Ambipolar Doping of Monolayer FeSe by Interface Engineering

We report on ambipolar modulation doping of monolayer FeSe epitaxial films grown by molecular beam epitaxy and in situ spectroscopic measurements via a cryogenic scanning tunneling microscopy. It is found that hole doping kills superconductivity in monolayer FeSe films on metallic Ir(001) substrates, whereas electron doping from polycrystalline IrO₂/SrTiO₃ substrate enhances significantly the superconductivity with an energy gap of 10.3 meV. By exploring substrate-dependent superconductivity, we elucidate the essential impact of substrate work functions on the superconductivity of monolayer FeSe films. Our results therefore offer a valuable reference guide for further enhancement of the transition temperature T_c in FeSe-based superconductors by interface engineering.