Characterization of community based-P2P systems and implications for traffic localization

In this paper, we present one of the first and most extensive characterizations of closed community-based P2P systems. Such systems are organic groups of peer-to-peer (P2P) clients, which can be joined only by users belonging to a certain network (e.g., connected to a given Internet Service Provider (ISP)). A number of factors motivate the growth of these communities, such as quality of content, anonymity of transfers, and the potential for better performance that enhances user experience. Our study is conducted in two contrasting environments—a campus network and a national ISP—located in different continents. In both cases, large-scale closed communities have been found to be the predominant P2P systems in use. We shed light both on the factors motivating the growth of such communities, and present results characterizing the extensiveness of their usage, the performance achievable by the systems, and the implications of such communities for network providers. While our findings are interesting in their own right, they also offer important lessons for ongoing research that seeks to localize traffic within ISP boundaries. In particular, our results suggest that (i) in ISPs with heterogeneous access technologies, the performance benefits to users on localizing P2P traffic is largely dependent on the degree of seed-like behavior of peers behind high-bandwidth access technologies; and (ii) while localization can reduce the traffic on Internet peering links, it has the potential to cause a significant increase in traffic on internal links of providers, potentially requiring upgrades of network links.     

Declarative Web services composition using proofs

Over the last decade, Web services composition has become a thriving area of research and development endeavors for application integration and interoperability. Although Web services composition has been heavily investigated, several issues still need to be addressed. In this paper, we mainly discuss two major bottlenecks in the current process of modeling compositions. The first bottleneck is related to the level of expertise required to achieve a composition process. Typical procedural style of modeling, inspired by workflow/business process paradigm, do not provide the required abstractions. Therefore, they fail to support dynamic and self-managed compositions able to adapt to unpredictable changes. The second bottleneck in current service compositions concerns their life cycle and their management, also called their governance. In this context, we propose a declarative proof-based approach to Web service composition. Based on the three stages of pre-composition, abstraction, and composition, our solution provides an easy way to specify functional and non-functional requirements of composite services in a precise and declarative manner. It guides the user through the composition process while allowing detection and recovery of violations at both design and run-time using proofs and planning. Experiment results clearly show the added value of the proof-based solution as a viable strategy to improve the composition process.     

Synthesis and sol-gel polymerization of [Si8O12](OCH3)8

Sol-gel polymerization of [Si₈O₁₂](OCH₃)₈ in an inert solvent under neutral conditions has been shown to yield very high-surface area silica xerogels [P. C. Cagleet al., Mater. Res. Soc. Symp. Proc. 180, 29 (1990)]. Two recent results relevant to this process are described here. First, a practical synthetic route to [Si₈O₁₂](OCH₃)₈ is described that allows multigram quantities to be conveniently prepared. Second, TEM studies are reported that confirm retention of the [Si₈O₁₂] cubic core structure in high-surface area, [Si₈O₁₂](OCH₃)₈ ^? derived xerogels.     

A framework for adaptive execution in grids

Grids offer a dramatic increase in the number of available processing and storing resources that can be delivered to applications. However, efficient job submission and management continue being far from accessible to ordinary scientists and engineers due to their dynamic and complex nature. This paper describes a new Globus based framework that allows an easier and more efficient execution of jobs in a ‘submit and forget’ fashion. The framework automatically performs the steps involved in job submission and also watches over its efficient execution. In order to obtain a reasonable degree of performance, job execution is adapted to dynamic resource conditions and application demands. Adaptation is achieved by supporting automatic application migration following performance degradation, ‘better’ resource discovery, requirement change, owner decision or remote resource failure. The framework is currently functional on any Grid testbed based on Globus because it does not require new system software to be installed in the resources. The paper also includes practical experiences of the behavior of our framework on the TRGP and UCM‐CAB testbeds. Copyright © 2004 John Wiley & Sons, Ltd.     

Engineering On‐Surface Spin Crossover: Spin‐State Switching in a Self‐Assembled Film of Vacuum‐Sublimable Functional Molecule

The realization of spin‐crossover (SCO)‐based applications requires study of the spin‐state switching characteristics of SCO complex molecules within nanostructured environments, especially on surfaces. Except for a very few cases, the SCO of a surface‐bound thin molecular film is either quenched or heavily altered due to: (i) molecule–surface interactions and (ii) differing intermolecular interactions in films relative to the bulk. By fabricating SCO complexes on a weakly interacting surface, the interfacial quenching problem is tackled. However, engineering intermolecular interactions in thin SCO active films is rather difficult. Here, a molecular self‐assembly strategy is proposed to fabricate thin spin‐switchable surface‐bound films with programmable intermolecular interactions. Molecular engineering of the parent complex system [Fe(H₂B(pz)₂)₂(bpy)] (pz = pyrazole, bpy = 2,2′‐bipyridine) with a dodecyl (C₁₂) alkyl chain yields a classical amphiphile‐like functional and vacuum‐sublimable charge‐neutral Fe^II complex, [Fe(H₂B(pz)₂)₂(C₁₂‐bpy)] (C₁₂‐bpy = dodecyl[2,2′‐bipyridine]‐5‐carboxylate). Both the bulk powder and 10 nm thin films sublimed onto either quartz glass or SiO_x surfaces of the complex show comparable spin‐state switching characteristics mediated by similar lamellar bilayer like self‐assembly/molecular interactions. This unprecedented observation augurs well for the development of SCO‐based applications, especially in molecular spintronics.     

Stability of an explicit high‐order spectral element method for acoustics in heterogeneous media based on local element stability criteria

This paper considers the stability of an explicit leapfrog time marching scheme for the simulation of acoustic wave propagation in heterogeneous media with high‐order spectral elements. The global stability criterion is taken as a minimum over local element stability criteria, obtained through the solution of element‐borne eigenvalue problems. First, an explicit stability criterion is obtained for the particular case of a strongly heterogeneous and/or rapidly fluctuating medium using asymptotic analysis. This criterion is only dependent upon the maximum velocity at the vertices of the mesh elements, and not on the velocity at the interior nodes of the high‐order elements. Second, in a more general setting, bounds are derived using statistics of the coefficients of the elemental dispersion matrices. Different bounds are presented, discussed, and compared. Several numerical experiments show the accuracy of the proposed criteria in one‐dimensional test cases as well as in more realistic large‐scale three‐dimensional problems.     

Monitoring Early‐Stage Nanoparticle Assembly in Microdroplets by Optical Spectroscopy and SERS

Microfluidic microdroplets have increasingly found application in biomolecular sensing as well as nanomaterials growth. More recently the synthesis of plasmonic nanostructures in microdroplets has led to surface‐enhanced Raman spectroscopy (SERS)‐based sensing applications. However, the study of nanoassembly in microdroplets has previously been hindered by the lack of on‐chip characterization tools, particularly at early timescales. Enabled by a refractive index matching microdroplet formulation, dark‐field spectroscopy is exploited to directly track the formation of nanometer‐spaced gold nanoparticle assemblies in microdroplets. Measurements in flow provide millisecond time resolution through the assembly process, allowing identification of a regime where dimer formation dominates the dark‐field scattering and SERS. Furthurmore, it is shown that small numbers of nanoparticles can be isolated in microdroplets, paving the way for simple high‐yield assembly, isolation, and sorting of few nanoparticle structures.     

Parallel Dislocation Networks and Cottrell Atmospheres Reduce Thermal Conductivity of PbTe Thermoelectrics

Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for materials with intrinsically low thermal conductivity, such as thermoelectrics, classical models require exceedingly high numbers of dislocations (>10¹² cm^–2) to further impede thermal transport. In this work, a significant reduction in thermal conductivity of Na_0.025Eu_0.03Pb_0.945Te is demonstrated at a moderate dislocation density of 1 × 10¹⁰ cm^–2. Further characteristics of dislocations, including their arrangement, orientation, and local chemistry are shown to be crucial to their phonon-scattering effect and are characterized by correlative microscopy techniques. Electron channeling contrast imaging reveals a uniform distribution of dislocations within individual grains, with parallel lines along four <111> directions. Transmission electron microscopy (TEM) shows the parallel networks are edge-type and share the same Burgers vectors within each group. Atom probe tomography reveals the enrichment of dopant Na at dislocation cores, forming Cottrell atmospheres. The dislocation network is demonstrated to be stable during in situ heating in the TEM. Using the Callaway transport model, it is demonstrated that both parallel arrangement of dislocations and Cottrell atmospheres make dislocations more efficient in phonon scattering. These two mechanisms provide new avenues to lower the thermal conductivity in materials for thermal-insulating applications.     

Second-generation octarellins: two new de novo ({beta}/{alpha})8 polypeptides designed for investigating the influence of {beta}-residue packing on the {alpha}/{beta}-barrel structure stability

The sequence of octarellin I, the first de novo (ß/)₈polypeptide, was revised according to several criteria, amongothers the symmetry of the sequence, ß-residue volumeand hydrophobicity, and charge distribution. These considerationsand the overall conclusions drawn from the first design ledto two new sequences, corresponding to octarellins II and III.Octarellin II retains perfect 8-fold symmetry. Octarellin IIIhas the same sequence as octarellin II, except for the ß-strandswhich exhibit a 4-fold symmetry. The two proteins were producedin Escherichia coli. Infrared and CD spectral analyses of octarellinsII and III reveal a high secondary structure content. Non-denaturinggel electrophoresis, molecular sieve chromatography and analyticalultracentrifugation suggest that both of these second-generationartificial polypeptides exist as a mixture of a monomer anda dimer form. Octarellins II and III are at least 10 times moresoluble than octarellin I. Ureainduced unfolding followed byfluorescence emission suggests that the tryptophan residues,designed to be buried in the (ß/)₈, are indeed packedin the hydrophobic core of both proteins. However, octarellinIII displays a higher stability towards urea denaturation, indicatingthat introducing 4-fold symmetry into the ß-barrelmight be important for stability of the overall folding.