  The latest discovery of possible high temperature superconductivity in the
single-layer FeSe film grown on a SrTiO3 substrate, together with the
observation of its unique electronic structure and nodeless superconducting
gap, has generated much attention. Initial work also found that, while the
single-layer FeSe/SrTiO3 film exhibits a clear signature of superconductivity,
the double-layer FeSe/SrTiO3 film shows an insulating behavior. Such a dramatic
difference between the single-layer and double-layer FeSe/SrTiO3 films is
surprising and the underlying origin remains unclear. Here we report our
comparative study between the single-layer and double-layer FeSe/SrTiO3 films
by performing a systematic angle-resolved photoemission study on the samples
annealed in vacuum. We find that, like the single-layer FeSe/SrTiO3 film, the
as-prepared double-layer FeSe/SrTiO3 film is insulating and possibly magnetic,
thus establishing a universal existence of the magnetic phase in the
FeSe/SrTiO3 films. In particular, the double-layer FeSe/SrTiO3 film shows a
quite different doping behavior from the single-layer film in that it is hard
to get doped and remains in the insulating state under an extensive annealing
condition. The difference 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 origin
of superconductivity and the doping mechanism in the FeSe/SrTiO3 films. The
property disparity between the single-layer and double-layer FeSe/SrTiO3 films
may facilitate to fabricate electronic devices by making superconducting and
insulating components on the same substrate under the same condition.
