Coarsening of mass-selected Au clusters on amorphous carbon at room temperature

Low-energy cluster beam deposition was used to deposit mass-selected Au_n clusters (n = 4, 6, 13 and 20) on amorphous carbon (a-C) substrates. The resulting samples were stored at room temperature under ambient conditions for time periods up to 32 months to analyze the coarsening behaviour of the clusters. Cluster-size distributions were measured in regular time intervals by transmission electron microscopy (TEM). The TEM experiments show a significant increase of the average cluster size with time analogous to classical surface Ostwald ripening (OR). The coarsening of Au clusters can be well described by steady-state diffusion-limited kinetics. The derived surface mass-transport diffusion coefficients at room temperature range between 1.1 and 3.8·10⁻²⁵ m² s⁻¹ for our samples. A detailed analysis of values suggests that, the rate of the surface OR for mass-selected Au_n clusters increases with the cluster size in the sequence: Au₄ ≈ Au₆ < Au₁₃ < Au₂₀ for the investigated range of Au clusters. Given that the initial, on-surface cluster-size distributions are nominally monodisperse, classical OR with cluster coarsening based only on the Gibbs-Thomson effect cannot explain the observed coarsening. The activation of the coarsening process is rationalized by initial variations of the cluster sizes due to the deposition process itself and/or the interaction of the clusters with the substrate. Moreover, the presence of initial deposited Au clusters as different isomers with slightly different chemical potential on the substrate, may also initiate the coarsening by surface OR. Furthermore, we find that the coarsening is most pronounced for the paucidispersed sample with Au_m (10 ? m ? 20) clusters. A possible explanation of this behaviour is the presence of an initial distribution of different cluster sizes directly after deposition.     

Non critical super strings on world sheets of constant curvature

We consider correlation functions in Neveu-Schwarz string theory coupled to two dimensional gravity. The actionfor the 2D gravity consists of the string induced Liouville action and the Jackiw-Teitelboim action describing pure 2D gravity. Then gravitational dressed dimensions of vertex operators are equal to their bare conformal dimensions. There are two possible interpretations of the model. Considering the 2D dilaton and the Liouville field as additional target space coordinates one gets ad+2-dimensional critical string. In thed-dimensional non critical string picture gravitational fields retain their original meaning and ford=4 one can get a mass spectrum via consistency requirements. In both cases a GSO projection is possible.     

Why Perform Code‐to‐Code Comparisons: A Vacuum Arc Discharge Simulation Case Study

Numerical modeling is increasingly becoming an indispensable tool for investigations in many fields of physics. Such modeling is especially useful in today's big science projects as a tool that can provide predictions and design parameters. The reliability of simulation results is thus essential. Code‐to‐code comparisons can help increase our confidence in simulation results, especially when other verification methods – such as comparison to theoretical models or experimental results – are limited or unavailable. In this paper, we describe a code‐to‐code comparison exercise wherein we compare one‐dimensional vacuum arc discharge simulation results from two independent particle‐in‐cell (PIC) codes. As part of our case study, we define a vacuum arc discharge test problem that can be used by other research groups for further comparison. Early disagreement between the two sets of our results motivated us to re‐examine the underlying methods in our codes. After remedying discrepancies, we observe good agreement in vacuum arc discharge time‐to‐breakdown, as well as in the time evolution of particle and current densities. This exercise demonstrates the usefulness of code‐to‐code comparisons and provides an example case study for the benefit of other research groups who may wish to carry out similar code‐to‐code comparisons (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase determination in interference-based superresolving microscopes through critical frequency analysis

Utilizing the interference of wave fronts of two opposing lenses, 4Pi-confocal and I(5)M microscopy improve the axial resolution of far-field fluorescence microscopy as much as threefold to sevenfold. However, establishing the phase difference of the wave fronts in the sample is a problem yet to be solved. Here we show that the phase difference is encoded in the microscope's transfer of the spatial frequencies that match the distance of the interference peaks. As a result the phase difference is readily extracted through a Fourier transform of the image. Our method is relevant to all microscopes that exploit the interference of counterpropagating waves to improve the axial and the lateral resolution.     

Sulphur and oxygen isotopic characters of dissolved sulphate in groundwater from the Pleistocene aquifer in the southern Jordan Valley (Jericho area,Palestine)

Sulphate and chloride concentrations in the shallow Pleistocene aquifer systems in the lower Jordan valley area indicate a general trend of increasing salinity eastward and southward. This study was conducted in one of the important sub-basins feeding the Pleo–Pleistocene aquifer in the Jericho area in the southern part of the valley using S and O isotopes of dissolved sulphate. The results show that sulphate has mainly two contributions to the groundwater. One is the surface seepage, which is present as a salty leachate form with the positive δ³⁴S_sulphate values of primary gypsum in Lisan and Samara formations, and the second is the upwelling saline water which was in contact with a deep secondary gypsum, aragonites and salty rocks and rose up under heavy abstraction with depleted ³⁴S in sulphate and relatively high sulphate and chloride content. The latest was clearly shown in the Arab Project wells to the east that is undergoing a continuous heavy abstraction. The isotopic signatures of S and O in these wells to the east show that this depleted ³⁴S and highly concentrated sulphate might also indicate a dissolved sulphate originating from pyrite oxidation that results from the interaction with a pyrite-rich aquifer, which can well up with salty water under heavy abstraction and is oxidised in the upper aerobic shallow aquifer.     

The Formation of Pd Nanocrystals from Pd2(dba)3 Microcrystals

As‐grown platelets formed from tris‐(dibenzylideneacetone) dipalladium(0) [Pd₂(dba)₃] precursor in the presence of Pd17 RNA are investigated before and after thermal annealing. Results show that as‐grown platelets are disordered crystals of Pd₂(dba)₃ containing 1?2 nm Pd clusters and platelets grown in the absence of RNA are morphologically and structurally similar to those formed with RNA. The initially formed crystals are so sensitive to environmental variables that the degree of crystallinity can not be determined accurately by electron diffraction. X‐ray crystallography on as‐grown platelets gives a crystal structure consistent with Pd₂(dba)₃, but reveals a composition of ≈Pd_1.07(dba)₃, indicating one Pd atom in Pd₂(dba)₃ is lost from the structure. Both electron beam and thermally induced decomposition of as‐grown Pd₂(dba)₃ platelets having a hexagonal habit on the micrometer scale produces elemental Pd platelets having a hexagonal habit on the nanometer scale. These hexagonal platelets are composed of a partially sparse form of Pd₂(dba)₃ that is initially crystalline but rapidly degrades due to the loss of Pd atoms from organic ligand cages. Once released, Pd atoms aggregate to form Pd clusters, which grow and transform into well‐formed Pd nanocrystals under electron‐beam irradiation or through thermal annealing.     

Traveling Salesman Problem with Clustering

In the original traveling salesman problem, the traveling salesman has the task to find the shortest closed tour through a proposed set of nodes, touching each node exactly once and returning to the initial node at the end. For the sake of the tour length to be minimized, nodes close to each other might not be visited one after the other but separated in the tour. However, for some practical applications, it is useful to group nodes to clusters, such that all nodes of a cluster are visited contiguously. Here we present an approach which leads to an automatic clustering with a clustering parameter governing the sizes of the clusters.