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 $\lambda =10, 41, \mathrm{and} 62$ Å. 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 $3\lambda$ is found in the $\lambda =41\mathrm{}$ Å sample, possibly resulting from stacking faults in the Ir layers. The superconducting transition temperature is significantly higher in the 41-Å sample ($T_c=0.19\mathrm{}$ K) than in the 62-Å sample ($T_c=0.15\mathrm{}$ K), whereas the 10-Å sample is not superconducting down to 20 mK.