Fermi Surface Nesting and the Origin of the Charge Density Wave in NbSe2_2

We use highly accurate density functional calculations to study the band structure and Fermi surfaces of NbSe2. We calculate the real part of the non-interacting susceptibility, Re chi_0(q), which is the relevant quantity for a charge density wave (CDW) instability and the imaginary part, Im chi_0(q), which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in Re chi_0(q) near the CDW wave vector, but that no such peaks are visible in Im chi_0(q), definitively eliminating FS nesting as a factor in CDW formation. Because the peak in Re chi_0(q) is broad and shallow, it is unlikely to be the direct cause of the CDW instability. We briefly address the possibility that electron-electron interactions (local field effects) produce additional structure in the total (renormalized) susceptibility, and we discuss the role of electron-ion matrix elements.Comment: Replacement of Table II values, minor changes to tex

{P2PPerf}: a framework for simulating and optimizing peer-to-peer distributed computing applications

International audiencePeer-to-Peer architectures are more and more studied by distributed computing community. Indeed, this type of architecture makes possible to inherit few properties, in particular the absence of centralized topology, fault-tolerance and dynamic reorganization of the network. However, the complexity of these networks is increased and the acceleration of the distributed applications is not ensured. That's why it is necessary to predict the performances as soon as possible in design and development phases, to bypass bottlenecks and correct program blocks which slow down the execution time. In this context, we propose P2PPerf: a simulation tool of which objective is to predict performances and execution times of a distributed application before its finalization. P2PPerf has been tested on JNGI: a P2P distributed computing application using the JXTA platfor

MODEL-K for prototyping and strategic reasoning at the knowledge level

To close the gap between knowledge level and symbol level, the MODEL-K language allows to specify KADS conceptual models and to refine them to operational systems. Since both activities may be arbitrarily interleaved, early prototyping is supported at the highest level. Systems written in MODEL-K contain their conceptual model, making them more transparent, easier to communicate to the expert, to explain to the user, and to maintain by the knowledge engineer. The strategy layer of KADS is supposed to control and possibly repair the activities being modeled by the lower layers. MODEL-K views this kind of strategic reasoning as a meta-activity. In the REFLECT project, we came to view meta-activities like resource-management or competence assessment as ordinary problem solving methods, that in turn can be described using KADS. Correspondingly, we extended MODEL-K to model and operationalize such meta-activities. In particular, the lower three layers and the system they mod..