Disorder limited superconductivity in electron doped FeSe

Although it is well established that electron doping is a key ingredient in raising the transition temperature (T_c) of the parent compound FeSe, its full phase diagram and superconducting dome has yet to be fully explored, in contrast to most other unconventional superconductors. This is due to the difficulty in uniformly doping the material in a continuous fashion up to high carrier concentrations past optimal doping via techniques such as bulk intercalation or electrostatic gating. Here, we employ a unique combination of molecular beam epitaxy synthesis, alkali surface doping, in-vacuum electrical transport, and angle-resolved photoemission spectroscopy to investigate the entire superconducting dome of electron-doped FeSe, achieving a fully metallic state where superconductivity is suppressed in the heavily overdoped regime. We discover a scaling between T_c and the residual resistivity which holds across the entire superconducting dome, indicating that T_c is primarily controlled by elastic scattering rather than carrier doping. This suggests that the superconducting dome in electron-doped FeSe is fundamentally different than that of other unconventional superconductors.