Superconductivity in a single layer alkali-doped FeSe: a weakly coupled   two-leg ladder system
  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. 
Wei Li, Hao Ding, Pengfei Zhang, Peng Deng, Kai Chang, Ke He, Shuaihua   Ji, Lili Wang, Xucun Ma, Jian Wu, Jiang-Ping Hu, Qi...
cond-mat.supr-con