Increasing Cluster Size Research Articles

Bayesian evaluation of informative hypotheses in cluster-randomized trials

Researchers often have informative hypotheses in mind when comparing means across treatment groups, such as H1 : μA < μB < μC and H2 : μB < μA < μC, and want to compare these hypotheses to each other directly. This can be done by means of Bayesian inference. This article discusses the disadvantages of the frequentist approach to null hypothesis testing and the advantages of the Bayesian approach. It demonstrates how to use the Bayesian approach to hypothesis testing in the setting of cluster-randomized trials. The data from a school-based smoking prevention intervention with four treatment groups are used to illustrate the Bayesian approach. The main advantage of the Bayesian approach is that it provides a degree of evidence from the collected data in favor of an informative hypothesis. Furthermore, a simulation study was conducted to investigate how Bayes factors behave with cluster-randomized trials. The results from the simulation study showed that the Bayes factor increases with increasing number of clusters, cluster size, and effect size, and decreases with increasing intraclass correlation coefficient. The effect of the number of clusters is stronger than the effect of cluster size. With a small number of clusters, the effect of increasing cluster size may be weak, especially when the intraclass correlation coefficient is large. In conclusion, the study showed that the Bayes factor is affected by sample size and intraclass correlation similarly to how these parameters affect statistical power in the frequentist approach of null hypothesis significance testing. Bayesian evaluation may be used as an alternative to null hypotheses testing.

Photoelectron velocity map imaging spectroscopic and theoretical study of heteronuclear vanadium-nickel carbonyl anions VNi(CO) n - (n = 2-6).

Mass-selected heteronuclear vanadium-nickel carbonyl anions VNi(CO) n - (n = 2-6) were investigated by photoelectron velocity-map imaging spectroscopy and quantum chemical calculations to obtain their chemical bonding and intrinsic electronic structure in the gas phase. The calculated energies (adiabatic detachment energies)/vertical detachment energies (VDEs) match well with experimental values: 1.30/1.49, 1.66/1.95, 2.22/2.48, 2.70/2.89, and 2.95/3.15 eV. The VDE value of VNi(CO) n - increases with an increase of cluster size, implying that the negative electron is stabilized upon the bonding of CO molecules. VNi(CO)2 - consists of one bridging carbonyl and one terminal carbonyl, whose feature is different from MNi(CO)2 - (M = Sc, Y, La, and Ce) with the involvement of one side-on-bonded carbonyl and one terminal CO carbonyl. The building block composed of three bridging carbonyls is favored for VNi(CO)3 -, the structure of which persists up to n = 6. The additional CO ligands are preferentially coordinated in the terminal mode to the Ni atom at n = 4 and then to the V atom at n = 5 and 6. The results obtained in this work would provide a molecular-level understanding about chemisorbed CO molecules on alloy surfaces/interfaces, which is important to understand CO molecule activation processes.

Simulations of NaCl Aggregation from Solution: Solvent Determines Topography of Free Energy Landscape.

The partition-enabled analysis of cluster histograms (PEACH) method is used to calculate the free energy surface of NaCl aggregation using cluster statistics from MD simulations of small systems (40-90 ions plus solvent) in four solvents. In all cases (pure methanol, pure water, and two methanol/water mixtures) NaCl clusters show a transition from amorphous to rocksalt structure with increasing cluster size. The crossover sizes, and the apparent kinetic barrier to ordering, increase with increasing water content. Implications for the proposed two-step mechanism of NaCl crystal nucleation (in which the ordered structure emerges from a large disordered cluster), and how this mechanism might depend on solvent and on degree of supersaturation, are discussed. In pure water, nonideal crowding effects that promote clustering are identified from systematic concentration-dependent deviations between simulation results and the PEACH model fit. In contrast, the ability of PEACH to fit aggregation statistics in mixed solvents is consistent with negligible interactions between ions in different clusters. © 2018 Wiley Periodicals, Inc.

Formation of positive and negative clusters of gold atoms inside helium nanodroplets close to zero K

We report the mass spectrometric detection of positive and negative cluster ions of gold atoms formed by the pickup of gold by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. Both singly and doubly charged positive cluster ions were observed. Ionization is initiated by the helium ions He+ and He*−, metastable helium atoms He* as well as electron attachment. We focus on the size distributions of the ions, electron energy dependence of ion yield, patterns of stability and the presence of “magic” clusters. As reported previously by others using a variety of other methodologies, a clear oscillation in the intensity of (Au)n+/- is seen, especially for n <21, with odd-numbered clusters being more intense than even-numbered clusters. The distributions peak early around n = 8. There is a suggestion of magic numbers at n = 35 and 58. Efficiencies of formation of the gold cluster anions were monitored as a function of the energy of the electrons; the measured intensity distributions exhibit a shift to higher energies with increasing cluster size that is attributed to the need to evaporate helium from the clusters for their observation in the nascent state.

Analysis of Packet Throughput in Small Cell Networks Under Clustered Dynamic TDD

Small cell networks under dynamic time-division duplex (D-TDD) transmission have emerged as a promising solution to accommodate the varied uplink (UL) and downlink (DL) traffic in next generation cellular mobile communication networks. By allowing each cell to individually configure its communication direction, D-TDD allocates resources to accommodate whichever transmission direction needs it most. However, with unaligned transmissions, the interference increases and limits the performance of mean packet throughput (MPT). In this paper, we study the small cell networks under D-TDD with cell clustering being the interference mitigation technique (clustered D-TDD). By leveraging stochastic geometry and queuing theory, we develop an analytical framework that captures both spatial and temporal randomness. We study the MPT whose analytical expression is verified via simulation, and based on the analysis, we explore the impact from different network and service parameters. In particular, numerical results show that there is an optimal cluster size for DL MPT, while UL MPT always benefits from increasing cluster size. By grouping cells into clusters, the clustered D-TDD can provide the flexible service compared with static time-division duplex (S-TDD), and provide significant improvement over a traditional D-TDD in terms of UL MPT at a small cost of DL MPT.

Inhomogeneous charge distribution across gold nanoclusters measured by scattered low energy alkali ions

The neutralization of low energy Na$^+$ and Li$^+$ ions scattered from Au nanoclusters formed by deposition onto oxide surfaces decreases as the cluster size increases. An explanation for this behavior is provided here, which is based on the notion that the atoms in the clusters are not uniformly charged, but that the edge atoms are positively charged while the center atoms are nearly neutral. This leads to upward pointing dipoles at the edge atoms that increase the neutralization probability of alkali ions scattered from those atoms. As the clusters increase in size, the number of edge atoms relative to the number of center atoms decreases, so that that the average neutralization also decreases. Calculations employing this model are compared to experimental data and indicate good agreement if the strengths of the dipoles at the edge atoms are assumed to decrease with cluster size. This model also explains differences in the neutralization probabilities of scattered Na$^+$ and Li$^+$.

Hydrogen bubble nucleation by self-clustering: density functional theory and statistical model studies using tungsten as a model system

Low-energy, high-flux hydrogen irradiation is known to induce bubble formation in tungsten, but its atomistic mechanisms remain little understood. Using first-principles calculations and statistical models, we studied the self-clustering behaviour of hydrogen in tungsten. Unlike previous speculations that the hydrogen self-clusters are energetically unstable owing to the general repulsion between two hydrogen atoms, we found that 2D platelet-like hydrogen self-clusters could form at high hydrogen concentrations. The attractive binding energy of the hydrogen self-cluster becomes larger as the cluster size increases and plateaus at 0.38 eV/H around size of 40. We found that hydrogen atoms would form 2D platelet-like structures along planes. These hydrogen self-clustering behaviours can be quantitatively understood by the competition between long-ranged elastic attraction and local electronic repulsion among hydrogens. Further analysis showed hydrogen self-clusters to be kinetically feasible and thermodynamically stable above a critical hydrogen concentration. Based on this critical hydrogen concentration, we predicted the hydrogen irradiation condition required for the formation of hydrogen self-clusters. Our predictions showed excellent agreement with the experimental results of hydrogen bubble formation in tungsten exposed to low-energy hydrogen irradiation. Finally, we proposed a possible mechanism for the hydrogen bubble nucleation via hydrogen self-clustering. This work provides mechanistic insights and quantitative models towards understanding of plasma-induced hydrogen bubble formation in plasma-facing tungsten.

Electronic Structure and Spin Transport Properties of a New Class of Semiconductor Surface-Confined One-Dimensional Half-Metallic [Eu-(CnHn–2)]N (n = 7–9) Sandwich Compounds and Molecular Wires: First Principle Studies

Transition-metal atom/π-conjugated ring sandwich compounds are promising candidates for application in molecular spintronics. However, a great challenge that has significantly restrained the practical application of these sandwich compounds is their fabrication on a well-characterized solid-state substrate in a controllable manner. In this work, we suggested a two-step self-assemble way to fabricate the Eu-CnHn–2 compounds on the hydrogen-terminated Si(100) surface and theoretically studied the geometric structure and electronic and magnetic properties. Theoretical results indicate that the silicon surface is an ideal substrate to support such kind of metal atom-encapsulated sandwich compounds as the lattice distance of silicon (100) surface is close to the inter-ring distances of freestanding gas-phase sandwich compounds. On the basis of the spin-polarized density functional theory calculations and ab initio molecular dynamics simulations, we find that the silicon surface-supported Si-[EuCh]N, Si-[EuCOT]N, and Si-[EuCnt]N sandwich compounds all process a ferromagnetic ground state. Moreover, the cycloheptatrienyl (Ch) and cyclononatetraenyl (Cnt) Eu sandwich compounds show half-metallic properties. The calculation of electron/spin transport properties using the nonequilibrium Green’s-function method confirms that the Ch Eu sandwich compounds are excellent spin filters, and the spin filter efficiency (SFE) is independent of the cluster size (N), whereas the SFE of Si-[EuCOT]N decreases rapidly with the increase of cluster size. The perfect half-metallic properties of these surface-supported sandwich compounds are promising for future application in spin devices. The present work suggests a way to fabricate the half-metallic sandwich compounds on a semiconductor silicon surface.

Variation of optical spectra of water clusters with size from many-body Green's function theory.

Water clusters are an important species in the environment and atmosphere and take part in various chemical and biological reactions. How their optical properties vary with size is still an open question. Using the GW method and Bethe-Salpeter equation within the ab initio many-body Green's function theory, we study the electronic excitations in a series of water clusters (H2O)n with n = 1-48. We find that their absorption peaks blueshift with increasing cluster size due to the reducing electron-hole binding energy which arises from the enhanced electronic screening and gradually delocalized excitonic spatial distribution. The position of the first absorption peak has a close relation to the average number of hydrogen bonds per molecule. Off-diagonal matrix elements of the self-energy operator have pronounced effects on the unoccupied electronic levels and optical absorption for small clusters with n ≤ 10 when using density functional theory as the starting point for GW calculations. Although the optical absorption is predominated by delocalized excitons, highly localized excitons on a single water molecule are always present on the cluster surface in the vicinity of the absorption edge. These localized excitons may facilitate the photodissociation of water molecules. This can provide inspiration on the excited-state dynamics and photolysis in water clusters.

Cluster truncated Wigner approximation in strongly interacting systems

We present a general method by which linear quantum Hamiltonian dynamics with exponentially many degrees of freedom is replaced by approximate classical nonlinear dynamics with the number of degrees of freedom (phase space dimensionality) scaling polynomially in the system size. This method is based on generalization of the truncated Wigner approximation (TWA) to a higher dimensional phase space, where phase space variables are associated with a complete set of quantum operators spanning finite size clusters. The method becomes asymptotically exact with increasing cluster size. The crucial feature of TWA is fluctuating initial conditions, which we approximate by a Gaussian distribution. We show that such fluctuations dramatically increase accuracy of TWA over traditional cluster mean-field approximations. In this way we can treat on equal footing quantum and thermal fluctuations as well as compute entanglement and various equal and non-equal time correlation functions. The main limitation of the method is exponential scaling of the phase space dimensionality with the cluster size, which can be significantly reduced by using the language of Schwinger bosons and can likely be further reduced by truncating the local Hilbert space variables. We demonstrate the power of this method analyzing dynamics in various spin chains with and without disorder and show that we can capture such phenomena as long time hydrodynamic relaxation, many-body localization and the ballistic spread of entanglement.

Quantification of endogenous and exogenous protein expressions of Na,K-ATPase with super-resolution PALM/STORM imaging.

Transient transfection of fluorescent fusion proteins is a key enabling technology in fluorescent microscopy to spatio-temporally map cellular protein distributions. Transient transfection of proteins may however bypass normal regulation of expression, leading to overexpression artefacts like misallocations and excess amounts. In this study we investigate the use of STORM and PALM microscopy to quantitatively monitor endogenous and exogenous protein expression. Through incorporation of an N-terminal hemagglutinin epitope to a mMaple3 fused Na,K-ATPase (α1 isoform), we analyze the spatial and quantitative changes of plasma membrane Na,K-ATPase localization during competitive transient expression. Quantification of plasma membrane protein density revealed a time dependent increase of Na,K-ATPase, but no increase in size of protein clusters. Results show that after 41h transfection, the total plasma membrane density of Na,K-ATPase increased by 63% while the endogenous contribution was reduced by 16%.

Nonrotating Convective Self-Aggregation in a Limited Area AGCM.

We present nonrotating simulations with the Goddard Earth Observing System (GEOS) atmospheric general circulation model (AGCM) in a square limited area domain over uniform sea surface temperature. As in previous studies, convection spontaneously aggregates into humid clusters, driven by a combination of radiative and moisture‐convective feedbacks. The aggregation is qualitatively independent of resolution, with horizontal grid spacing from 3 to 110 km, with both explicit and parameterized deep convection. A budget for the spatial variance of column moist static energy suggests that longwave radiative and surface flux feedbacks help establish aggregation, while the shortwave feedback contributes to its maintenance. Mechanism‐denial experiments confirm that aggregation does not occur without interactive longwave radiation. Ice cloud radiative effects help support the humid convecting regions but are not essential for aggregation, while liquid clouds have a negligible effect. Removing the dependence of parameterized convection on tropospheric humidity reduces the intensity of aggregation but does not prevent the formation of dry regions. In domain sizes less than (5,000 km)2, the aggregation forms a single cluster, while larger domains develop multiple clusters. Larger domains initialized with a single large cluster are unable to maintain them, suggesting an upper size limit. Surface wind speed increases with domain size, implying that maintenance of the boundary layer winds may limit cluster size. As cluster size increases, large boundary layer temperature anomalies develop to maintain the surface pressure gradient, leading to an increase in the depth of parameterized convective heating and an increase in gross moist stability.

Investigation of nucleation kinetics in H2SO4 vapor through modeling of gas phase kinetics coupled with particle dynamics.

We have developed a new model utilizing our existing kinetic gas phase models to simulate experimental particle size distributions emerging in dry supersaturated H2SO4 vapor homogeneously produced by rapid oxidation of SO2 through stabilized Criegee-Intermediates from 2-butene ozonolysis. We use a sectional method for simulating the particle dynamics. The particle treatment in the model is based on first principles and takes into account the transition from the kinetic to the diffusion-limited regime. It captures the temporal evolution of size distributions at the end of the ozonolysis experiment well, noting a slight underrepresentation of coagulation effects for larger particle sizes. The model correctly predicts the shape and the modes of the experimentally observed particle size distributions. The predicted modes show an extremely high sensitivity to the H2SO4 evaporation rates of the initially formed H2SO4 clusters (dimer to pentamer), which were arbitrarily restricted to decrease exponentially with increasing cluster size. In future, the analysis presented in this work can be extended to allow a direct validation of quantum chemically predicted stabilities of small H2SO4 clusters, which are believed to initiate a significant fraction of atmospheric new particle formation events. We discuss the prospects and possible limitations of the here presented approach.

Competing Effects of Indirect Protection and Clustering on the Power of Cluster-Randomized Controlled Vaccine Trials.

Power considerations for trials evaluating vaccines against infectious diseases are complicated by indirect protective effects of vaccination. While cluster-randomized controlled trials (cRCTs) are less statistically efficient than individually randomized controlled trials (iRCTs), a cRCT's ability to measure direct and indirect vaccine effects may mitigate the loss of efficiency due to clustering. Within cRCTs, the number and size of clusters affects 3 determinants of power: the effect size being measured, disease incidence, and intracluster correlation. We simulated trials conducted in a collection of small communities to assess how indirect protection and clustering affected the power of cRCTs and iRCTs during an emerging epidemic. Across diverse parameters, we found that within the same trial population, cRCTs were never more powerful than iRCTs, although the difference can be small. We also identified 2 effects that attenuated the loss of cRCT power traditionally associated with increased cluster size. First, if enrollment of fewer, larger clusters was performed to achieve higher vaccine coverage within vaccinated communities, this increased the effect to be measured and, consequently, power. Second, the greater rate of imported transmission in larger communities may increase the attack rate and similarly mitigate loss of power relative to a trial in many, smaller communities.

Geometries, stabilities and electronic properties of bimetallic AlnPdm (n = 1–10, m = 1,2) clusters

The geometries, stabilities and electronic properties of Al[Formula: see text]Pd[Formula: see text] (n = 1–10, m = 1, 2) have been systematically investigated by using the DFT method at B3PW91/GENECP level. The optimized results indicate that the lowest-energy structures of Al[Formula: see text]Pd clusters prefer to form three-dimensional (3D) structures and the Pd atom occupies a peripheral position of Al[Formula: see text]Pd clusters. The most stable Al[Formula: see text]Pd2 clusters can be obtained by adding one Al atom to the most stable structure of Al[Formula: see text]Pd2 clusters except for n = 7 and 10. The two Pd atoms are found to occupy the exclusive surface sites. The analysis of stabilities reveals that Al3Pd[Formula: see text] and Al7Pd[Formula: see text] clusters are more stable than their neighbors. The doping of Pd atoms enhances the stabilities of aluminum clusters. The charges always transfer from Al atoms to Pd atoms in Al[Formula: see text]Pd[Formula: see text] clusters. There exists strong spd orbital hybridization between Pd and Al. The results of polarizability imply that the nucleus and electron cloud of these clusters are easily affected by the external field and the nonlinear optical effect of Al[Formula: see text]Pd and Al[Formula: see text]Pd2 clusters is enhanced with the increase of cluster size.

Nature of bonding and cooperativity in linear DMSO clusters: A DFT, AIM and NCI analysis

This study aims to cast light on the nature of interactions and cooperativity that exists in linear dimethyl sulfoxide (DMSO) clusters using dispersion corrected density functional theory. In the linear DMSO, DMSO molecules in the middle of the clusters are bound strongly than at the terminal. The plot of the total binding energy of the clusters vs the cluster size and mean polarizabilities vs cluster size shows an excellent linearity demonstrating the presence of cooperativity effect. The computed incremental binding energy of the clusters remains nearly constant, implying that DMSO addition at the terminal site can happen to form an infinite chain. In the linear clusters, two σ-hole at the terminal DMSO molecules were found and the value on it was found to increase with the increase in cluster size. The quantum theory of atoms in molecules topography shows the existence of hydrogen and SO⋯S type in linear tetramer and larger clusters. In the dimer and trimer SO⋯OS type of interaction exists. In 2D non-covalent interactions plot, additional peaks in the regions which contribute to the stabilization of the clusters were observed and it splits in the trimer and intensifies in the larger clusters. In the trimer and larger clusters in addition to the blue patches due to hydrogen bonds, additional, light blue patches were seen between the hydrogen atom of the methyl groups and the sulphur atom of the nearby DMSO molecule. Thus, in addition to the strong H-bonds, strong electrostatic interactions between the sulphur atom and methyl hydrogens exists in the linear clusters.

VUV photon emission from Ne clusters of varying sizes following photon and photoelectron excitations

Vacuum ultraviolet (VUV) fluorescence emission from neon clusters of different sizes is investigated after excitation with photons of energies between 34 and 49 eV, i.e. near and far below the Ne 2s-electron photoionization threshold. Undispersed VUV fluorescence (<120 nm) excitation functions in the Ne 2s-regime display a series of distinct features attributed to 2s → np resonant cluster excitations with subsequent cluster-specific decays. Features connected with resonant interatomic Coulombic decay are visible for all cluster sizes. For larger clusters they appear to be less prominent due to additional emissions. This emission has a threshold energy of 35.8 eV and increases with exciting-photon energy. It also increases with increasing cluster size and is interpreted as being caused by inelastically scattered 2p-photoelectrons (photoelectron impact induced luminescence).

Studying effect of carrier fluid viscosity in magnetite based ferrofluids using optical tweezers

Ferrofluids with varying viscosities of carrier fluids have been prepared with magnetite (Fe3O4) nanoparticles. The nanoparticles were synthesized by chemical co-precipitation and characterized using X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). They were found to be nearly spherical in shape with an almost uniform size of 13nm. The superparamagnetic nature of the water based ferrofluids at room temperature was established by SQUID magnetometry. Dynamic light scattering (DLS) was carried out to establish the size of the nanoparticle clusters in the ferrofluids synthesized. The results indicate an increase in cluster size with increase in carrier fluid viscosity. This is supported by results from Raman Spectroscopy. A further attempt to characterise these ferrofluids was made by studying the behaviour of well characterised non-magnetic micron sized probes that are optically trapped while suspended in the ferrofluid. An increase in carrier fluid viscosity results in a decrease in corner frequency when only the carrier fluid is used as the suspending medium. When the magnetic component is also present the corner frequency is higher than with just the carrier fluid. This relative increase happens at all laser powers at the trapping plane. This trend is also found to be independent of the size and material of the probe particle. Comparisons of various parameters that influence optical trapping lead us to believe that the enhancement could be due to a directed motion of the magnetic clusters in the presence of an optical trap.

Electronic, nonlinear optical and thermodynamic properties of (CdS)n clusters: A first principle study

Under the formalism of density functional theory, here we present a detailed study on the electronic, nonlinear and thermodynamic properties of (CdS)n (n = 1–8) cluster. All the structures are found to be energetically stable. The nonlinear optical properties including the polarizability, first and second order hyperpolarizability have been calculated and the results are analyzed with increase in cluster size. The Cd7S7 cluster shows exceptional behavior with respect to its other counterparts. The remarkable behavior in the nonlinear optical properties of these clusters makes it feasible as a potential candidate in the field of nonlinear optics. The optical spectra of these clusters produce absorption peaks in the UV and visible region, which may be a reason behind considerable nonlinear optical responses as shown by the clusters. Apart from these, it has been noticed that thermodynamic properties are significantly enhanced with the increase in temperature and size of the cluster.

Electron scattering in large water clusters from photoelectron imaging with high harmonic radiation.

Low-energy electron scattering in water clusters (H2O)n with average cluster sizes of n < 700 is investigated by angle-resolved photoelectron spectroscopy using high harmonic radiation at photon energies of 14.0, 20.3, and 26.5 eV for ionization from the three outermost valence orbitals. The measurements probe the evolution of the photoelectron anisotropy parameter β as a function of cluster size. A remarkably steep decrease of β with increasing cluster size is observed, which for the largest clusters reaches liquid bulk values. Detailed electron scattering calculations reveal that neither gas nor condensed phase scattering can explain the cluster data. Qualitative agreement between experiment and simulations is obtained with scattering calculations that treat cluster scattering as an intermediate case between gas and condensed phase scattering.