π-SQUIDs based on Josephson contacts between high-T_c and low-T_c superconductors

For phase-sensitive studies on the pairing-symmetry in a given superconductor, its combination with a well-characterized isotropic superconductor into a superconducting quantum interference device (SQUID) provides a very fruitful method. With a well-controlled orientation and position of the hybrid Josephson junctions involved, the angular dependence of both the phase and the magnitude of the order parameter wave function can be determined. In view of this, and motivated by recent proposals for quantum-logic devices, we have investigated low-inductance dc SQUIDs with different geometries. These SQUIDs are based on ramp-type YBa2Cu3O7/Au/Nb junctions oriented toward the , <100>YBa2Cu3O7−δ crystal axes. For SQUIDs with perpendicularly oriented junctions, maxima in the critical current are observed at applied fluxes of ±1/2 Φ0 , confirming the predominant dx2-y2 pairing symmetry in YBa2Cu3O7. The measured critical current-modulations for these π-SQUIDs correspond well to simulations. No indications are found for any time-reversal symmetry breaking, for instance resulting from imaginary symmetry admixtures.