Superconducting energy gap in Coulomb staircase tunneling structures

A sharp Coulomb staircase I-V characteristic has been observed in tunneling structures formed between a tungsten scanning tunneling microscope (STM) tip and a superconducting lead film. The actual geometry is that of a single isolated particle acting as a common electrode in a series configuration of two low-capacitance tunneling junctions between the bulk of the lead film, the particle, and the STM tip. The voltage width of the step that crossed zero bias was larger than that of other steps in the staircase. This is due to the superconductivity of lead, and is consistent with theory.