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 μm. In these small grains the Josephson contacts with negative critical current (π-junctions) generally involved to explain the WE must be able to carry a current density of order j ≈ 105 − 106 A/cm2, even close to Tc. 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.