Granular superconductivity inR1Ba2Cu3O7−δthin films

Abstract

Present methods of fabricating thin films and bulk samples of the high-$T_c$ oxide superconductor $R_1{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ frequently produce a granular morphology with stoichiometric grains separated by weakly superconducting material, Josephson, or proximity effect junctions. In these films, the bulk superconducting properties (transition width, critical current density) are controlled primarily by the strength of coupling between the grains and not by the superconductivity of the grains themselves. We have examined the $I-V$ characteristics of thin-film oxide superconductors as a function of temperature and find evidence for a coherence transition in which isolated grains couple to form a bulk superconductor. The transition manifests itself in the scaling behavior $V\sim {(I-I_c)}^x$ where $x$ changes abruptly at coherence. The magnetotransport is in support of the model.