Wohlleben effect in small grains of Bi-based high-temperature superconductors: evidence for intrinsic nature of spontaneous currents

We report on the observation of the Wohlleben effect (WE) in powders of Bi-based high-temperature superconductors consisting of isolated grains with sizes of order 1 mu m In these small grains the Josephson contacts with negative critical current (pi-junctions) generally involved to explain the WE must be able to carry a current density of order j approximate to 10(5)-10(6) A/cm(2) even close to T-c. Such high spontaneous currents can be explained, if such contacts are not ordinary tunneling Josephson junctions but superconductor-normal-metal-superconductor junctions and if the superconducting order parameter has d-wave symmetry.