Resistive state of superconducting structures with fractal clusters of a normal phase

The effect of morphologic factors on magnetic flux dynamics and critical currents in percolative superconducting structures is considered. The superconductor contains the fractal clusters of a normal phase, which act as pinning centers. The properties of these clusters are analyzed in the general case of gamma-distribution of their areas. The statistical characteristics of the normal phase clusters are studied, the critical current distribution is derived, and the dependencies of the main statistical parameters on the fractal dimension are found. The effect of fractal clusters of a normal phase on the electric field induced by the motion of the magnetic flux after the vortices have been broken away from pinning centers is considered. The voltage-current characteristics of fractal superconducting structures in a resistive state for an arbitrary fractal dimension are obtained. It is found that the fractality of the boundaries of normal phase clusters intensifies magnetic flux trapping and thereby increases the current-carrying capability of the superconductor.Comment: 15 pages with 8 figures, revtex3, alternative e-mail of author is [email protected]

Dynamics of the magnetic flux trapped in fractal clusters of normal phase in a superconductor

The influence of geometry and morphology of superconducting structure on critical currents and magnetic flux trapping in percolative type-II superconductor is considered. The superconductor contains the clusters of a normal phase, which act as pinning centers. It is found that such clusters have significant fractal properties. The main features of these clusters are studied in detail: the cluster statistics is analyzed; the fractal dimension of their boundary is estimated; the distribution of critical currents is obtained, and its peculiarities are explored. It is examined thoroughly how the finite resolution capacity of the cluster geometrical size measurement affects the estimated value of fractal dimension. The effect of fractal properties of the normal phase clusters on the electric field arising from magnetic flux motion is investigated in the case of an exponential distribution of cluster areas. The voltage-current characteristics of superconductors in the resistive state for an arbitrary fractal dimension are obtained. It is revealed that the fractality of the boundaries of the normal phase clusters intensifies the magnetic flux trapping and thereby raises the critical current of a superconductor.Comment: revtex, 16 pages with 1 table and 5 figures; text and figures are improved; more detailed version with geometric probability analisys of the distribution of entry points into weak links over the perimeter of a normal phase clusters and one additional figure is published in Phys.Rev.B; alternative e-mail of author is [email protected]

Rewriting Conjunctive Queries over Description Logic Knowledge Bases

Abstract. We consider the problems of conjunctive query answering and rewriting for information integration systems in which a Description Logic ontology is used to provide a global view of the data. We present a resolution-based query rewriting algorithm for DL-Lite + ontologies, and use it to show that query answering in this setting is NLogSpacecomplete with respect to data complexity. We also show that our algorithm produces an optimal rewriting when the input ontology is expressed in the language DL-Lite. Finally, we sketch an extended version of the algorithm that would, we are confident, be optimal for several DL languages with data complexity of query answering ranging from LogSpace to PTime-complete.

Computational Space Efficiency and Minimal Model Generation for Guarded Formulae

This paper describes two hyperresolution-based decision procedures for a subfragment of the guarded fragment. The rst procedure is a polynomial space decision procedure which eectively corresponds to polynomial space tableaux-based algorithms without blocking. The second procedure is a minimal model generation procedure which constructs all and only minimal Herbrand models for guarded formulae. This procedure is based on hyperresolution, complement splitting and a model constraint propagation rule. Both procedures have concrete application domains and are relevant for all multi-modal and description logics that can be embedded into the guarded fragment