  The accurate theoretical description of the underlying electronic structures
is essential for understanding the superconducting mechanism of iron-based
superconductors. Compared to bulk FeSe, the superconducting single-layer
FeSe/SrTiO3 films exhibit a distinct electronic structure consisting of only
electron Fermi pockets, due to the formation of a new band gap at the Brillouin
zone (BZ) corners and an indirect band gap between the BZ center and corners.
Although intensive studies have been carried out, the origin of such a distinct
electronic structure and its connection to bulk FeSe remain unclear. Here we
report a systematic study on the temperature evolution of the electronic
structure in single-layer FeSe/SrTiO3 films by angle-resolved photoemission
spectroscopy. A temperature-induced electronic phase transition was clearly
observed at 200 K, above which the electronic structure of single-layer
FeSe/SrTiO3 films restored to that of bulk FeSe, characterized by the closing
of the new band gap and the vanishing of the indirect band gap. Moreover, the
interfacial charge transfer effect induced band shift of ~ 60 meV was
determined for the first time. These observations not only show the first
direct evidence that the electronic structure of single-layer FeSe/SrTiO3 films
originates from bulk FeSe through a combined effect of an electronic phase
transition and an interfacial charge transfer, but also provide a quantitative
basis for theoretical models in describing the electronic structure and
understanding the superconducting mechanism in single-layer FeSe/SrTiO3 films.
