Showing papers on "Cluster (physics) published in 2013"
TL;DR: It is found that the charge state of the Au particle is negative in a reducing chemical environment whereas in the presence of oxidizing species coadsorbed to the oxide surface the cluster obtained a net positive charge.
Abstract: To probe metal particle/reducible oxide interactions density functional theory based ab initio molecular dynamics studies were performed on a prototypical metal cluster (Au20) supported on reducible oxides (rutile TiO2(110)) to implicitly account for finite temperature effects and the role of excess surface charge in the metal oxide. It is found that the charge state of the Au particle is negative in a reducing chemical environment whereas in the presence of oxidizing species coadsorbed to the oxide surface the cluster obtained a net positive charge. In the context of the well-known CO oxidation reaction, charge transfer facilitates the plasticization of Au20, which allows for a strong adsorbate induced surface reconstruction upon addition of CO leading to the formation of mobile Au–CO species on the surface. The charging/discharging of the cluster during the catalytic cycle of CO oxidation enhances and controls the amount of O2 adsorbed at oxide/cluster interface and strongly influences the energetics of...
280 citations
TL;DR: This review summarizes the rich structural variety of copper and silver chalcogenide clusters with protecting ligand shells of phosphane and/or organic ligands that were generated starting out from silylated chalCogenide sources, demonstrating the clusters to be understood as intermediates between mononuclear complexes and binary bulk phases.
Abstract: This review summarizes the rich structural variety of copper and silver chalcogenide clusters with protecting ligand shells of phosphane and/or organic ligands that were generated starting out from silylated chalcogenide sources. This route turned out to be fairly selective and thus allows for the isolation of uniform, polynuclear to nanosized cluster molecules that can consist of only a few or up to hundreds of metal atoms, being bridged by the chalcogen atoms. However, all of these clusters are only kinetically stable with respect to the formation of the binary coinage metal chalcogen phases, but do not collapse into the solid M2E materials owing to the protection by bulky ligands on the surface. Upon a more detailed analysis of the development of the structural properties with the cluster size, one recognizes differences for the particular M/E combinations: whereas copper chalcogenide and silver selenide clusters show a clear tendency to approach structural patterns of the Cu2E bulk, most obvious for the Cu/Se combination, this is not visible for silver sulfide clusters, even not at the largest species with 490 silver and 302 sulfur atoms. Besides the discussion on the structures of title compounds, the review presents insight into the bonding properties, reactivity, thermal and photophysical properties. The latter can be interpreted in terms of the quantum confinement effect, thus demonstrating the clusters to be understood as intermediates between mononuclear complexes and binary bulk phases.
254 citations
TL;DR: D density functional theory calculations on highly parallel computing resources are used to study size-dependent changes in the chemical and electronic properties of platinum (Pt) for a number of fixed freestanding clusters and find that the surface catalytic properties of the clusters converge to the single crystal limit.
Abstract: In this paper, we use density functional theory (DFT) calculations on highly parallel computing resources to study size-dependent changes in the chemical and electronic properties of platinum (Pt) for a number of fixed freestanding clusters ranging from 13 to 1415 atoms, or 0.7-3.5 nm in diameter. We find that the surface catalytic properties of the clusters converge to the single crystal limit for clusters with as few as 147 atoms (1.6 nm). Recently published results for gold (Au) clusters showed analogous convergence with size. However, this convergence happened at larger sizes, because the Au d-states do not contribute to the density of states around the Fermi-level, and the observed level fluctuations were not significantly damped until the cluster reached ca. 560 atoms (2.7 nm) in size.
245 citations
TL;DR: In this paper, a new interatomic pair potential for W-He is described, which includes a short range modification to the Ackland-Thetford tungsten potential, and molecular dynamics simulations using these potentials accurately reproduce ab initio results of the formation energies and ground state positions of He point defects and self interstitial atoms.
223 citations
TL;DR: Recent experimental and theoretical advances of this new class of aromatic borometallic compounds, which contain a highly coordinated central transition metal atom inside a monocyclic boron ring are discussed.
Abstract: Atomic clusters have intermediate properties between that of individual atoms and bulk solids, which provide fertile ground for the discovery of new molecules and novel chemical bonding. In addition, the study of small clusters can help researchers design better nanosystems with specific physical and chemical properties. From recent experimental and computational studies, we know that small boron clusters possess planar structures stabilized by electron delocalization both in the σ and π frameworks. An interesting boron cluster is B9–, which has a D8h molecular wheel structure with a single boron atom in the center of a B8 ring. This ring in the D8h-B9– cluster is connected by eight classical two-center, two-electron bonds. In contrast, the cluster’s central boron atom is bonded to the peripheral ring through three delocalized σ and three delocalized π bonds. This bonding structure gives the molecular wheel double aromaticity and high electronic stability. The unprecedented structure and bonding pattern i...
213 citations
TL;DR: Using first-principles calculations, stability and growth mechanism of various boron sheets on Cu(111) substrate are explored and theoretical predictions would stimulate further experiments of synthesizing borons sheets on metal substrates and thus enrich the variety of 2D monolayer materials.
Abstract: As attractive analogue of graphene, boron monolayers have been theoretically predicted. However, due to electron deficiency of boron atom, synthesizing boron monolayer is very challenging in experiments. Using first-principles calculations, we explore stability and growth mechanism of various boron sheets on Cu(111) substrate. The monotonic decrease of formation energy of boron cluster BN with increasing cluster size and low diffusion barrier for a single B atom on Cu(111) surface ensure continuous growth of two-dimensional (2D) boron cluster. During growth process, hexagonal holes can easily arise at the edge of a 2D triangular boron cluster and then diffuse entad. Hence, large-scale boron monolayer with mixed hexagonal-triangular geometry can be obtained via either depositing boron atoms directly on Cu(111) surface or soft landing of small planar BN clusters. Our theoretical predictions would stimulate further experiments of synthesizing boron sheets on metal substrates and thus enrich the variety of 2D monolayer materials.
207 citations
TL;DR: ASolid-state material formed from binary assembly of atomically precise molecular clusters able to interact electronically to produce a magnetically ordered phase at low temperature, akin to atoms in a solid-state compound is described.
Abstract: We describe a solid-state material formed from binary assembly of atomically precise molecular clusters. [Co6Se8(PEt3)6][C60]2 and [Cr6Te8(PEt3)6][C60]2 assembled into a superatomic relative of the cadmium iodide (CdI2) structure type. These solid-state materials showed activated electronic transport with activation energies of 100 to 150 millielectron volts. The more reducing cluster Ni9Te6(PEt3)8 transferred more charge to the fullerene and formed a rock-salt-related structure. In this material, the constituent clusters are able to interact electronically to produce a magnetically ordered phase at low temperature, akin to atoms in a solid-state compound.
197 citations
INAF1, European Southern Observatory2, University of Copenhagen3, Johns Hopkins University4, Carnegie Institution for Science5, Academia Sinica6, Space Telescope Science Institute7, Heidelberg University8, University of Vienna9, University of Milan10, Paris Observatory11, Spanish National Research Council12, University of the Basque Country13, University of Concepción14, Michigan State University15, Princeton University16, University of California, Berkeley17, Ohio State University18, California Institute of Technology19, CERN20, Max Planck Society21
TL;DR: In this article, the mass, velocity-anisotropy, and pseudo-phase-space density profiles of the z = 0.44 CLASH cluster MACS J1206.2-0847 were derived using the projected phase-space distribution of cluster galaxies.
Abstract: Aims. We constrain the mass, velocity-anisotropy, and pseudo-phase-space
density profiles of the z = 0.44 CLASH cluster MACS J1206.2-0847, using the projected phase-space
distribution of cluster galaxies in combination with gravitational lensing.Methods. We use an unprecedented data-set of ≃600 redshifts for cluster
members, obtained as part of a VLT/VIMOS large program, to constrain the cluster mass
profile over the radial range ~0–5 Mpc (0–2.5 virial radii) using the MAMPOSSt and Caustic
methods. We then add external constraints from our previous gravitational lensing
analysis. We invert the Jeans equation to obtain the velocity-anisotropy profiles of
cluster members. With the mass-density and velocity-anisotropy profiles we then obtain the
first determination of a cluster pseudo-phase-space density profile.Results. The kinematics and lensing determinations of the cluster mass
profile are in excellent agreement. This is very well fitted by a NFW model with mass
M200 = (1.4 ± 0.2) × 1015 M⊙
and concentration c200 = 6 ± 1, only slightly higher than
theoretical expectations. Other mass profile models also provide acceptable fits to our
data, of (slightly) lower (Burkert, Hernquist, and Softened Isothermal Sphere) or
comparable (Einasto) quality than NFW. The velocity anisotropy profiles of the passive and
star-forming cluster members are similar, close to isotropic near the center and
increasingly radial outside. Passive cluster members follow extremely well the theoretical
expectations for the pseudo-phase-space density profile and the relation between the slope
of the mass-density profile and the velocity anisotropy. Star-forming cluster members show
marginal deviations from theoretical expectations.Conclusions. This is the most accurate determination of a cluster mass
profile out to a radius of 5 Mpc, and the only determination of the velocity-anisotropy
and pseudo-phase-space density profiles of both passive and star-forming galaxies for an
individual cluster. These profiles provide constraints on the dynamical history of the
cluster and its galaxies. Prospects for extending this analysis to a larger cluster sample
are discussed.
193 citations
TL;DR: The ongoing study of zirconium- and hafnium-porphyrinic metal-organic frameworks led to the discovery of isostructural MOFs based on Zr8 and Hf8 clusters, which are unknown in both cluster and MOF chemistry.
Abstract: The ongoing study of zirconium– and hafnium–porphyrinic metal–organic frameworks (MOFs) led to the discovery of isostructural MOFs based on Zr8 and Hf8 clusters, which are unknown in both cluster and MOF chemistry. The Zr8O6 cluster features an idealized Zr8 cube, in which each Zr atom resides on one vertex and each face of the cube is capped by one μ4-oxygen atom. On each edge of the cube, a carboxylate from a porphyrinic ligand bridges two Zr atoms to afford a 3D MOF with a very rare (4,12)-connected ftw topology, in which two types of polyhedral cages with diameters of ∼1.1 and ∼2.0 nm and a cage opening of ∼0.8 nm are found. The isostructural Zr– and Hf–MOFs exhibit high surface areas, gas uptakes, and catalytic selectivity for cyclohexane oxidation.
190 citations
TL;DR: In this article, the mass profile of the z=0.44 cluster MACS J1206.2-0847 over the radial range 0-5 Mpc (0-2.5 virial radii) using the MAMPOSSt and Caustic methods was determined.
Abstract: We use an unprecedented data-set of about 600 redshifts for cluster members, obtained as part of a VLT/VIMOS large programme, to constrain the mass profile of the z=0.44 cluster MACS J1206.2-0847 over the radial range 0-5 Mpc (0-2.5 virial radii) using the MAMPOSSt and Caustic methods. We then add external constraints from our previous gravitational lensing analysis. We invert the Jeans equation to obtain the velocity-anisotropy profiles of cluster members. With the mass-density and velocity-anisotropy profiles we then obtain the first determination of a cluster pseudo-phase-space density profile. The kinematics and lensing determinations of the cluster mass profile are in excellent agreement. This is very well fitted by a NFW model with mass M200=(1.4 +- 0.2) 10^15 Msun and concentration c200=6 +- 1, only slightly higher than theoretical expectations. Other mass profile models also provide acceptable fits to our data, of (slightly) lower (Burkert, Hernquist, and Softened Isothermal Sphere) or comparable (Einasto) quality than NFW. The velocity anisotropy profiles of the passive and star-forming cluster members are similar, close to isotropic near the center and increasingly radial outside. Passive cluster members follow extremely well the theoretical expectations for the pseudo-phase-space density profile and the relation between the slope of the mass-density profile and the velocity anisotropy. Star-forming cluster members show marginal deviations from theoretical expectations. This is the most accurate determination of a cluster mass profile out to a radius of 5 Mpc, and the only determination of the velocity-anisotropy and pseudo-phase-space density profiles of both passive and star-forming galaxies for an individual cluster [abridged]
189 citations
TL;DR: In this paper, a method for assembling noble metal nanoparticles into stable, three-dimensional (3-D) clusters, whose optical properties can be highly sensitive or remarkably independent of cluster orientation, depending on particle number and cluster geometry.
Abstract: Assembling nanoparticles into well-defined structures is an important way to create and tailor the optical properties of materials. Most advances in metamaterials research to date have been based on structures fabricated in two-dimensional planar geometries. Here, we show an efficient method for assembling noble metal nanoparticles into stable, three-dimensional (3-D) clusters, whose optical properties can be highly sensitive or remarkably independent of cluster orientation, depending on particle number and cluster geometry. Some of the clusters, such as tetrahedra and icosahedra, could serve as the optical kernels for metafluids, imparting metamaterial optical properties into disordered media such as liquids, glasses, or plastics, free from the requirement of nanostructure orientation.
TL;DR: It is reported that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13).
Abstract: A relationship between the size of metal particles and their catalytic activity has been established over a nanometer scale (2–10 nm). However, application on a subnanometer scale (0.5–2 nm) is difficult, a possible reason being that the activity no longer relies on the size but rather the geometric structure as a cluster (or superatomic) compound. We now report that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13). The synthesis with an atomic-level precision was successfully achieved by using a dendrimer ligand as the macromolecular template strictly defining the number of metal atoms. It was quite surprising that the Pt12 cluster exhibited more than 2-fold catalytic activity compared with that of the Pt13 cluster. ESI-TOF-mass and EXAFS analyses provided information about the structures. These analyses suggested that the Pt12 has a deformed coordination, while the Pt13 h...
TL;DR: A remarkably uniform iron abundance, as a function of radius and azimuth, that is statistically consistent with a constant value of ZFe = 0.306 ± 0.012 in solar units out to the edge of the nearby Perseus cluster is reported.
Abstract: A uniform iron abundance in the intracluster gas of the Perseus cluster suggests that the metal enrichment of the intergalactic medium occurred before the cluster formed, probably more than ten billion years ago, rather than after the cluster formed. Most of the metals (elements heavier than helium) produced by stars in the member galaxies of clusters are found in the hot, X-ray-emitting gas between the galaxies. If the metals are uniformly distributed, then they likely were put in place early in the cluster's history. The alternative, where metals appear after cluster formation, is expected to introduce significant spatial variation of metallicity. To test the early enrichment model by eliminating the effects of potential inhomogeneities, it is necessary to measure abundances out to large radii along multiple directions in clusters. Norbert Werner et al. have done just that on a data set of 86 measurements in the Perseus cluster. They find an iron abundance of ZFe = 0.306, which is remarkably uniform as a function of radius and azimuth right to the edge of the cluster. This distribution requires that most of the metal enrichment of the intergalactic medium occurred during the period of maximal star formation and black hole activity, more than 10 billion years ago. Most of the metals (elements heavier than helium) produced by stars in the member galaxies of clusters currently reside within the hot, X-ray-emitting intra-cluster gas. Observations of X-ray line emission from this intergalactic medium have suggested a relatively small cluster-to-cluster scatter outside the cluster centres1,2 and enrichment with iron out to large radii3,4,5, leading to the idea that the metal enrichment occurred early in the history of the Universe3. Models with early enrichment predict a uniform metal distribution at large radii in clusters, whereas those with late-time enrichment6,7 are expected to introduce significant spatial variations of the metallicity. To discriminate clearly between these competing models, it is essential to test for potential inhomogeneities by measuring the abundances out to large radii along multiple directions in clusters, which has not hitherto been done. Here we report a remarkably uniform iron abundance, as a function of radius and azimuth, that is statistically consistent with a constant value of ZFe = 0.306 ± 0.012 in solar units out to the edge of the nearby Perseus cluster. This homogeneous distribution requires that most of the metal enrichment of the intergalactic medium occurred before the cluster formed, probably more than ten billion years ago, during the period of maximal star formation and black hole activity.
TL;DR: In this article, high resolution XPS spectra (4f) of mass-selected Au N -clusters supported by a thin natural silica layer are presented in the size range N ǫ = 1 − 35 atoms per cluster.
TL;DR: In this article, an adaptive mesh refinement simulation of the magnetohydrodynamic evolution of a galaxy cluster from cosmological initial conditions is used to locate shock fronts and apply models of cosmic-ray electron acceleration that are then input into radio emission models.
Abstract: Non-thermal radio emission from cosmic-ray electrons in the vicinity of merging galaxy clusters is an important tracer of cluster merger activity, and is the result of complex physical processes that involve magnetic fields, particle acceleration, gas dynamics, and radiation. In particular, objects known as radio relics are thought to be the result of shock-accelerated electrons that, when embedded in a magnetic field, emit synchrotron radiation in the radio wavelengths. In order to properly model this emission, we utilize the adaptive mesh refinement simulation of the magnetohydrodynamic evolution of a galaxy cluster from cosmological initial conditions. We locate shock fronts and apply models of cosmic-ray electron acceleration that are then input into radio emission models. We have determined the thermodynamic properties of this radio-emitting plasma and constructed synthetic radio observations to compare observed galaxy clusters. We find a significant dependence of the observed morphology and radio relic properties on the viewing angle of the cluster, raising concerns regarding the interpretation of observed radio features in clusters. We also find that a given shock should not be characterized by a single Mach number. We find that the bulk of the radio emission comes from gas with T > 5 ? 107 K, ? ~ 10?28-10?27 g cm?3, with magnetic field strengths of 0.1-1.0 ?G, and shock Mach numbers of . We present an analysis of the radio spectral index which suggests that the spatial variation of the spectral index can mimic synchrotron aging. Finally, we examine the polarization fraction and position angle of the simulated radio features, and compare to observations.
TL;DR: Three novel bimetallic Au-Cu nanoclusters stabilized by a mixed layer of thiolate and phosphine ligands bearing pyridyl groups are synthesized and fully characterized by X-ray single crystal analysis and density functional theory computations and suggest that the selective release of organic ligands from the clusters is possible.
Abstract: Three novel bimetallic Au-Cu nanoclusters stabilized by a mixed layer of thiolate and phosphine ligands bearing pyridyl groups are synthesized and fully characterized by X-ray single crystal analysis and density functional theory computations. The three clusters have an icosahedral Au13 core face-capped by two, four, and eight Cu atoms, respectively. All face-capping Cu atoms in the clusters are triply coordinated by thiolate or pyridyl groups. The surface ligands control the exposure of Au sites in the clusters. In the case of the Au13Cu8 cluster, the presence of 12 2-pyridylthiolate ligands still leaves open space for catalysis. All the 3 clusters are 8-electron superatoms displaying optical gaps of 1.8–1.9 eV. The thermal decomposition studies suggest that the selective release of organic ligands from the clusters is possible.
TL;DR: The TCC detects local structures with bond topologies similar to isolated clusters which minimise the potential energy for a number of monatomic and binary simple liquids with m ≤ 13 particles.
Abstract: We describe the topological cluster classification (TCC) algorithm. The TCC detects local structures with bond topologies similar to isolated clusters which minimise the potential energy for a number of monatomic and binary simple liquids with m ⩽ 13 particles. We detail a modified Voronoi bond detection method that optimizes the cluster detection. The method to identify each cluster is outlined, and a test example of Lennard-Jones liquid and crystal phases is considered and critically examined.
TL;DR: In this paper, a review of basic theoretical concepts to describe submonolayer growth kinetics under nonequilibrium conditions is given, which can be extended and further developed to treat self-organized cluster formation in material systems of current interest, such as nanoalloys and molecular clusters in organic thin-film growth.
Abstract: Understanding and control of cluster and thin-film growth on solid surfaces is a subject of intensive research to develop nanomaterials with new physical properties. In this Colloquium a review of basic theoretical concepts to describe submonolayer growth kinetics under nonequilibrium conditions is given. It is shown how these concepts can be extended and further developed to treat self-organized cluster formation in material systems of current interest, such as nanoalloys and molecular clusters in organic thin-film growth. The presentation is focused on ideal flat surfaces to limit the scope and to discuss key ideas in a transparent way. Open experimental and theoretical challenges are pointed out.
TL;DR: It is found that most icosahedral clusters with a particular composition and arrangement of large and small particles are structural elements of the crystal and thus local crystalline ordering makes only a limited contribution to this process.
Abstract: We study the relationship between local structural ordering and dynamical heterogeneities in a model glass-forming liquid, the Wahnstrom mixture. A novel cluster-based approach is used to detect local energy minimum polyhedral clusters and local crystalline environments. A structure-specific time correlation function is then devised to determine their temporal stability. For our system, the lifetime correlation function for icosahedral clusters decays far slower than for those of similarly sized but topologically distinct clusters. Upon cooling, the icosahedra form domains of increasing size and their lifetime increases with the size of the domains. Furthermore, these long-lived domains lower the mobility of neighboring particles. These structured domains show correlations with the slow regions of the dynamical heterogeneities that form on cooling towards the glass transition. Although icosahedral clusters with a particular composition and arrangement of large and small particles are structural elements of the crystal, we find that most icosahedral clusters lack such order in composition and arrangement and thus local crystalline ordering makes only a limited contribution to this process. Finally, we characterize the spatial correlation of the domains of icosahedra by two structural correlation lengths and compare them with the four-point dynamic correlation length. All the length scales increase upon cooling, but in different ways.
TL;DR: The formation and stabilization of silver clusters using DNA templates are described and the distinct spectroscopic and photophysical properties of the resulting hybrid fluorophores are highlighted, suggesting strategies for fine tuning and stabilizing cluster-based emitters in a host of sensing and biolabeling applications that would benefit from brighter, more photostable, and quantifiable emiters in high background environments.
Abstract: Conductive and plasmon-supporting noble metals exhibit an especially wide range of size-dependent properties, with discrete electronic levels, strong optical absorption, and efficient radiative relaxation dominating optical behavior at the ∼10-atom cluster scale. In this Perspective, we describe the formation and stabilization of silver clusters using DNA templates and highlight the distinct spectroscopic and photophysical properties of the resulting hybrid fluorophores. Strong visible-to-near-IR emission from DNA-encapsulated silver clusters ranging in size from 5 to 11 atoms has been produced and characterized. Importantly, this strong Ag cluster fluorescence can be directly modulated and selectively recovered by optically controlling the dark state residence, even when faced with an overwhelming background. The strength and sequence sensitivity of the oligonucleotide-Ag interaction suggests strategies for fine-tuning and stabilizing cluster-based emitters in a host of sensing and biolabeling applicatio...
TL;DR: In this article, a large, mass-limited sample of massive galaxy clusters from a high-resolution hydrodynamical cosmological simulation is used to show that acceleration of gas introduces biases in the hydrostatic mass estimate of galaxy clusters.
Abstract: The use of galaxy clusters as cosmological probes hinges on our ability to measure their masses accurately and with high precision. Hydrostatic mass is one of the most common methods for estimating the masses of individual galaxy clusters, which suffer from biases due to departures from hydrostatic equilibrium. Using a large, mass-limited sample of massive galaxy clusters from a high-resolution hydrodynamical cosmological simulation, in this work we show that in addition to turbulent and bulk gas velocities, acceleration of gas introduces biases in the hydrostatic mass estimate of galaxy clusters. In unrelaxed clusters, the acceleration bias is comparable to the bias due to non-thermal pressure associated with merger-induced turbulent and bulk gas motions. In relaxed clusters, the mean mass bias due to acceleration is small (<3%), but the scatter in the mass bias can be reduced by accounting for gas acceleration. Additionally, this acceleration bias is greater in the outskirts of higher redshift clusters where mergers are more frequent and clusters are accreting more rapidly. Since gas acceleration cannot be observed directly, it introduces an irreducible bias for hydrostatic mass estimates. This acceleration bias places limits on how well we can recover cluster masses from future X-ray and microwave observations. We discuss implications for cluster mass estimates based on X-ray, Sunyaev-Zeldovich effect, and gravitational lensing observations and their impact on cluster cosmology.
TL;DR: In this paper, the authors present the results of a full dynamical simulation of a globular cluster containing many stellar-mass BHs with a realistic mass spectrum, and they find no evidence for the Spitzer instability.
Abstract: Globular clusters should be born with significant numbers of stellar-mass black holes (BHs). It has been thought for two decades that very few of these BHs could be retained through the cluster lifetime. With masses {approx}10 M{sub Sun }, BHs are {approx}20 times more massive than an average cluster star. They segregate into the cluster core, where they may eventually decouple from the remainder of the cluster. The small-N core then evaporates on a short timescale. This is the so-called Spitzer instability. Here we present the results of a full dynamical simulation of a globular cluster containing many stellar-mass BHs with a realistic mass spectrum. Our Monte Carlo simulation code includes detailed treatments of all relevant stellar evolution and dynamical processes. Our main finding is that old globular clusters could still contain many BHs at present. In our simulation, we find no evidence for the Spitzer instability. Instead, most of the BHs remain well mixed with the rest of the cluster, with only the innermost few tens of BHs segregating significantly. Over the 12 Gyr evolution, fewer than half of the BHs are dynamically ejected through strong binary interactions in the cluster core. The presence of BHs leads to long-termmore » heating of the cluster, ultimately producing a core radius on the high end of the distribution for Milky Way globular clusters (and those of other galaxies). A crude extrapolation from our model suggests that the BH-BH merger rate from globular clusters could be comparable to the rate in the field.« less
TL;DR: The current work suggests that the 24-atom boron cluster continues to be quasi-2D, albeit the tendency to form filled pentagonal units, characteristic of 3D cage-like structures of bulk borons, is observed.
Abstract: The structure and chemical bonding of the 24-atom boron cluster are investigated using photoelectron spectroscopy and ab initio calculations. The joint experimental and theoretical investigation shows that B24 − possesses a quasi-planar structure containing fifteen outer and nine inner atoms with six of the inner atoms forming a filled pentagonal moiety. The central atom of the pentagonal moiety is puckered out of plane by 0.9 A, reminiscent of the six-atom pentagonal caps of the wellknown B12 icosahedral unit. The next closest isomer at the ROCCSD(T) level of theory has a tubular double-ring structure. Comparison of the simulated spectra with the experimental data shows that the global minimum quasi-planar B24 − isomer is the major contributor to the observed photoelectron spectrum, while the tubular isomer has no contribution to the experiment. Chemical bonding analyses reveal that the periphery of the quasi-planar B24 constitutes 15 classical 2c-2e B-B σ-bonds, whereas delocalized σ- and π-bonds are found in the interior of the cluster with one unique 6c-2e π-bond responsible for bonding in the B-centered pentagon. The current work suggests that the 24atom boron cluster continues to be quasi-2D, albeit the tendency to form filled pentagonal units, characteristic of 3D cage-like structures of bulk boron, is observed. © 2013 AIP Publishing LLC.
TL;DR: For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved, indicating that the covalency of the metal-ligand bonds is correctly described by the calculations.
Abstract: A detailed study of the electronic and geometric structure of V2O5 and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V2O5. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L2,3-edge NEXAFS spectra of V2O5 were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal–ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids.
TL;DR: This paper considers general graph topologies, which might be time-varying, and proves that if the graph has cluster spanning trees and all vertices self-linked, then the static linear system can realize intra-cluster synchronization.
Abstract: In this paper, cluster consensus of multiagent systems is studied via inter-cluster nonidentical inputs. Here, we consider general graph topologies, which might be time-varying. The cluster consensus is defined by two aspects: intracluster synchronization, the state at which differences between each pair of agents in the same cluster converge to zero, and inter-cluster separation, the state at which agents in different clusters are separated. For intra-cluster synchronization, the concepts and theories of consensus, including the spanning trees, scramblingness, infinite stochastic matrix product, and Hajnal inequality, are extended. As a result, it is proved that if the graph has cluster spanning trees and all vertices self-linked, then the static linear system can realize intra-cluster synchronization. For the time-varying coupling cases, it is proved that if there exists T > 0 such that the union graph across any T-length time interval has cluster spanning trees and all graphs has all vertices self-linked, then the time-varying linear system can also realize intra-cluster synchronization. Under the assumption of common inter-cluster influence, a sort of inter-cluster nonidentical inputs are utilized to realize inter-cluster separation, such that each agent in the same cluster receives the same inputs and agents in different clusters have different inputs. In addition, the boundedness of the infinite sum of the inputs can guarantee the boundedness of the trajectory. As an application, we employ a modified non-Bayesian social learning model to illustrate the effectiveness of our results.
TL;DR: Magnetic clusters, i.e., assemblies of a finite number (between two or three and several hundred) of interacting spin centers which are magnetically decoupled from their environment, can be found in many materials ranging from inorganic compounds, magnetic molecules, artificial metal structures formed on surfaces to metalloproteins as discussed by the authors.
Abstract: Magnetic clusters, i.e., assemblies of a finite number (between two or three and several hundred) of interacting spin centers which are magnetically decoupled from their environment, can be found in many materials ranging from inorganic compounds, magnetic molecules, artificial metal structures formed on surfaces to metalloproteins. The magnetic excitation spectra in them are determined by the nature of the spin centers, the nature of the magnetic interactions, and the particular arrangement of the mutual interaction paths between the spin centers. Small clusters of up to four magnetic ions are ideal model systems to examine the fundamental magnetic interactions which are usually dominated by Heisenberg exchange, but often complemented by anisotropic and/or higher-order interactions. In large magnetic clusters which may potentially deal with a dozen or more spin centers, the possibility of novel many-body quantum states and quantum phenomena are in focus. In this review the necessary theoretical concepts and experimental techniques to study the magnetic cluster excitations and the resulting characteristic magnetic properties are introduced, followed by examples of small clusters demonstrating the enormous amount of detailed physical information which can be retrieved. The current understanding of the excitations and their physical interpretation in the molecular nanomagnets which represent large magnetic clusters is then presented, with an own section devoted to the subclass of the single-molecule magnets which are distinguished by displaying quantum tunneling of the magnetization. Finally, some quantum many-body states are summarized which evolve in magnetic insulators characterized by built-in or field-induced magnetic clusters. The review concludes addressing future perspectives in the field of magnetic cluster excitations.
TL;DR: In this paper, the ligand-protected mixed-metal Au25-xAgx(SC2H4Ph)18 cluster (abbreviated as Au25−xAgX, where x = 0-5 Ag atoms) is compared with the unsubstituted Au25(SC 2H4P)18 (ABbrevated as AU25).
Abstract: Recent synthetic advances have produced very small (sub-2 nm), ligand-protected mixed-metal clusters. Realization of such clusters allows the investigation of fundamental questions: (1) Will heteroatoms occupy specific sites within the cluster? (2) How will the inclusion of heteroatoms affect the electronic structure and chemical properties of the cluster? (3) How will these very small mixed-metal systems differ from larger, more traditional alloy materials? In this report we provide experimental and computational characterization of the ligand-protected mixed-metal Au25–xAgx(SC2H4Ph)18 cluster (abbreviated as Au25–xAgx, where x = 0–5 Ag atoms) compared with the unsubstituted Au25(SC2H4Ph)18 cluster (abbreviated as Au25). Density functional theory analysis has predicted that Ag heteroatoms will preferentially occupy sites on the surface of the cluster core. X-ray photoelectron spectroscopy revealed Au–Ag state mixing and charge redistribution within the Au25–xAgx cluster. Optical spectroscopy and nonaqueo...
TL;DR: In this article, the authors developed a method to diagnose the substructure and dynamical state of galaxy clusters by using photometric data of the Digital Sky Survey (SDSS), where the brightness distribution of meber galaxies is smoothed by using a Gaussian kernel with a weight of their optical luminosities.
Abstract: Dynamical state of galaxy clusters is closely related to the ir observational properties in X-ray, optical and radio wavelengths. We develop a method to diagnose the substructure and dynamical state of galaxy clusters by using photometric data of Sl oan Digital Sky Survey (SDSS). To trace mass distribution, the brightness distribution of me mber galaxies is smoothed by using a Gaussian kernel with a weight of their optical luminosities. After deriving the asymmetry, the ridge flatness and the normalized deviation of the smooth ed optical map, we define a relaxation parameter, , to quantify dynamical state of clusters. This method is applied to a test sample of 98 clusters of 0.05 0) and unrelaxed ( 0. We find that the dominance and absolute magnitude of the brigh test cluster galaxies closely correlate with dynamical state of clusters. The emission power of radio halos is quantitatively related to cluster dynamical state, beside the known dependence on the X-ray luminosity.
TL;DR: An unprecedented air-stable, nanospheric polyhydrido copper cluster, [Cu20H11(S2P(O(i)Pr)2)9] (1H], which is the first example of an elongated triangular orthobicupola array of Cu atoms having C3h symmetry, was synthesized and characterized.
Abstract: An unprecedented air-stable, nanospheric polyhydrido copper cluster, [Cu20H11(S2P(OiPr)2)9] (1H), which is the first example of an elongated triangular orthobicupola array of Cu atoms having C3h symmetry, was synthesized and characterized. Its composition was primarily determined by electrospray ionization mass spectrometry, and it was fully characterized by 1H, 2H, and 31P NMR spectroscopy and single-crystal X-ray diffraction (XRD). The structure of complex 1H can be expressed in terms of a trigonal-bipyramidal [Cu2H5]3– unit anchored within an elongated triangular orthobicupola containing 18 Cu atoms, which is further stabilized by 18 S atoms from nine dithiophosphate ligands and six capping hydrides. The positions of the 11 hydrides revealed by low temperature XRD were supported by a density functional theory investigation on the simplified model [Cu20H11(S2PH2)9] with C3h symmetry. 1H is capable of releasing H2 gas upon irradiation with sunlight, under mild thermal conditions (65 °C), or in the presen...