Origin of charge transfer and enhanced electron–phonon coupling in single unit-cell FeSe films on SrTiO 3

Interface charge transfer and electron–phonon coupling have been suggested to play a crucial role in the recently discovered high-temperature superconductivity of single unit-cell FeSe films on SrTiO 3 . However, their origin remains elusive. Here, using ultraviolet photoemission spectroscopy and element-sensitive X-ray photoemission spectroscopy, we identify the strengthened Ti–O bond that contributes to the interface enhanced electron–phonon coupling and unveil the band bending at the FeSe/SrTiO 3 interface that leads to the charge transfer from SrTiO 3 to FeSe films. We also observe band renormalization that accompanies the onset of superconductivity. Our results not only provide valuable insights into the mechanism of the interface-enhanced superconductivity, but also point out a promising route toward designing novel superconductors in heterostructures with band bending-induced charge transfer and interfacial enhanced electron–phonon coupling.