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

摘要

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 YBa_2Cu_3O_7/Au/Nb junctions oriented toward the < 100 >-YBa_2Cu_3O_(7-δ) crystal axes. For SQUIDs with perpendicularly oriented junctions, maxima in the critical current are observed at applied fluxes of ±1/2 Φ_0, confirming the predominant d_(x~2-y~2) pairing symmetry in YBa_2Cu_3O_7. 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.