  We prepare single layer potassium-doped iron selenide (110) film by molecular
beam expitaxy. Such a single layer film can be viewed as a two-dimensional
system composed of weakly coupled two-leg iron ladders. Scanning tunneling
spectroscopy reveals that superconductivity is developed in this two-leg ladder
system. The superconducting gap is similar to that of the multi-layer films.
However, the Fermi surface topology given by first-principles calculation is
remarkably different from that of the bulk materials. Our results suggest that
superconducting pairing is very short-ranged or takes place rather locally in
iron-chalcogenides. The superconductivity is most likely driven by
electron-electron correlation effect and is insensitive to the change of Fermi
surfaces.
