Fractal boundaries of vortex pinning clusters in copper-oxide superconductors in magnetic field

The influence of fractal boundaries of a normal phase clusters on the flux creep in the copper-oxide high-temperature superconductors YBCO was investigated. A model of the magnetic-field dependence of the voltage due to flux creep for different transport currents was developed. To describe the experimental curves is proposed to use an exponential-hyperbolic function in which the argument is direct current. Empirical dependence of the fractal dimension of the cluster boundaries on the magnetic field strength was determined. The characteristic values of the magnetic field were found. The index of connectivity of the flux creep paths at the percolation threshold was calculated.