Well-defined zigzag-shaped ramp-type Josephson junctions between YBa 2 Cu 3 O 7 and Nb have been studied. The magnetic field dependencies of the critical currents provide evidence for d-wave-induced alternations in the direction of the Josephson current between neighboring sides of the zigzag structure. The arrays present controllable model systems to study the influences of p facets in high-angle high-T c grain boundaries. From the characteristics, we estimate a possible imaginary s-wave admixture to the order parameter of the YBa 2 Cu 3 O 7 to be below 1%.

We report on an ab anisotropy of Jc parallel b/Jc parallel a approximately/= 1.8 IcRn parallelb/IcRn parallel a approximately/= 1.2 and in ramp-edge junctions between untwinned YBa2Cu3O7 and s-wave Nb. For these junctions, the angle theta with the YBa2Cu3O7 crystal b axis is varied as a single parameter. The RnA(theta) dependence presents twofold symmetry. The minima in IcRn at theta approximately/= 50 degrees suggest a real s-wave subdominant component and negligible d(xy)-wave or imaginary s-wave admixtures. The IcRn(theta) dependence is well fitted by 83% dx2-y2-, 15% isotropic s-, and 2% anisotropic s-wave order parameter symmetry, consistent with deltab/deltaa approximately/= 1.5.

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We report experiments in which one dimensional (1D) and two dimensional (2D) arrays of YBa2Cu3O 7−δ -Nb π-rings are cooled through the superconducting transition temperature of the Nb in various magnetic fields. These π-rings have degenerate ground states with either clockwise or counter-clockwise spontaneous circulating supercurrents. The final flux state of each ring in the arrays was determined using scanning SQUID microscopy. In the 1D arrays, fabricated as a single junction with facets alternating between alignment parallel to a [100] axis of the YBCO and rotated 90 o to that axis, half-fluxon Josephson vortices order strongly into an arrangement with alternating signs of their magnetic flux. We demonstrate that this ordering is driven by phase coupling and model the cooling process with a numerical solution of the Sine-Gordon equation. The 2D ring arrays couple to each other through the magnetic flux generated by the spontaneous supercurrents. Using π-rings for the 2D flux coupling experiments eliminates one source of disorder seen in similar experiments using conventional superconducting rings, since π-rings have doubly degenerate ground states in the absence of an applied field. Although anti-ferromagnetic ordering occurs, with larger negative bond orders than previously reported for arrays of conventional rings, long-range order is never observed, even in geometries without geometric frustration. This may be due to dynamical effects. Monte-Carlo simulations of the 2D array cooling process are presented and compared with experiment.

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