  The effect of fractal normal-phase clusters on vortex dynamics in a
percolative superconductor is considered. The superconductor contains
percolative superconducting cluster carrying a transport current and clusters
of a normal phase, acting as pinning centers. A prototype of such a structure
is YBCO film, containing clusters of columnar defects, as well as the BSCCO/Ag
sheathed tape, which is of practical interest for wire fabrication. Transition
of the superconductor into a resistive state corresponds to the percolation
transition from a pinned vortex state to a resistive state when the vortices
are free to move. The dependencies of the free vortex density on the fractal
dimension of the cluster boundary as well as the resistance on the transport
current are obtained. It is revealed that a mixed state of the vortex glass
type is realized in the superconducting system involved. The current-voltage
characteristics of superconductors containing fractal clusters are obtained and
their features are studied.
