INFLUENCE OF TWIN BOUNDARIES ON JOSEPHSON-JUNCTIONS BETWEEN HIGH-TEMPERATURE AND CONVENTIONAL SUPERCONDUCTORS

We give a consistent explanation of the c-axis Josephson tunneling exp eriment by Sun and co-workers between YBa2Cu3O6+x and Pb within the d- wave pairing scenario. Using a Ginzburg-Landau formulation, orthorhomb ic deformation and twinning of the crystal lattice are taken into acco unt. In the presence of orthorhombic distortion, symmetry arguments al low a c-axis Josephson coupling between YBa2Cu3O6+x and Pb. However, f or a highly twinned YBa2Cu3O6+x sample, the Josephson coupling is weak ened due to destructive interference effects. On the other hand, we de monstrate that destructive effects due to twinning can be:overcome, if twin boundaries support a state which locally breaks time reversal sy mmetry and leads to a channel which adds constructively to the total J osephson coupling. Properties of the Josephson junctions measured in e xperiment, such as the Fraunhofer pattern and the Fiske resonance, kee p their standard form for such a junction. The existence of a twin bou ndary state with broken time reversal symmetry can directly be tested, for example, by observing vortices with fractional flux quanta on twi n boundaries.