Stacking structure and superconductivity in ruthenium-iridium bicrystal superlattices

The stacking structure and epitaxial ordering are studied in molecular-beam-epitaxy-grown Ru-Ir superlattices of wavelengths λ=10, 41, and 62 {}. These samples span the fcc-hcp crossover previously found to occur when the Ir component is thinner than about 10 monolayers. Evidence for a new stacking sequence of period 3λ is found in the λ=41 {} sample, possibly resulting from stacking faults in the Ir layers. The superconducting transition temperature is significantly higher in the 41-{} sample (Tc=0.19 K) than in the 62-{} sample (Tc=0.15 K), whereas the 10-{} sample is not superconducting down to 20 mK.