117,583 research outputs found
Surface theory of melting
No full textWe demonstrate that melting is a surface initiated process. The surface becomes unstable before the bulk and the process of melting consists in the unstable surface that proceeds into the otherwise stable bul
The Fractal Structure of the Universe: Correlations of Galaxies and Clusters and the Average Mass Density
No full textStability and melting of simple ionic systems
No full textA melting criterion for binary ionic compounds is formulated in terms of an instability due to the overlap between the larger ions (generally anions). This model provides a simple explanation of the linear behavior of the ratio of the ionic radii versus the square root of the melting temperature observed for alkali halides and pointed out here for the first time. It is then shown that the variation of the melting temperature with pressure is mainly due to the change of the force constants between the ions and that covalent or polarization effects are essential for the stability of binary superionic conductors like AgI
Nonadiabatic breakdown and pairing in high-Tc compounds
No full textThe electron-phonon interaction plays a fundamental role in the superconducting and normal-state properties of all the high-Tc materials, from cuprates to fullerenes. Another common element of these compounds is in addition the extremely small Fermi energy EF, which is comparable with the range ωph of the phonon frequencies. In such a situation the adiabatic principle ωph/EF⪡1, on which the standard theory of the electron-phonon interaction and of the superconductivity relies, breaks down. In this contribution we discuss the physical consequences of the breakdown of the adiabatic assumption, with special interest on the superconducting properties. We review the microscopic derivation of the nonadiabatic theory of the electron-phonon coupling which explicitly takes into account higher-order electron-phonon scattering not included in the conventional picture. Within this context we discuss also the role of the repulsive electron-electron correlation and the specific phenomenology of cuprates and fullerides
Assortative model for social networks
No full textAn assortative graph growth model was developed in order to describe the behavior of social networks using the case study of the preprint archive at cul.arxiv.org. The model allowed to weight the addition of new nodes and addition of new edges in the microscopial behavior of the network. It was observed that the assortativity can effect the topology of the network significantly. It was also observed that most of the features of the real assortative networks can be reproduced using simulation
Statistical properties of dislocation mutual interactions
No full textWe compute the probability distribution density of mutual forces between edge and screw dislocation assemblies. The distribution is directly related to the internal stress distribution and can be revealed experimentally from x-ray profiles. Using numerical simulations, we compare the tail of the probability distribution for homogeneous and fractal dislocation arrangements
NONADIABATIC ELECTRON-PHONON EFFECTS IN LOW CARRIER DENSITY SUPERCONDUCTORS
No full textDifferent families of unconventional superconductors present a low charge carrier density as a common trait, suggesting that the low charge density can be at the basis of a unifying picture for different superconductors. In the past years we have suggested that the electron–phonon interaction can be responsible for a high-Tc superconducting pairing in a nonadiabatic regime, where nonadiabatic effects are triggered on by the small electronic Fermi energy associated with the low charge density character. A coherent picture of such a framework requires however reconciling the low charge density and the small Fermi energy with a finite metallic character (sizable density of states and large Fermi surfaces). In this paper we investigate the peculiar conditions which are needed to be encountered in order to fulfill these requirements. We discuss the specific case of fullerenes, cuprates and MgB2 alloys by analyzing their specific structural and electronic properties The comparison between these materials and simple instructive models permits to underline the different routes to reconcile these characteristics in different compounds. In cuprates and fullerenes the interplay between small Fermi energies and large Fermi surface is strictly connected with strong electronic correlation effects. A comprehensive understanding of these issues can be useful to the future search for new nonadiabatic high-Tc materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Food-web topology: universal scaling in food-web structure? (reply)
No full textAlthough Camacho and Arenas raise potentially interesting points, we believe that some of their arguments are flawed or undermined by poor statistics, and therefore that they do not invalidate our result
Fixed scale transformation for fracture growth processes governed by vectorial fields
No full textWe use the Fixed Scale Transformation (FST) approach to study the problem of fractal growth in fracture patterns generated by using the Born Model. The application of the method to this model is very complex because of the vectorial nature of the system considered. In particular, the implementation of this scheme requires a careful choice of the fracture path and the identification of the appropriate way to take into account screening effects. The good agreements of our results with computer simulations shows the validity and flexibility of the FST method which represents a general theoretical approach for the study of fractal patterns evolution. © 1995
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