Superconducting FeSe monolayer with millielectronvolt Fermi energy

Iron selenide (FeSe) is an iron-based superconductor which shows unique properties, including strongly anisotropic superconducting gap, paramagnetism in undoped compound, and extremely small Fermi pocket size. In this work, we demonstrate that the sizes of electron hole pockets FeSe monolayer become much smaller than those bulk. The energy on order a few meV can be fine tuned by thickness graphene layers underneath. Despite low carrier density, monolayers grown trilayer or multilayer are superconducting. gap size sensitive to band. Remarkably, provides opportunity study physics crossover regime where comparable each other.