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Josephson junctions with ultrathin (25–40 Å) barriers were fabricated using high TC, 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, Bi2Sr2(Ca, Sr, Bi, Dy)n−1CunO2n+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.

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We report the use of pseudo-double-beam atomic absorption spectroscopy to make very accurate (0.1%–1%) measurements of the beam flux from Knudsen effusion cells in a molecular beam epitaxy system. This system has been used to grow Bi–Sr–Ca–Cu–O–based superconducting thin films and heterostructures in an atomic-layer-by-layer fashion. The resulting material displays excellent crystallographic and transport properties. Atomic absorption measurement of beam fluxes may also be of benefit for other materials systems, such as InGaAs/InP.

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