Electronic evidence of an insulator–superconductor crossover in single-layer FeSe/SrTiO3 films

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 twodimensionality

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.