The effect of the fractal clusters of a normal phase, which act as pinning centers, on the dynamics of magnetic flux in percolative type-II superconductor is considered. 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 found that there is the range of fractal dimension where this distribution has anomalous statistical properties, specifically, its dispersion becomes infinite. It is examined how the finite resolution capacity of the cluster geometric size measurement affects the estimated value of fractal dimension. The effect of fractal properties of the normal phase clusters on the electric field arisen from magnetic flux motion is investigated for the cluster area distribution of different kinds. The voltage-current characteristics of fractal superconducting structures in the resistive state are obtained for an arbitrary fractal dimension. 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.

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Electronic structure, magnetic and optical properties of intermetallic compounds R 2 Fe 17 (R = Pr, Gd)

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The effect of Cu-doping on the electronic structure and optical properties of intermetallic compounds (Ho,Er)Ni 5Àx Cu x (x ¼ 0, 1, 2) has been studied. Band structure and interband optical conductivity were investigated theoretically using the LSDA þ U method supplementing local spin density approximation with a correction for strong electron interaction on the 4f shell of rare earth element. Optical properties were studied by the ellipsometry method in the spectral range 0.22-15 mm. It was found that substitution of Cu for Ni atoms in these alloys leads to significant changes in the frequency dependence of the optical conductivity. Both the theoretical calculations and spectroscopic experiments revealed the appearance of a new absorption maximum near 4.5 eV formed by Cu 3d ! Ni 3d interband transitions.