Thin films of Bi and Ga deposited at 300 K on thin Ni underlayers are strongly coupled superconductors at liquid-helium temperature. A layer of 0.1–5.0 nm of Ni apparently seeds a superconducting phase in each of these metals. For Bi films ${\mathit{T}}_{\mathit{c}}$≤4 K, with an energy gap 2Δ≤1.30 meV; for Ga films ${\mathit{T}}_{\mathit{c}}$≤7 K and 2Δ≤2.30 meV. For both elements 2Δ/${\mathit{kT}}_{\mathit{c}}$≊4.2. The superconductivity of the Bi film was not confined to the interface, but extended throughout the film up to thicknesses of at least 40 nm, as was shown by tunneling studies on each surface of the film. X-ray diffraction shows that the Bi films grow pseudomorphically as an fcc phase with the lattice parameter of 5.71 Å, in contrast to the orthorhombic structure of bulk Bi. Upper critical fields were measured as a function of temperature and were several teslas. Ga films with or without seeding with Ni show only a disordered structure by x-ray diffraction. Electron-spin-polarization tunneling studies indicate that the Ni underlayers up to a thickness of about 2.0 nm are not ferromagnetic.

• Received 25 January 1990

DOI:https://doi.org/10.1103/PhysRevB.42.179