Properties of evaporated Mo-Re thin-film superconductors

Mo-Re films have been deposited by electron-beam coevaporation in ultrahigh vacuum in the composition range between 25 and 40 at % Re. The films had either a single-crystal A15 structure, single-crystal -Mo (bcc) structure, or polycrystalline -Mo structure, depending on the substrate temperature during deposition. The superconducting transition temperature was approximately 12 K for films with both the A15 and -Mo structures, the same as literature values for bulk samples of the -Mo structure, even for films as thin as 20 nm. XPS measurements showed that the surface oxide thickness of samples exposed to ambient air was approximately 0.5 nm. Artificial tunnel barriers of oxidized Al were used with Pb or Mo-Re counterelectrodes to form low-leakage tunnel junctions for measurements of the superconducting energy gap.