Showing papers on "Cluster (physics) published in 2007"
TL;DR: In this article, the authors compared the ICM properties outside cluster cores with the observed X-ray observations of nearby relaxed clusters to assess the impact of galaxy formation, and found that the observed ICM characteristics outside cluster core are well reproduced in the simulations that include cooling and star formation, while the non-radiative simulations predict an overall shape of ICM profiles inconsistent with observations.
Abstract: We present detailed comparisons of the intracluster medium (ICM) in cosmological Eulerian cluster simulations with deep Chandra observations of nearby relaxed clusters. To assess the impact of galaxy formation, we compare two sets of simulations, one performed in the nonradiative regime and another with radiative cooling and several physical processes critical to various aspects of galaxy formation: star formation, metal enrichment, and stellar feedback. We show that the observed ICM properties outside cluster cores are well reproduced in the simulations that include cooling and star formation, while the nonradiative simulations predict an overall shape of the ICM profiles inconsistent with observations. In particular, we find that the ICM entropy in our runs with cooling is enhanced to the observed levels at radii as large as half of the virial radius. We also find that outside cluster cores entropy scaling with the mean ICM temperature in both simulations and Chandra observations is consistent with being self-similar within current error bars. We find that the pressure profiles of simulated clusters are also close to self-similar and exhibit little cluster-to-cluster scatter. We provide analytic fitting formulae for the pressure profiles of the simulated and observed clusters. The X-ray observable mass relations for our simulated sample agree with the Chandra measurements to ≈10%-20% in normalization. We show that this systematic difference could be caused by the subsonic gas motions, unaccounted for in X-ray hydrostatic mass estimates. The much improved agreement of simulations and observations in the ICM profiles and scaling relations is encouraging, and the existence of tight relations of X-ray observables, such as YX, and total cluster mass and the simple redshift evolution of these relations hold promise for the use of clusters as cosmological probes. However, the disagreement between the predicted and observed fractions of cluster baryons in stars remains a major puzzle.
715 citations
TL;DR: The novel findings suggest that good size control during the synthesis of MoS2 nanostructures may be used for the production of chemically or optically active MoS1 nanomaterials with superior performance.
Abstract: Molybdenum disulphide nanostructures are of interest for a wide variety of nanotechnological applications ranging from the potential use of inorganic nanotubes in nanoelectronics to the active use of nanoparticles in heterogeneous catalysis. Here, we use atom-resolved scanning tunnelling microscopy to systematically map and classify the atomic-scale structure of triangular MoS2 nanocrystals as a function of size. Instead of a smooth variation as expected from the bulk structure of MoS2, we observe a very strong size dependence for the cluster morphology and electronic structure driven by the tendency to optimize the sulphur excess present at the cluster edges. By analysing of the atomic-scale structure of clusters, we identify the origin of the structural transitions occurring at unique cluster sizes. The novel findings suggest that good size control during the synthesis of MoS2 nanostructures may be used for the production of chemically or optically active MoS2 nanomaterials with superior performance.
639 citations
TL;DR: In this article, the authors present mock Chandra analyses of cosmological cluster simulations and assess X-ray measurements of galaxy cluster properties using a model and procedure essentially identical to that used in real data analysis, and show that reconstruction of three-dimensional ICM density and temperature profiles is excellent for relaxed clusters, but still reasonably accurate for unrelaxed systems.
Abstract: X-ray observations of galaxy clusters potentially provide powerful cosmological probes if systematics due to our incomplete knowledge of the ICM physics is understood and controlled. In this paper we present mock Chandra analyses of cosmological cluster simulations and assess X-ray measurements of galaxy cluster properties using a model and procedure essentially identical to that used in real data analysis. We show that reconstruction of three-dimensional ICM density and temperature profiles is excellent for relaxed clusters, but still reasonably accurate for unrelaxed systems. The total ICM mass is measured quite accurately (6%) in all clusters, while the hydrostatic estimate of the gravitationally bound mass is biased low by about 5%-20% through the virial region, primarily due to additional pressure support provided by subsonic bulk motions in the ICM, ubiquitous in our simulations even in relaxed systems. Gas fraction determinations are therefore biased high; the bias increases toward cluster outskirts and depends sensitively on its dynamical state, but we do not observe significant trends of the bias with cluster mass or redshift. We also find that different average ICM temperatures, such as the X-ray spectroscopic Tspec and gas-mass-weighted Tmg, are related to each other by a constant factor with a relatively small object-to-object scatter and no systematic trend with mass, redshift, or the dynamical state of clusters. We briefly discuss direct applications of our results for different cluster-based cosmological tests.
570 citations
TL;DR: The use of the cluster-wise stability assessment is used to distinguish between meaningful stable and spurious clusters, but it is also shown that clusters are sometimes only stable because of the inflexibility of certain clustering methods.
514 citations
TL;DR: The design, synthesis, and application of a novel mesh-adjustable molecular sieve (MAMS-1) that possesses an infinite number of mesh sizes is reported, which in principle, by precise temperature control, any two gases with a size difference can be separated by a MAMS.
Abstract: Gas separation using molecular sieves (MSs) is a environmentally benign, energy-conserving alternative to traditional separation processes, such as distillation and absorption. When using zeolite MSs, an accurate one-on-one match between the mesh size and the separation need is essential. However, when the size disparity of the two gases to be separated is small, a MS with the optimum mesh size is not always readily available. A mismatch inevitably leads to an inefficient separation. Recently, titanosilicate was shown to possess superior flexibility over that of traditional zeolites; a few MSs with discrete mesh sizes were made based on the degree of dehydration of this material at various temperatures. Nevertheless, a MS with more than one mesh size has never been made in the past. Herein, we report the design, synthesis, and application of a novel mesh-adjustable molecular sieve (MAMS-1) that possesses an infinite number of mesh sizes. MAMS-1 is based on a metal–organic framework (MOF), compounds known for their dynamic porous properties. However, the concept of a MAMS has never appeared in the literature prior to the present work. MAMS-1 represents a MOF-based MS whose mesh can be adjusted continuously. The mesh range of MAMS-1 falls between 2.9 and 5.0 5, which covers the size range of almost all commercially important gas separations. When the temperature is precisely controlled, any mesh size within this range can be accurately attained. Gas separations such as those of N2/O2 and N2/CH4, which are normally difficult to achieve, are readily attainable by using MAMS-1. In principle, by precise temperature control, any two gases with a size difference can be separated by a MAMS. MOFs have attracted a great deal of attention because of their unique structures 7] and potential applications in catalysis, separation, and gas storage. In particular, flexible MOFs have caught enormous attention lately. Numerous studies have indicated that the key to constructing a flexible MOF lies in the utilization of weak interactions, such as hydrogen bonding, p–p stacking, and hydrophobic interaction, in addition to strong covalent and coordinative bonding. Flexible MOFs based on hydrogen bonding have been widely studied, but those originating from p–p stacking and hydrophobic interaction have rarely been explored. To make a MAMS, two factors must be taken into account: the material must have permanent porosity to hold gas molecules, and the pores must be flexible. The former usually requires strong bonds, while the latter implies weak interactions in the framework. These two seemingly irreconcilable prerequisites for a MAMS can be met simultaneously by using a graphitic structure, in which atoms in each layer are connected covalently but the layers are held together by weak interactions. One approach to such a graphitic MOF is to apply an amphiphilic ligand that consists of hydrophobic and hydrophilic ends, similar to a surfactant, but with the hydrophilic end functionalized. The functional group at the hydrophilic end of the ligand will bind metal ions/clusters, and the structure will propagate into a 2D layer. Two layers of ligands will sandwich a metal-ion/cluster layer, thus giving rise to a trilayer, and these trilayers will pack through van der Waals interaction. The ligand adopted for the aforementioned purposes is 5tert-butyl-1,3-benzenedicarboxylate (bbdc), which was previously used in our laboratory to build a micelle-like cuboctahedral cage to adjust the solubility of a 24-molybdenum cluster. Recently, it was also used in a zinc microporous MOF. In fact, solvothermal reaction between H2(bbdc) and Ni(NO3)2 in H2O/ethylene glycol in a Teflon-lined autoclave afforded such a graphitic structure, [Ni8(5-bbdc)6(m3-OH)4] (designated MAMS-1 for convenience). Desolvated MAMS1 demonstrates temperature-induced molecular-gating effects in which the size of the gates can be tuned continuously from 2.9 to 5.0 5 for the first time. Commercially relevant gas separations, such as those of H2/N2, H2/CO, N2/O2, N2/CH4, CH4/C2H4, and C2H4/C3H6, can be achieved by MAMS-1. In principle, by precise temperature control, any mesh size within this range can be achieved. In fact, all the pairs of gases listed above have been separated by using MAMS-1. Single-crystal X-ray analysis revealed that MAMS-1 contains an octanickel [Ni8(m3-OH)4] cluster as one of the two secondary building units (SBUs; Figure 1a), the other being the bbdc ligand. The eight octahedral Ni atoms are divided into four pairs by a twofold axis through the center of the cluster. Ni1 binds five carboxylate O atoms from four bbdc ligands and one m3-OH group. Ni2 is coordinated by three carboxylate O atoms and three m3-OH groups. Ni3 is bound to four carboxylate O atoms, one m3-OH group, and an aqua [*] S. Ma, Dr. D. Sun, Dr. X.-S. Wang, Prof. Dr. H.-C. Zhou Department of Chemistry and Biochemistry Miami University Oxford, OH 45056 (USA) Fax: (+01)513-529-8091 E-mail: zhouh@muohio.edu
342 citations
TL;DR: The chemical reaction between [Au11(PPh3)8Cl2]- and n-alkanethiol CnH2n+1SH (n = 2, 8, 10, 12, 14, 16, and 18) serendipitously yielded stable Au25 cluster compounds with the formula, [au25(Pph3)10(SCnH 2n+ 1)5Cl2]2] 2.
Abstract: The chemical reaction between [Au11(PPh3)8Cl2]+ and n-alkanethiol CnH2n+1SH (n = 2, 8, 10, 12, 14, 16, and 18) serendipitously yielded stable Au25 cluster compounds with the formula, [Au25(PPh3)10(SCnH2n+1)5Cl2]2+. Single-crystal X-ray structural analysis of [Au25(PPh3)10(SC2H5)5Cl2](SbF6)2 revealed that the Au25 core is constructed by bridging two icosahedral Au13 clusters with thiolates sharing a vertex atom. Optical absorption spectroscopy showed that coupling between the Au13 building blocks gives rise to new electronic levels in addition to those of the Au13 constituents.
330 citations
TL;DR: It is shown that all cages with even carbon atoms from C(68) to C(98) are available as endohedral nitride cluster structures (with the exception of C(72), C(74), and C(76).
Abstract: The world of endohedral fullerenes was significantly enlarged over the past seven years by the cluster fullerenes, which contain structures such as the M(2)C(2) carbides and the M(3)N nitrides. While the carbide clusters are generated under the standard arc-burning conditions according to stabilization conditions, the nitride cluster fullerenes (NCFs) are formed by varying the composition of the cooling gas atmosphere in the arc-burning process. The special conditions for NCF synthesis is described in detail and the optimum conditions for the production of NCFs as the main product in fullerene syntheses are given. A general review of all NCFs reported to date consists of the structures, properties, and stability of the NCFs as well as the abundance of the NCFs in the fullerene soot. It is shown that all cages with even carbon atoms from C(68) to C(98) are available as endohedral nitride cluster structures (with the exception of C(72), C(74), and C(76)). Specifically, the NCFs form the largest number of structures that violate the isolated pentagon rule (IPR). Finally some practical applications of these cluster fullerenes are illustrated and an outlook is given, taking the superior stability of these endohedral fullerenes into account.
324 citations
TL;DR: In this article, the authors present a statistical study of the occurrence and effects of cooling cores in the clusters of galaxies in a flux-limited sample, HIFLUGCS, based on ROSAT and ASCA observations.
Abstract: We present a statistical study of the occurrence and effects of the cooling cores in the clusters of galaxies in a flux-limited sample, HIFLUGCS, based on ROSAT and ASCA observations. About 49% of the clusters in this sample have a significant, classically-calculated cooling-flow, mass-deposition rate. The upper envelope of the derived mass-deposition rate is roughly proportional to the cluster mass, and the fraction of cooling core clusters is found to decrease with it. The cooling core clusters are found to have smaller core radii than non-cooling core clusters, while some non-cooling core clusters have high beta values (> 0.8). In the relation of the X-ray luminosity vs. the temperature and the mass, the cooling core clusters show a significantly higher normalization. A systematic correlation analysis, also involving relations of the gas mass and the total infrared luminosity, indicates that this bias is shown to be mostly due to an enhanced X-ray luminosity for cooling core clusters, while the other parameters, like temperature, mass, and gas mass may be less affected by the occurrence of a cooling core. These results may be explained by at least some of the non-cooling core clusters being in dynamically young states compared with cooling core clusters, and they may turn into cooling core clusters in a later evolutionary stage.
288 citations
TL;DR: In this paper, the structural and scaling properties of the temperature distribution of the hot, X-ray emitting intra-cluster medium of galaxy clusters, and its dependence on dynamical state, can give insights into the physical processes governing the formation and evolution of structure.
Abstract: Context. A study of the structural and scaling properties of the temperature distribution of the hot, X-ray emitting intra-cluster medium of galaxy clusters, and its dependence on dynamical state, can give insights into the physical processes governing the formation and evolution of structure. Aims. Accurate temperature measurements are a pre-requisite for a precise knowledge of the thermodynamic properties of the intra-cluster medium. Methods. We analyse the X-ray temperature profiles from XMM-Newton observations of 15 nearby (z < 0.2) clusters, drawn from a statistically representative sample. The clusters cover a temperature range from 2.5 keV to 8.5 keV, and present a variety of X-ray morphologies. We derive accurate projected temperature profiles to ∼0.5 R 200 , and compare structural properties (outer slope, presence of cooling core) with a quantitative measure of the X-ray morphology as expressed by power ratios. We also compare the results to recent cosmological numerical simulations. Results. Once the temperature profiles are scaled by an average cluster temperature (excluding the central region) and the estimated virial radius, the profiles generally decline in the region 0.1 R 200 ≤ R ≤ 0.5 R 200 . The central regions show the largest scatter, attributable mostly to the presence of cool core clusters. There is good agreement with numerical simulations outside the core regions. We find no obvious correlations between power ratio and outer profile slope. There may however be a weak trend with the existence of a cool core, in the sense that clusters with a central temperature decrement appear to be slightly more regular. Conclusions. The present results lend further evidence to indicate that clusters are a regular population, at least outside the core region.
258 citations
TL;DR: In this paper, a sample of 33 galaxy clusters, selected in the redshift range z=0.055 to 0.183 from the this paperLEX Cluster Survey, having X-ray luminosities above 0.4 X 10^44 h_70^-2 erg s^-1 in the 0.1 - 2.4 keV band, is presented.
Abstract: The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass. To improve on this situation we have started an XMM-Newton Large Programme for the in-depth study of a representative sample of 33 galaxy clusters, selected in the redshift range z=0.055 to 0.183 from the REFLEX Cluster Survey, having X-ray luminosities above 0.4 X 10^44 h_70^-2 erg s^-1 in the 0.1 - 2.4 keV band. This paper introduces the sample, compiles properties of the clusters, and provides detailed information on the sample selection function. We describe the selection of a nearby galaxy cluster sample that makes optimal use of the XMM-Newton field-of-view, and provides nearly homogeneous X-ray luminosity coverage for the full range from poor clusters to the most massive objects in the Universe. For the clusters in the sample, X-ray fluxes are derived and compared to the previously obtained fluxes from the ROSAT All-Sky Survey. We find that the fluxes and the flux errors have been reliably determined in the ROSAT All-Sky Survey analysis used for the REFLEX Survey. We use the sample selection function documented in detail in this paper to determine the X-ray luminosity function, and compare it with the luminosity function of the entire REFLEX sample. We also discuss morphological peculiarities of some of the sample members. The sample and some of the background data given in this introductory paper will be important for the application of these data in the detailed studies of cluster structure, to appear in forthcoming publications.
223 citations
TL;DR: It is demonstrated that one cannot ignore σ‐electrons and that the presence of two‐center two‐electron (2c2e) peripheral BB bonds together with the globally delocalized σ-electrons must be taken into consideration when the shape of pure boron cluster is discussed.
Abstract: We present a comprehensive analysis of chemical bonding in pure boron clusters. It is now established in joint experimental and theoretical studies that pure boron clusters are planar or quasi-planar at least up to twenty atoms. Their planarity or quasi-planarity was usually discussed in terms of pi-delocalization or pi-aromaticity. In the current article, we demonstrated that one cannot ignore sigma-electrons and that the presence of two-center two-electron (2c--2e) peripheral B--B bonds together with the globally delocalized sigma-electrons must be taken into consideration when the shape of pure boron cluster is discussed. The global aromaticity (or global antiaromaticity) can be assigned on the basis of the 4n+2 (or 4n) electron counting rule for either pi- or sigma-electrons in the planar structures. We showed that pure boron clusters could have double (sigma- and pi-) aromaticity (B3-, B4, B5+, B6(2+), B7+, B7-, B8, B(8)2-, B9-, B10, B11+, B12, and B13+), double (sigma- and pi-) antiaromaticity (B6(2-), B15), or conflicting aromaticity (B5-,sigma-antiaromatic and pi-aromatic and B14, sigma-aromatic and pi-antiaromatic). Appropriate geometric fit is also an essential factor, which determines the shape of the most stable structures. In all the boron clusters considered here, the peripheral atoms form planar cycles. Peripheral 2c--2e B--B bonds are built up from s to p hybrid atomic orbitals and this enforces the planarity of the cycle. If the given number of central atoms (1, 2, 3, or 4) can perfectly fit the central cavity then the overall structure is planar. Otherwise, central atoms come out of the plane of the cycle and the overall structure is quasi-planar.
TL;DR: In this article, a detailed investigation into the properties of CDM halos make them effective strong gravitational lenses is presented, where the authors measure concentrations, axis ratios, orientations, and substructure of each cluster, and compare the lensing-weighted distribution of each quantity to that of the cluster population as a whole.
Abstract: We present a detailed investigation into which properties of CDM halos make them effective strong gravitational lenses. Strong-lensing cross sections of 878 clusters from an N-body simulation are measured by ray-tracing through 13,594 unique projections. We measure concentrations, axis ratios, orientations, and the substructure of each cluster, and compare the lensing-weighted distribution of each quantity to that of the cluster population as a whole. The concentrations of lensing clusters are on average 34% larger than the typical cluster in the universe. Despite this bias, the anomalously high concentrations (c > 14) recently measured by several groups appear to be inconsistent with the concentration distribution in our simulations, which predict that 0.6) lenses is in good agreement with ΛCDM, although our simulations predict more low-redshift (z < 0.6) lenses than observed.
TL;DR: In this paper, the structural features and third-order optical nonlinearities of the principal classes of transition heterothiometallic clusters are discussed and examined, and recent advances and perspectives of these clusters as NLO materials are also described.
TL;DR: A detailed analysis of the underlying electronic structure shows that the chemical reactivity of the gold cluster anions can be elucidated in terms of a partial-jellium picture, where delocalized electrons occupying electronic shells move over the ionic skeleton, whose geometric structure is strongly influenced by the directional bonding associated with the highly localized "d-band" electrons.
Abstract: Ground-state structures and other experimentally relevant isomers of Au(15) (-) to Au(24) (-) clusters are determined through joint first-principles density functional theory and photoelectron spectroscopy measurements. Subsequent calculations of molecular O(2) adsorption to the optimal cluster structures reveal a size-dependent reactivity pattern that agrees well with earlier experiments. A detailed analysis of the underlying electronic structure shows that the chemical reactivity of the gold cluster anions can be elucidated in terms of a partial-jellium picture, where delocalized electrons occupying electronic shells move over the ionic skeleton, whose geometric structure is strongly influenced by the directional bonding associated with the highly localized "d-band" electrons.
TL;DR: In this paper, the authors used star density maps obtained from the Two-Micron All-Sky Survey (2MASS) to obtain a sample of star clusters in the entire Galactic Plane with vertical bar b vertical bar < 20 degrees.
Abstract: We used star density maps obtained from the Two-Micron All-Sky Survey (2MASS) to obtain a sample of star clusters in the entire Galactic Plane with vertical bar b vertical bar < 20 degrees. A total of 1788 star cluster candidates are identified in this survey. Among those are 681 previously known open clusters and 86 globular clusters. A statistical analysis indicates that our sample of 1021 new cluster candidates has a contamination of about 50 per cent. Star cluster parameters are obtained by fitting a King profile to the star density. These parameters are used to statistically identify probable new globular cluster candidates in our sample. A detailed investigation of the projected distribution of star clusters in the Galaxy demonstrates that they show a clear tendency to cluster on spatial scales in the order of 12-25 pc, a typical size for molecular clouds.
TL;DR: In this article, the authors presented a comprehensive mass reconstruction of the rich galaxy cluster Cl 0024+17 at z 0.4 from ACS data, unifying both strong and weak-lensing constraints.
Abstract: We present a comprehensive mass reconstruction of the rich galaxy cluster Cl 0024+17 at z 0.4 from ACS data, unifying both strong- and weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies (~120 arcmin-2) across the cluster enables the derivation of a high-resolution parameter-free mass map. The strongly lensed objects tightly constrain the mass structure of the cluster inner region on an absolute scale, breaking the mass-sheet degeneracy. The mass reconstruction of Cl 0024+17 obtained in such a way is remarkable. It reveals a ringlike dark matter substructure at r ~ 75'' surrounding a soft, dense core at r 50''. We interpret this peculiar substructure as the result of a high-speed line-of-sight collision of two massive clusters ~1-2 Gyr ago. Such an event is also indicated by the cluster velocity distribution. Our numerical simulation with purely collisionless particles demonstrates that such density ripples can arise by radially expanding, decelerating particles that originally comprised the precollision cores. Cl 0024+17 can be likened to the bullet cluster 1E 0657-56, but viewed along the collision axis at a much later epoch. In addition, we show that the long-standing mass discrepancy for Cl 0024+17 between X-ray and lensing can be resolved by treating the cluster X-ray emission as coming from a superposition of two X-ray systems. The cluster's unusual X-ray surface brightness profile that requires a two isothermal sphere description supports this hypothesis.
TL;DR: In this article, the current state of understanding pertaining to various factors that control the reactivity and catalytic activity of such nanostructures, using the CO oxidation reaction catalyzed by the gold nanoclusters adsorbed on MgO as a paradigm.
Abstract: Focusing on size-selected gold clusters consisting of up to 20 atoms, that is, in the size regime where properties cannot be obtained from those of the bulk material through scaling considerations, we discuss the current state of understanding pertaining to various factors that control the reactivity and catalytic activity of such nanostructures, using the CO oxidation reaction catalyzed by the gold nanoclusters adsorbed on MgO as a paradigm. These factors include the role of the metal-oxide support and its defects, the charge state of the cluster, structural fluxionality of the clusters, electronic size effects, the effect of an underlying metal support on the dimensionality, charging and chemical reactivity of gold nanoclusters adsorbed on the metal-supported metal-oxide, and the promotional effect of water. We show that through joined experimental and first-principles quantum mechanical calculations and simulations, a detailed picture of the reaction mechanism emerges.
TL;DR: In this paper, the authors present a statistical study of the occurrence and effects of cooling cores in the clusters of galaxies in a flux-limited sample, HIFLUGCS, based on ROSAT and ASCA observations.
Abstract: We present a statistical study of the occurrence and effects of the cooling cores in the clusters of galaxies in a flux-limited sample, HIFLUGCS, based on ROSAT and ASCA observations. About 49% of the clusters in this sample have a significant, classically-calculated cooling-flow, mass-deposition rate. The upper envelope of the derived mass-deposition rate is roughly proportional to the cluster mass, and the fraction of cooling core clusters is found to decrease with it. The cooling core clusters are found to have smaller core radii than non-cooling core clusters, while some non-cooling core clusters have high $\beta$ values (> 0.8). In the relation of the X-ray luminosity vs. the temperature and the mass, the cooling core clusters show a significantly higher normalization. A systematic correlation analysis, also involving relations of the gas mass and the total infrared luminosity, indicates that this bias is shown to be mostly due to an enhanced X-ray luminosity for cooling core clusters, while the other parameters, like temperature, mass, and gas mass may be less affected by the occurrence of a cooling core. These results may be explained by at least some of the non-cooling core clusters being in dynamically young states compared with cooling core clusters, and they may turn into cooling core clusters in a later evolutionary stage.
TL;DR: Evaluating the proposed complete link cluster method for science mapping showed that the proposed method could provide with valid depictions of current research, though some severe restrictions would adhere to its application.
TL;DR: It is demonstrated that superatom compounds formed by combining superhalogens (Al(13)) with superalkalis (K(3)O and Na( 3)O) can exhibit novel chemical and tunable electronic features.
Abstract: It has recently been demonstrated that chosen clusters of specific size and composition can exhibit behaviors reminiscent of atoms in the periodic table and hence can be regarded as superatoms forming a third dimension. An Al13 cluster has been shown to mimic the behavior of halogen atoms. Here, we demonstrate that superatom compounds formed by combining superhalogens (Al13) with superalkalis (K3O and Na3O) can exhibit novel chemical and tunable electronic features. For example, Al13(K3O)3 is shown to have low first and second ionization potentials of 2.49 and 4.64 eV, respectively, which are lower than alkali atoms and can be regarded as ultra alkali motifs. Al13K3O is shown to be a strongly bound molecule that can be assembled into stable superatom assemblies (Al13K3O)n with Al13 and K3O as the superatom building blocks. The studies illustrate the potential of creating new materials with an unprecedented control on physical and electronic properties.
TL;DR: The structures and stabilities of gold clusters with up to 14 atoms have been determined by density-functional theory and are characterized by the maximum hardness principle of chemical reactivity and minimum polarizability principle.
Abstract: The structures and stabilities of gold clusters with up to 14 atoms have been determined by density-functional theory. The structure optimizations and frequency analysis are performed with the Perdew-Wang 1991 gradient-corrected functional combined with the effective core potential and corresponding valence basis set (LANL2DZ). The turnover point from two-dimensional to three-dimensional geometry for gold clusters occurs at Au12. The energetic and electronic properties of the small gold clusters are strongly dependent on sizes and structures, which are in good agreement with experiment and other theoretical calculations. The even-odd oscillation in cluster stability and electronic properties predicted that the clusters with even numbers of atoms were more stable than the neighboring clusters with odd numbers of atoms. The stability and electronic structure properties of gold clusters are also characterized by the maximum hardness principle of chemical reactivity and minimum polarizability principle.
TL;DR: In this article, the authors present a sample of 29 galaxy clusters from the XMM-LSS survey over an area of some 5deg2 out to a redshift of z=1.05.
Abstract: We present a sample of 29 galaxy clusters from the XMM-LSS survey over an area of some 5deg2 out to a redshift of z=1.05. The sample clusters, which represent about half of the X-ray clusters identified in the region, follow well defined X-ray selection criteria and are all spectroscopically confirmed. For all clusters, we provide X-ray luminosities and temperatures as well as masses. The cluster distribution peaks around z=0.3 and T =1.5 keV, half of the objects being groups with a temperature below 2 keV. Our L-T(z) relation points toward self-similar evolution, but does not exclude other physically plausible models. Assuming that cluster scaling laws follow self-similar evolution, our number density estimates up to z=1 are compatible with the predictions of the concordance cosmology and with the findings of previous ROSAT surveys. Our well monitored selection function allowed us to demonstrate that the inclusion of selection effects is essential for the correct determination of the evolution of the L-T relation, which may explain the contradictory results from previous studies. Extensive simulations show that extending the survey area to 10deg2 has the potential to exclude the non-evolution hypothesis, but that constraints on more refined ICM models will probably be limited by the large intrinsic dispersion of the L-T relation. We further demonstrate that increasing the dispersion in the scaling laws increases the number of detectable clusters, hence generating further degeneracy [in addition to sigma8, Omega_m, L(M,z) and T(M,z)] in the cosmological interpretation of the cluster number counts. We provide useful empirical formulae for the cluster mass-flux and mass-count-rate relations as well as a comparison between the XMM-LSS mass sensitivity and that of forthcoming SZ surveys.
TL;DR: A recent structural model reconciles apparently conflicting features of randomness, short-range order, and medium-range ordering that coexist in metallic glasses as mentioned in this paper, including icosahedra.
Abstract: A recent structural model reconciles apparently conflicting features of randomness, short-range order, and medium-range order that coexist in metallic glasses In this efficient cluster packing model, short-range order can be described by efficiently packed solute-centered clusters, producing more than a dozen established atomic clusters, including icosahedra The observed preference for icosahedral short-range order in metallic glasses is consistent with the theme of efficient atomic packing and is further favored by solvent-centered clusters Driven by solute–solute avoidance, medium-range order results from the organization in space of overlapping, percolating (via connected pathways), quasi-equivalent clusters Cubic-like and icosahedral-like organization of these clusters are consistent with measured medium-range order New techniques such as fluctuation electron microscopy now provide more detailed experimental studies of medium-range order for comparison with model predictions Microscopic free volume in the efficient cluster packing model is able to represent experimental and computational results, showing free volume complexes ranging from subatomic to atomic-level sizes Free volume connects static structural models to dynamic processes such as diffusion and deformation New approaches dealing with “free” and “anti-free” microscopic volume and coordinated atomic motion show promise for modeling the complex dynamics of structural relaxations such as the glass transition Future work unifying static and dynamic structural views is suggested
TL;DR: In this paper, the authors present a sample of 29 galaxy clusters from the XMM-LSS survey over an area of some 5deg2 out to a redshift of z=1.05.
Abstract: We present a sample of 29 galaxy clusters from the XMM-LSS survey over an area of some 5deg2 out to a redshift of z=1.05. The sample clusters, which represent about half of the X-ray clusters identified in the region, follow well defined X-ray selection criteria and are all spectroscopically confirmed. For all clusters, we provide X-ray luminosities and temperatures as well as masses. The cluster distribution peaks around z=0.3 and T =1.5 keV, half of the objects being groups with a temperature below 2 keV. Our L-T(z) relation points toward self-similar evolution, but does not exclude other physically plausible models. Assuming that cluster scaling laws follow self-similar evolution, our number density estimates up to z=1 are compatible with the predictions of the concordance cosmology and with the findings of previous ROSAT surveys. Our well monitored selection function allowed us to demonstrate that the inclusion of selection effects is essential for the correct determination of the evolution of the L-T relation, which may explain the contradictory results from previous studies. Extensive simulations show that extending the survey area to 10deg2 has the potential to exclude the non-evolution hypothesis, but that constraints on more refined ICM models will probably be limited by the large intrinsic dispersion of the L-T relation. We further demonstrate that increasing the dispersion in the scaling laws increases the number of detectable clusters, hence generating further degeneracy [in addition to sigma8, Omega_m, L(M,z) and T(M,z)] in the cosmological interpretation of the cluster number counts. We provide useful empirical formulae for the cluster mass-flux and mass-count-rate relations as well as a comparison between the XMM-LSS mass sensitivity and that of forthcoming SZ surveys.
TL;DR: In this paper, the authors present a systematic study on the possible mechanisms of hydrogen spillover on several carbon-based materials using density functional theory (DFT) and show that the migration of H atoms from a Pt cluster catalyst to the substrates is facile at ambient conditions with a small energy barrier.
Abstract: We present a systematic study on the possible mechanisms of hydrogen spillover onto several carbon-based materials using density functional theory (DFT). Adsorption and diffusion of atomic hydrogen on a graphene sheet, single-walled carbon nanotubes, and a polyaromatic compound, hexabenzocoronene, were calculated, and the potential energies along the selected adsorption and diffusion minimum energy pathways were mapped out. We show that the migration of H atoms from a Pt cluster catalyst to the substrates is facile at ambient conditions with a small energy barrier, although the process is slightly endothermic, and that the H atoms can be either physisorbed or chemisorbed on carbon surfaces. Our results indicate that diffusion of H atoms in a chemisorbed state is energetically difficult since it requires C−H bond breaking and hydrogen spillover would occur likely via physisorption of H atoms. The curvature of the carbon materials is found to have a pronounced influence on the mobility of H atoms. The role ...
TL;DR: The results suggest that reductive coupling of adjacent [2Fe-2S]2+ clusters assembled on IscU provides a general mechanism for the final step in the biosynthesis of [4Fe-4S]3+ clusters, which may provide an effective way to populate appropriately cluster-loaded forms of Isc U for maturation of different types of [Fe-S] proteins.
Abstract: Rapid and quantitative reductive coupling of two [2Fe-2S]2+ clusters to form a single [4Fe-4S]2+ cluster on the homodimeric IscU Fe-S cluster scaffold protein has been demonstrated by UV-visible absorption, Mossbauer, and resonance Raman spectroscopies, using dithionite as the electron donor. Partial reductive coupling was also observed using reduced Isc ferredoxin, which raises the possibility that Isc ferredoxin is the physiological reductant. The results suggest that reductive coupling of adjacent [2Fe-2S]2+ clusters assembled on IscU provides a general mechanism for the final step in the biosynthesis of [4Fe-4S]2+ clusters. The [4Fe-4S]2+ center on IscU can be reduced to a S = 1/2[4Fe-4S]+ cluster (g parallel = 2.06 and g perpendicular = 1.92), but the low midpoint potential (< -570 mV) and instability of the reduced cluster argue against any physiological relevance for the reduced cluster. On exposure to O2, the [4Fe-4S]2+ cluster on IscU degrades via a semistable [2Fe-2S]2+ cluster with properties analogous to those of the [2Fe-2S]2+ center in [2Fe-2S]2+ IscU. It is suggested that the ability of IscU to accommodate either [2Fe-2S]2+ or [4Fe-4S]2+ clusters in response to cellular redox status and/or oxygen levels may provide an effective way to populate appropriately cluster-loaded forms of IscU for maturation of different types of [Fe-S] proteins.
TL;DR: In this article, a two-stage clustering methodology was developed and applied to back trajectories arriving in three European cities: Athens, Madrid and Birmingham, which experience large, moderate and small numbers of daily PM10 episodes, respectively.
TL;DR: This work explored the use of an electronic closing of a silicon caged cluster and variations in its cavity size to facilitate metal-atom encapsulation by means of a systematic doping with transition metal atoms of groups 3, 4, and 5.
Abstract: Silicon clusters mixed with a transition metal atom, MSin, were generated by a double-laser vaporization method, and the electronic and geometric stabilities for the resulting clusters with transition metal encapsulated by silicon were examined experimentally. By means of a systematic doping with transition metal atoms of groups 3, 4, and 5 (M = Sc, Y, Lu, Ti, Zr, Hf, V, Nb, and Ta), followed by changes of charge states, we explored the use of an electronic closing of a silicon caged cluster and variations in its cavity size to facilitate metal-atom encapsulation. Results obtained by mass spectrometry, anion photoelectron spectroscopy, and adsorption reactivity toward H2O show that the neutral cluster doped with a group 4 atom features an electronic and a geometric closing at n = 16. The MSi16 cluster with a group 4 atom undergoes an electronic change in (i) the number of valence electrons when the metal atom is substituted by the neighboring metals with a group 3 or 5 atom and in (ii) atomic radii with t...
TL;DR: In this paper, the authors investigated a new model of AGN heating, in which the intracluster medium is efficiently heated by cosmic-rays, which are injected into the ICM through diffusion or the shredding of the bubbles by Rayleigh-Taylor or Kelvin-Helmholtz instabilities.
Abstract: Recent observations show that the cooling flows in the central regions of galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles are a leading candidate for suppressing cooling, usually via some form of mechanical heating. At the same time, observed X-ray cavities and synchrotron emission point toward a significant non-thermal particle population. Previous studies have focused on the dynamical effects of cosmic-ray pressure support, but none have built successful models in which cosmic-ray heating is significant. Here we investigate a new model of AGN heating, in which the intracluster medium is efficiently heated by cosmic-rays, which are injected into the ICM through diffusion or the shredding of the bubbles by Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal conduction as well. Using numerical simulations, we show that the cooling catastrophe is efficiently suppressed. The cluster quickly relaxes to a quasi-equilibrium state with a highly reduced accretion rate and temperature and density profiles which match observations. Unlike the conduction-only case, no fine-tuning of the Spitzer conduction suppression factor f is needed. The cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within observational bounds. Cosmic ray heating is a very attractive alternative to mechanical heating, and may become particularly compelling if GLAST detects the gamma-ray signature of cosmic-rays in clusters.
TL;DR: In this paper, the authors study the dynamical evolution of globular clusters containing populations of primordial binaries, using their newly updated Monte Carlo cluster evolution code with the inclusion of direct integration of binary scattering interactions.
Abstract: We study the dynamical evolution of globular clusters containing populations of primordial binaries, using our newly updated Monte Carlo cluster evolution code with the inclusion of direct integration of binary scattering interactions. We describe the modifications we have made to the code, as well as improvements we have made to the core Monte Carlo method. We present several test calculations to verify the validity of the new code and perform many comparisons with previous analytical and numerical work in the literature. We simulate the evolution of a large grid of models, with a wide range of initial cluster profiles and with binary fractions ranging from 0 to 1, and compare with observations of Galactic globular clusters. We find that our code yields very good agreement with direct N-body simulations of clusters with primordial binaries, but yields some results that differ significantly from other approximate methods. Notably, the direct integration of binary interactions reduces their energy generation rate relative to the simple recipes used in Paper III and yields smaller core radii. Our results for the structural parameters of clusters during the binary-burning phase are now in the tail of the range of parameters for observed clusters, implying that either clusters are born significantly more or less centrally concentrated than has been previously considered or there are additional physical processes beyond two-body relaxation and binary interactions that affect the structural characteristics of clusters.