Low-temperature STM Measurements of Granular Pb films*

Using low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) we have investigated the electronic properties of granular Pb films grown on HOPG at low temperature. Films grown under these conditions form a two-dimensional array of disconnected grains with a similar distribution of sizes. Local spectroscopy measurements as a function of field and temperature reveal a grain size dependent competition between the repulsive electron-electron interaction and the attractive superconducting pairing interaction. Our results show the presence of an increased depletion of states around the Fermi energy for all grain sizes, with a complete suppression of the superconducting state below a critical grain size. We compare these results to those found on 9ML and 100ML continuous films also grown on HOPG, where the superconducting state completely dominates the electronic properties.