizations can accurately reproduce some computer-generated density of states.

A kinetic model for clustering of water on hydrated protons in a supersonic free jet expansion

Abstract: A simple model with three variable parameters is developed for calculating observed clustering data from a free jet expansion. One parameter, σR1, can be associated with a reactive collision diameter for a monomer associating with a cluster to form a larger, excited cluster. A second parameter, σR3, can be associated with a reactive collision diameter for energy transfer from the excited cluster to the surrounding gas. The third parameter, K′, is a constant in a unimolecular decay rate expression for the excited cluster. The reactive cross sections that give the best fit of experimental data are given by (π/4) σ2R1 =9.90×10−16 cm2 and (π/4) σ2R3=5.22 ×10−17 cm2. No value could be assigned to K′. Apparently the excited clusters are stabilized by energy exchange collisions in this particular flow field before there is appreciable unimolecular decay. Good fits of clustering data are obtained even though these same values are used for clusters varying in size from 6 to 26 water molecules. Since the clustering...

Small periodic cluster calculation on point defect problems in hexagonal layered solids

Abstract: A model is proposed for discussing deep defect levels in covalent solids, based on the representation of the one−electron energies of the crystal by the eigenvalue spectrum of a small periodic cluster of atoms. Calculations of this model by semiempirical MO−LCAO methods for a vacancy problem in hexagonal boron nitride and graphite and substitutional boron impurity in graphite, yield satisfactory results when compared with experimental EPR, thermoluminescence, and thermally−stimulated−currents data.

A stochastic theory of cluster growth in homogeneous nucleation

Abstract: A critical cluster in homogeneous nucleation can be defined in two ways. Thermodynamically, the size of the critical cluster corresponds to a maximum of the free energy of the system; kinetically the critical cluster is defined such that all supercritical clusters grow and all others decay. We describe the evolution of a cluster by a postulated stochastic master equation in which the transition probabilities are given by a formula proposed by Langer. We derive from these assumptions, by a path integral method, an equation for the most probable evolution of the size of the cluster and show thereby that under these circumstances the two definitions are equivalent.

A SCF−Xα−SW investigation of chemisorption bonding of chalcogens on nickel (001)

Abstract: The electronic levels and the total energies for a series of chalcogen−nickel clusters are calculated by the self−consistent field Xα scattered−wave method. In accord with experimental results the chalcogen atoms are assumed to be located in a pyramidal position above a cluster of nickel atoms whose geometry simulates the (001) surface of nickel. Binding energy curves of the clusters are calculated for various positions of each of the chalcogens above the Ni ’’surface.’’ The resulting favored positions are compared with those obtained experimentally. The characteristics of the orbitals directly involved in the binding of the chalcogen to the nickel cluster are discussed.

Cluster compounds of gold

Abstract: In this paper the author reviews the work carried out under his direction in the laboratory of the Italian Research Council on the structure of an unusual class of compounds in which metal atoms in zero or low oxidation states are bonded to one another. The resolution of the configuration of the atoms in these interesting compounds may well improve our understanding of the electronic structure of metals and alloys.

Cluster calculations of the electronic d-states in VO2

Abstract: An ab-initio cluster calculation of the electronic d-states in VO$sub 2$ has been made by using the self-consistent statistical exchange multiple scattering method. VO$sub 6$$sup 8-$ octaedron was used to represent the tetragonal phase, and a V$sub 2$O$sub 10$$sup 12-$ cluster with paired vanadium atoms was used for the monoclinic phase. Our results are in conflict with an explanation of the semiconducting gap in terms of simple distortion-induced band splitting. (FR)

STATISTICS: Cluster Analysis.

Abstract: source of authority was an ordinary man without official position, if two sources of authority gave incompatible orders, and if there were other people around who protested at the idea of carrying on with the punishment. However, to the world at large and to Milgrani himself the most startling finding of the whole series of experiments was the willingness of a majority of subjects under standard conditions to administer what they believed to be painful and dangerous electric shocks, even when the victim was †̃¿ crying out in protest and agony'. Why this should so surprise us who have lived in a twentieth century of totalitarian states is itself something of a mystery. The experiments are well worth writing up in book form, in that they confront us with a dramatic presentation, within the laboratory, of a fundamental human situation. They have generated fierce argu ment—a proper purpose of psychological experi.. ments. The argument has included strong attacks on Milgram's own morality in deceiving (some say cruelly deceiving) his subjects. It may be that in so doing Milgram was, like his subjects, only manifesting obedience to the authority of †̃¿ Science'. The wealmess of the work lies in its lack of an elaborated theoretical base. †̃¿ Obedience' is a concept which obviously covers a multitude of psychological sins and is not to be explained by another equally global concept of †̃¿ authority'. Milgram's subjects were not simply obeying the authority of an experi menter in a white coat—they were obeying the authority of a whole culture which has given them