Universal Quenching of Superconductivity in Two-dimensional Nano-islands

We systematically address superconductivity of Pb nano-islands with different thicknesses and lateral sizes via a scanning tunneling microscopy/spectroscopy. Reduction of the superconducting gap is observed even when the island is larger than the bulk coherence length and becomes very fast below ~ 50 nm lateral size. The suppression of gap with size depends to a good approximation only on the volume of the island and is independent of its shape. Theoretical analysis indicates that the universal quenching behavior is primarily manifested by the mean number of electronic orbitals within the pairing energy window.