title: Effects of Surface Electron Doping and Substrate on the Superconductivity of Epitaxial FeSe Films.
abstract: Superconductivity in FeSe is greatly enhanced in films grown on SrTiO3 substrates, although the mechanism behind remains unclear. Recently, surface potassium (K) doping has also proven able to enhance the superconductivity of FeSe. Here, by using scanning tunneling microscopy, we compare the K doping dependence of the superconductivity in FeSe films grown on two substrates: SrTiO3 (001) and graphitized SiC (0001). For thick films (20 unit cells (UC)), the optimized superconducting (SC) gaps are of similar size (∼9 meV) regardless of the substrate. However, when the thickness is reduced to a few UC, the optimized SC gap is increased up to ∼15 meV for films on SrTiO3, whereas it remains unchanged for films on SiC. This clearly indicates that the FeSe/SrTiO3 interface can further enhance the superconductivity, beyond merely doping electrons. Intriguingly, we found that this interface enhancement decays exponentially as the thickness increases, with a decay length of 2.4 UC, which is much shorter than the length scale for relaxation of the lattice strain, pointing to interfacial electron-phonon coupling as the likely origin.
authors: State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University , Shanghai 200433, China.
authors: W H
authors: WH
authors: Zhang
authors: State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University , Shanghai 200433, China.
authors: X
authors: X
authors: Liu
authors: State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University , Shanghai 200433, China.
authors: C H P
authors: CH
authors: Wen
authors: State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University , Shanghai 200433, China.
authors: R
authors: R
authors: Peng
authors: State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University , Shanghai 200433, China.
authors: S Y
authors: SY
authors: Tan
authors: State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University , Shanghai 200433, China.|Collaborative Innovation Center of Advanced Microstructures, Fudan University , Shanghai 200433, China.
authors: B P
authors: BP
authors: Xie
authors: State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University , Shanghai 200433, China.|Collaborative Innovation Center of Advanced Microstructures, Fudan University , Shanghai 200433, China.
authors: T
authors: T
authors: Zhang
authors: State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University , Shanghai 200433, China.|Collaborative Innovation Center of Advanced Microstructures, Fudan University , Shanghai 200433, China.
authors: D L
authors: DL
authors: Feng
keywords: FeSe; electron−phonon coupling; interfacial effect; iron-based superconductor; scanning tunneling microscopy