Superconductivity of nanometer-size Pb islands studied by low-temperature scanning tunneling microscopy

Using a low-temperature (1.2 K) scanning tunneling microscopy, the tunneling spectra showing the superconducting gap was taken on Pb island structures, whose dimension ranges from 80 to 300 nm in diameter and 7-12 monolayers in thickness. There is no considerable spatial variation in the tunneling spectra taken on a single island regardless of local geometry (center or peripheral) and thickness of the measured sites. The superconducting gap increases with the island size, and the size dependence is enhanced at higher temperature (3.9 K). The behavior of the gap is explained qualitatively by considering the superconducting fluctuation in the small islands.