ac susceptibility and transport critical-current density of polycrystalline c-axis-orientedYBa2Cu3O7−δnfilms:fJosephson tunneling and d-wave pairing

We determined the intergranular critical-current density ${\mathrm{j}}_{\mathrm{cJ}}$(T) of thin films of polycrystalline c-axis-oriented ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ by measuring the isothermal ac susceptibility χ(${\mathrm{h}}_{\mathrm{ac}}$) at zero dc magnetic field. The temperature dependence of ${\mathrm{j}}_{\mathrm{cJ}}$ was obtained from the temperature-dependent position of the intergrain peak in ${\mathrm{\chi }}^{|\mathrm{I}\mathrm{J}}$(${\mathrm{h}}_{\mathrm{ac}}$). We confirmed our results by measurements of the transport critical current performed under self-field conditions. The observed temperature dependence of the critical current was compared with calculations of the Josephson critical current across rough interfaces. The experimental data can be explained by either assuming d-wave symmetry of the superconducting order parameter or, alternatively, by assuming s-wave symmetry and unusually strong pair breaking at the grain boundaries. The calculated critical currents show temperature dependences significantly different from the Ambegaokar-Baratoff formula, and agree well with recent measurements on single grain boundary junctions.