Electron Tunneling into Superconducting Mercury Films

Tunneling measurements have been made on junctions consisting of evaporated films of aluminum and mercury separated by an aluminum oxide insulating layer. The width of the superconducting mercury energy gap extrapolated to $T=0\mathrm{}°$K has been determined to be $2\Delta \mathrm{}\left(0\right)=(1.65\mathrm{}\pm 0.04)\times {10}^{-3\mathrm{}} \mathrm{eV}=(4.60\mathrm{}\pm 0.11)k_B{T}_c$ from the current-voltage characteristic with the junction in the superconductor-insulator-superconductor configuration. Comparison is made with theory, and with thermodynamic and spectroscopic measurements of the gap. Measurements of the differential conductance in the vicinity of the gap in the metal-insulator-superconductor configuration show qualitative agreement with curves calculated from a BCS density of states, although the experimental curves are more peaked and have a larger tail at low bias voltages. The temperature dependence of the gap, as indicated by the differential conductance at zero bias, has been found to agree closely with the BCS variation, provided that the BCS curve is scaled by a constant multiplier. Structure, thought to be related to the phonon spectrum of mercury, has been observed in the first and second derivatives of the tunneling current.