Tuning superconductivity in highly crystalline Pb1−xBix alloy ultrathin films at atomic level

Achieving highly crystalline superconductors with reduced dimensionality and fine-tuning their superconductivity have been daunting challenges in condensed matter physics. Here, we have successfully grown two-monolayer highly crystalline ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$(111) films with controllable Bi concentrations $x$ by molecular beam epitaxy and systematically studied their structural and superconducting properties by scanning tunneling microscopy. We first show that the superconducting transition temperatures of ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$ films exhibit a domelike behavior with increasing Bi concentration $x$. Our first-principles calculations reveal that Bi doping can promote the electronic states and the electron-phonon coupling strength at lower $x$ and suppress the electron-phonon coupling strength and superconductivity with largely Bi electronic states when Bi doping increases over a critical ratio. Our findings not only demonstrate a quantum phenomenon of superconductivity in highly crystalline alloyed films but also provide a practical pathway to tune the superconductivity at the atomic level.