Dichotomy of the electronic structure and superconductivity between single-layer and double-layer FeSe/SrTiO3 films

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.