Scanning tunneling microscopy study of the superconducting properties of three-atomic-layer Pb films
Ultrathin Pb films with a thickness of three monolayers (ML) were prepared on α-3×3Pb/Si(111) (Pb-SIC) substrate by molecular beam epitaxy. Despite significant defect scattering, low temperature scanning tunneling microscopy reveals a high superconducting transition temperature Tc of 6.9 K, compared with the bulk Tc (7.2 K). By applying external magnetic field, magnetic vortices were directly imaged, which demonstrates the robustness of superconductivity. By comparing to nearly free-standing Pb films on graphitized SiC (0001) substrate, we suggest that the higher Tc of 3 ML Pb films on Pb-SIC originates from the combined effects of quantum confinement and substrate-enhanced electron-phonon coupling.