Abstract
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The latest discovery of possible high-temperature superconductivity in the single-layer FeSe
film grown on a SrTiO3 substrate has generated much attention. Initial work found that,
while the single-layer FeSe/SrTiO3 film exhibits a clear signature of superconductivity, the
double-layer film shows an insulating behaviour. Such a marked layer-dependent difference is
surprising and the underlying origin remains unclear. Here we report a comparative angleresolved
photoemission study between the single-layer and double-layer FeSe/SrTiO3 films
annealed in vacuum. We find that, different from the single-layer FeSe/SrTiO3 film, the
double-layer FeSe/SrTiO3 film is hard to get doped and remains in the semiconducting/
insulating state under an extensive annealing condition. Such a behaviour originates from the
much reduced doping efficiency in the bottom FeSe layer of the double-layer FeSe/SrTiO3 film
from the FeSe–SrTiO3 interface. These observations provide key insights in understanding the
doping mechanism and the origin of superconductivity in the FeSe/SrTiO3 films.