AC SUSCEPTIBILITY AND TRANSPORT CRITICAL-CURRENT DENSITY OF POLYCRYSTALLINE C-AXIS-ORIENTED YBA2CU3O7-DELTA FILMS - JOSEPHSON TUNNELING ANDD-WAVE PAIRING

We determined the intergranular critical-current density j(cJ)(T) of t hin films of polycrystalline c-axis-oriented YBa2Cu3O7-delta by measur ing the isothermal ac susceptibility chi(h(ac)) at zero de magnetic fi eld. The temperature dependence of j,, was obtained from the temperatu re-dependent position of the intergrain peak in chi ''(h(ac)). We conf irmed our results by measurements of the transport critical current pe rformed under self-field conditions. The observed temperature dependen ce of the critical current was compared with calculations of the Josep hson critical current across rough interfaces. The experimental data c an be explained by either assuming d-wave symmetry of the superconduct ing order parameter or, alternatively, by assuming s-wave symmetry and unusually strong pair breaking at the grain boundaries. The calculate d critical currents show temperature dependences significantly differe nt from the Ambegaokar-Baratoff formula, and agree well with recent me asurements on single grain boundary junctions.