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

Achieving highly crystalline superconductors with reduced dimensionality and fine-tuning their supercon-ductivity have been daunting challenges in condensed matter physics. Here, we have successfully grown two-monolayer highly crystalline Pb1-xBix(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 Pb1-xBix 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.