Discrete Superconducting Phases in FeSe-derived Superconductors

A general feature of unconventional superconductors is the existence of a superconducting dome in the phase diagram, i.e., the continuous evolution of the transition temperature T_c with tuning parameters such as doping and pressure. For the simplest iron-based superconductor FeSe (T_c > 8 K), its T_c can be greatly enhanced by doping electrons via many ways, even up to 65 K in monolayer FeSe/SrTiO₃ with the help of phonons. However, it remains controversial whether there exists a superconducting dome in these electron-doped FeSe-derived superconductors. Here, we report the observation of a series of discrete superconducting phases in FeSe thin flakes by finely tuning the intercalation of Li and Na ions with a solid ion gating technique. Such discrete superconducting phases contrast sharply to the superconducting dome in most unconventional superconductors. Our results demonstrate that the existence of discrete superconducting phases is intrinsic and universal in FeSe intercalated by alkali/alkaline earth/rear earth metals. It likely comes from the spontaneous phase separation of these ions between FeSe layers.