Persistent supercurrents in composite rings of high-T_c and conventional superconductors

We have measured substantial supercurrents in rings formed by segments of epitaxial Y1Ba2Cu3O7 and lead thin films. The supercurrent density across the planar interface of the conventional and high-Tc superconductor has been found to be larger than 600 A/cm2 at 4.2 K. Our experimental observations can place powerful constraints on the up to now unknown nature of the superconducting order parameter in high-temperature superconductors. © 1990.