In high-temperature cuprate superconductors, it is now generally agreed that superconductivity is realized by doping an antiferromagnetic Mott (charge transfer) insulator. The doping-induced insulator-to-superconductor transition has been widely observed in cuprates, which provides important information for understanding the superconductivity mechanism. In the iron-based superconductors, however, the parent compound is mostly antiferromagnetic bad metal, raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture. No evidence of doping-induced insulator–superconductor transition (or crossover) has been reported in the iron-based compounds so far. Here, we report an electronic evidence of an insulator–superconductor crossover observed in the single-layer FeSe film grown on a SrTiO3 substrate. By taking angle-resolved photoemission measurements on the electronic structure and energy gap, we have identified a clear evolution of an insulator to a superconductor with increasing carrier concentration. In particular, the insulator–superconductor crossover in FeSe/SrTiO3 film exhibits similar behaviors to that observed in the cuprate superconductors. Our results suggest that the observed insulator–superconductor crossover may be associated with the two-dimensionality that enhances electron localization or correlation. The reduced dimensionality and the interfacial effect provide a new pathway in searching for new phenomena and novel superconductors with a high transition temperature.

We here report our experimental study of the quantum size effect modulated metalation reaction of phthalocyanine (H2Pc) by low temperature scanning tunneling microscopy. When iron atoms were deposited onto Pb(111) thin films (2−5 nm thick) precovered by a self-assembled H2Pc monolayer, a surface metalation reaction to iron phthalocyanine (FePc) was observed. The amount of the FePc products was found to change prominently whenever the film thickness varies by one atomic layer and exhibits thickness-dependent oscillatory behavior. We show that the oscillation can be well-understood by the quantum size effect in the Pb thin films. The present study gives direct proof for tailoring a surface chemical reaction by quantum confinement.