Atomic-layer fabrication of high-T_c tunnel junctions

Using ALL-MBE, we have engineered a novel, metastable cuprate superconductor, BiSr₂Ca_7-xDy_xCu₈O_19+y' in which only the central Ca layer is doped by Dy. This provides, within a single unit cell, the bottom superconducting electrode, an insulating barrier layer (only few Å thick), and the top superconducting electrode, thus constituting an artificial intra-cell Josephson junction. In this way, we have fabricated the first high-T_c tunnel (SIS) junctions. They exhibit very sharp quasiparticle tunneling I-V characteristics, consistent with tunneling between 2D superconductors with d-wave pairing.