Growth and Superconductivity of Pb and Pb-Bi Alloys in the Quantum Regime

Superconductivity is a collective quantum phenomenon that is inevitably suppressed in reduced dimensionality. Questions of how thin superconducting wires or films can be before they lose their superconducting properties have important technological ramifications and go to the heart of understanding formation, coherence, and robustness of the superconducting state in quantum confined geometries. Suppression of superconductivity in low dimensions is usually attributed to thermal or quantum fluctuations, or to pair-breaking Coulomb interactions in the presence of strong disorder. Control and quantification of a film’s disorder length scale remained a critical experimental obstacle, however. Here, we exploit quantum confinement of itinerant electrons in a soft metal (Pb), to stabilize atomically-flat superconductors with lateral dimensions of the order of a few millimeters and vertical dimensions of only a few atomic layers. These extremely thin superconductors show no indication of defect- or fluctuation-driven suppression of superconductivity and sustain macroscopic super- currents of up to ~10% of the theoretical depairing current density. The extreme hardness of the critical state can be attributed to the presence of intrinsic vortex traps that are stabilized by quantum confinement. We furthermore show that the quantum growth and superconductive properties of the films can be tailored by Fermi surface engineering via controlled alloying. The present study paints a conceptually appealing, elegant picture of a model nano-scale superconductor with calculable critical state properties. It furthermore indicates the intriguing possibility of achieving and exploiting superconductivity in the ultimate low-dimensional limit.