High-temperature superconducting multilayers and heterostructures grown by atomic layer-by-layer molecular beam epitaxy

Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) of high-temperature superconductors (HTSC) and other complex oxides has been developed. Thin films with atomically flat surfaces and abrupt interfaces can be produced an atomic layer at a time. Samples are engineered by stacking molecular layers of different compounds, by adding or omitting atomic monolayers, and by doping within specified monolayers. Novel artificial HTSC compounds such as Bi₂Sr₂Ca₇Cu₈O_x (2278), as well as various heterostructures, have been synthesized in this way. This unique synthetic capability has allowed several fundamental physics issues, such as the dimensionality of the HTSC state, long-range proximity effects, resonant tunneling with a specified number of hops, etc, to be addressed. Trilayer Josephson junctions with uniform and reproducible properties have been fabricated; phase-locked operation of two junctions has also been demonstrated. Finally, titanate slabs only 4-Å thick have been grown without pin-holes and shown to provide tunneling barriers for c-axis transport.