Metriplectic framework for dissipative magneto-hydrodynamics

The metriplectic framework, which allows for the formulation of an algebraic structure for dissipative systems, is applied to visco-resistive Magneto-Hydrodynamics (MHD), adapting what had already been done for non-ideal Hydrodynamics (HD). The result is obtained by extending the HD symmetric bracket and free energy to include magnetic field dynamics and resistive dissipation. The correct equations of motion are obtained once one of the Casimirs of the Poisson bracket for ideal MHD is identified with the total thermodynamic entropy of the plasma. The metriplectic framework of MHD is shown to be invariant under the Galileo Group. The metriplectic structure also permits us to obtain the asymptotic equilibria toward which the dynamics of the system evolves. This scheme is finally adapted to the two-dimensional incompressible resistive MHD, that is of major use in many applications.     

Fano varieties of cubic fourfolds containing a plane

We prove that the Fano variety of lines of a generic cubic fourfold containing a plane is isomorphic to a moduli space of twisted stable complexes on a K3 surface. On the other hand, we show that the Fano varieties are always birational to moduli spaces of twisted stable coherent sheaves on a K3 surface. The moduli spaces of complexes and of sheaves are related by wall-crossing in the derived category of twisted sheaves on the corresponding K3 surface.     

Information dynamics algorithm for detecting communities in networks

The problem of community detection is relevant in many scientific disciplines, from social science to statistical physics. Given the impact of community detection in many areas, such as psychology and social sciences, we have addressed the issue of modifying existing well performing algorithms by incorporating elements of the domain application fields, i.e. domain-inspired. We have focused on a psychology and social network-inspired approach which may be useful for further strengthening the link between social network studies and mathematics of community detection. Here we introduce a community-detection algorithm derived from the van Dongen’s Markov Cluster algorithm (MCL) method [4] by considering networks’ nodes as agents capable to take decisions. In this framework we have introduced a memory factor to mimic a typical human behavior such as the oblivion effect. The method is based on information diffusion and it includes a non-linear processing phase. We test our method on two classical community benchmark and on computer generated networks with known community structure. Our approach has three important features: the capacity of detecting overlapping communities, the capability of identifying communities from an individual point of view and the fine tuning the community detectability with respect to prior knowledge of the data. Finally we discuss how to use a Shannon entropy measure for parameter estimation in complex networks.     

Assessment of the illumination dependency of Al2O3 and SiO2 rear‐passivated p‐type silicon solar cells

This Letter discusses an important difference between positively charged SiO₂ and negatively charged Al₂O₃ rear‐passivated p‐type Si solar cells: their illumination level dependency. For positively charged SiO₂ rear‐passivated p‐type Si solar cells, a loss in short circuit current (J_SC) and open circuit voltage (V_OC) as a function of illumination level is mainly caused by parasitic shunting and a decrease in surface recombination, respectively. Hence, the relative loss in cell conversion efficiency, J_SC, and V_OC as a function of the illumination level for SiO₂ compared to Al₂O₃ rear‐passivated p‐type Si solar cells has been measured and discussed. Subsequently, an exponential decay fit of the loss in cell efficiency is applied in order to estimate the difference in the energy output for both cell types in three different territories: Belgium (EU), Seattle and Austin (US). The observed trends in the difference in energy output between both cells, as a function of time of the year and region, are as expected and discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Large-area phoswich balloon experiment for hard-X-ray astronomy

Summary We describe a balloon experiment, currently in the marking, devoted to the observation of celestial X-ray sources. The main features of the X-ray telescope are summarized as follows. It operates in the energy band from 20 to 300 keV. Its energy resolution is about 17% at 60 keV. Under the hypothesis of 10⁴ s of observing time, 3 mbar of residual atmospheric pressure and 3σ of statistical significance, the expected sensitivity of the instrument is 2·10⁻⁶ photons/cm² s keV in the (20÷200) keV energy band, corresponding to about 1 milliCrab. Its high sensitivity allows us to detect both time variability in the flux and cyclotron lines in the spectra of X-ray sources. It has a field of view of 3° FWHM and has the possibility of resolving complex fields by using multipitch modulation collimators. With such a configuration its angular resolution is about 10′. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Response of a designed metalloprotein to changes in metal ion coordination, exogenous ligands, and active site volume determined by X-ray crystallography

The de novo protein DF1 is a minimal model for diiron and dimanganese metalloproteins, such as soluble methane monooxygenase. DF1 is a homodimeric four-helix bundle whose dinuclear center is formed by two bridging Glu side chains, two chelating Glu side chains, and two monodentate His ligands. Here, we report the di-Mn(II) and di-Co(II) derivatives of variants of this protein. Together with previously solved structures, 23 crystallographically independent four-helix bundle structures of DF1 variants have been determined, which differ in the bound metal ions and size of the active site cavity. For the di-Mn(II) derivatives, as the size of the cavity increases, the number and polarity of exogenous ligands increases. This collection of structures was analyzed to determine the relationship between protein conformation and the geometry of the active site. The primary mode of backbone movement involves a coordinated tilting and sliding of the first helix in the helix-loop-helix motif. Sliding depends on crystal-packing forces, the steric bulk of a critical residue that determines the dimensions of the active site access cavity, and the intermetal distance. Additionally, a torsional motion of the bridging carboxylates modulates the intermetal distance. This analysis provides a critical evaluation of how conformation, flexibility, and active site accessibility affect the geometry and ligand-binding properties of a metal center. The geometric parameters defining the DF structures were compared to natural diiron proteins; DF proteins have a restricted active site cavity, which may have implications for substrate recognition and chemical stability.     

Iron stains on paper. Can electrophoretic removal become an effective alternative to chemical cleaning?

Abstract

Research in restoration and conservation is directed vs. more sustainable working materials, methods and technologies. Electrophoretic removal, from porous material, of undesired stains due to charged species is theoretically an interesting alternative to chemical cleaning methods, but the lack of specific and comprehensive research work leads to controversial opinions about the efficiency and the needed harmfulness for the treated objects. In this work paper, samples with artificial rust stains were subjected to electrophoretic cleaning treatments in mineral water as electrolyte. Treatments were carried out either in a bath by complete sample immersion between the distanced electrodes or by sample wetting and sandwiching between the electrodes. Evaluation of cleaning efficiency and treatment effects was based on colour change measurements (image analysis of scanned paper samples before and after the treatment and by colorimetric data via spectrophotometric measurements), investigation of morphological changes by SEM observations and folding endurance measurements.