Engineering of ultrathin barriers in high T_C, trilayer Josephson junctions

Josephson junctions with ultrathin (25-40 Å) barriers were fabricated using high T_C, trilayer films grown by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The films consisted of top and bottom electrodes of Bi₂Sr₂CaCu₂O₈, separated by a single molecular layer of a metastable compound, Bi ₂Sr₂(Ca, Sr, Bi, Dy)_n-1Cu_nO _2n+4, with n=5 to 11. Systematic variation of Bi or Dy doping on Ca sites in the barrier layer provided four orders of magnitude of tuning of both the junction critical current and the normal state resistance, while keeping their product approximately constant, near 0.5 mV. Barrier and junction engineering, on an atomic length scale, has been demonstrated for the first time.