Measurement of the Superconducting Energy Gap and Fermi Velocity in Single-Crystal Lead Films by Electron Tunneling

The superconducting energy gap and reduced Fermi velocity have been measured as a function of crystallographic orientation by tunneling experiments using single-crystal thin films of Pb. Double gaps were observed in all directions except [100] in which a triple gap was found. Except in this last direction, the values measured agree with those of earlier experiments on bulk single crystals of Pb. Fermi velocities due to electrons from the second and third Brillouin zones are evaluated from a study of Tomasch oscillations in these films. Data are reported for the following orientations: [100], [110], [111], [211], and 5^∘{} off [100].