Origin of charge transfer and enhanced electron-phonon coupling in single unit-cell FeSe films on SrTiO₃

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 SrTiO₃. 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/SrTiO₃ interface that leads to the charge transfer from SrTiO₃ 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.The origin of interface charge transfer and electron-phonon coupling in single unit-cell FeSe on SrTiO₃ remains elusive. Here, Zhang et al. report strengthened Ti-O bond and band bending at the FeSe/SrTiO₃ interface, which leads to several important processes.