Abstract
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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.