We prepare single-unit-cell FeSe films on insulating ${\text{SrTiO}}_3$ substrates by molecular beam epitaxy and investigate the evolution of their superconducting properties with annealing by in situ scanning tunneling microscopy and scanning tunneling spectroscopy and ex situ transport measurements. We find that through an annealing process, the superconductivity of 1-uc FeSe films on ${\text{SrTiO}}_3$ substrates develops with the formation of stoichiometric FeSe films and is further enhanced by charge transfer from ${\text{SrTiO}}_3$ substrates to FeSe films. Moreover, the superconductivity is independent of the bulk property of the ${\text{SrTiO}}_3$ substrate, regardless of whether it is insulating or conductive. Our results reveal that the high-temperature superconductivity of 1-uc FeSe films on ${\text{SrTiO}}_3$ substrates indeed occurs at the FeSe/${\text{SrTiO}}_3$ interface, where the electron doping at FeSe films plays an important role in this interfacial superconductivity.

• Received 31 December 2013
• Revised 12 February 2014

DOI:https://doi.org/10.1103/PhysRevB.89.060506