Robust superconductivity in quantum-confined Pb: Equilibrium and irreversible superconductive properties

Strong quantum size effects enable the formation of crystalline Pb films that are atomically flat on a macroscopic length scale. The superconducting properties of 5--18--monolayer-(ML) thick Pb films were investigated in a superconducting quantum interference device (SQUID) magnetometer using combined ac and dc methods. Even the thinnest films (5 ML) are extraordinarily robust type-II superconductors. Despite the extreme two-dimensional geometry, the thermodynamic parameters Tc and upper critical field Hc2 are primarily controlled by the physical boundary conditions of the film and show no evidence for disorder-driven or fluctuation-driven quenching of superconductivity. A magnetically hard critical state is established as a consequence of vortex trapping by quantum growth defects.