Superconductivity in Cu-Nb with extremely fine structure

Electrodynamic properties of three-dimensional composites CuNb with extremely fine structure and different concentration of components are investigated. The samples have been fabricated from the melt using slow and extremely fast cooling rate. The slowly cooled samples (SCS) are characterized by a fractal niobium grid with a characteristic size of the micron scale immersed into a copper matrix. The fast melt cooling results in the nano-scale composite. It has a rather pure copper matrix in which almost pure isolated nano-inclusions of niobium are distributed. We have found the electromagnetic properties of these two (micron and nano scale) CuNb composites have some qualitative difference. The SCS shows features of superconducting transition, typical for granular superconductors. The superconducting transition in quenched samples (QS) carries the feature of an effective superconducting media which a critical temperature T_c are renormalized to a rather lowered value for the small size of superconducting regions. On the basis of the experimental data we have estimated a coherence length of the effective superconducting media of the QS that occurs close to the Nb droplet diameter. Voltage-current characteristics (VCC) of the QS in magnetic fields parallel and perpendicular to the current show abnormally low slope in some intermediate current region while at lower and higher currents the VCC features are characteristic for a hard superconductor. The opportunity of a relation of the abnormal dissipation in the QS with the Josephson generation is discussed.