  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.
