Two-band superconductivity of bulk and surface states in Ag thin films on Nb

Abstract

We use epitaxial strain to spatially tune the bottom of the surface-state band ${E}_{\mathrm{SS}}$ of Ag(111) islands on Nb(110). Bulk and surface-state contributions to the Ag(111) local density of states (LDOS) can be separated with scanning tunneling spectroscopy. For thick islands ($\ensuremath{\approx}20$ nm), the Ag surface states are decoupled from the Ag bulk states and the superconductive gap induced by proximity to Nb is due to bulk states only. However, for thin islands (3--4 nm), surface-state electrons develop superconducting correlations as identified by a complete energy gap in the LDOS when ${E}_{\mathrm{SS}}$ is smaller than but close to the Fermi level. The induced superconductivity in this case is of a two-band nature and appears to occur when the surface-state wave function reaches down to the Ag/Nb interface.