CURRENT TRANSFER AND INITIAL DISSIPATION IN HIGH-T-C SUPERCONDUCTORS

Various aspects of the problem of current transfer in high-T-c superco nductors (HTSs) are reviewed. The spatial inhomogeneities of various t ypes are identified as a primary cause of non-uniformity of both norma l currents and supercurrents in real samples of HTSs. The role these i nhomogeneities play in transport features of the samples is discussed. The case of grain boundaries in polycrystalline samples is elaborated in detail. The local structural and transport properties of isolated grain boundaries are first reviewed and then integrated into the knowl edge of global (macroscopic) charge transport. The paper emphasizes th e common ingredients characterizing the transport in various forms and families of HTS samples in small magnetic fields. The phenomenon of p ercolation is identified as the most obvious one and is shown to domin ate a large number of observations covered by this report. The experim ental results focused on by this report elaborate primarily the proble ms of critical currents, initial dissipation and current-voltage chara cteristics, penetration depth, resistive and metal-insulator transitio n, resistance noise and magneto-optical studies of current paths. Vari ous models for current transfer (disordered bonds, brick wall and rail way switch) are also reviewed and discussed.