Critical currents and vortex dynamics in percolative superconductors   containing fractal clusters of a normal phase
  The effect of fractal clusters on magnetic and transport properties of percolative superconductors is considered. The superconductor contains percolative superconducting cluster, carrying a transport current, as well as clusters of a normal phase, which act as pinning centers. A prototype of such a structure is a high-temperature superconducting wire. It is found that normal-phase clusters can have fractal features that affect the vortex dynamics. The fractal dimension of the normal-phase cluster boundary in YBCO films is estimated. Depinning and transport of vortices in fractal superconducting structures are investigated. The current-voltage characteristics of superconductors containing fractal clusters are obtained. Dependencies of free vortex density on the fractal dimension as well as the resistance on the transport current are studied. It is found that the fractality of the cluster boundary intensifies pinning. This feature enables the current-carrying capability of a superconductor to be enhanced without changing of its chemical composition. 
Yuriy I. Kuzmin