Abstract
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.
Highlights
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
The barrier across the FeSe/STO junction becomes smaller with annealing
Apart from the electron-doping effect, the Ti–O bond becomes strengthened with annealing in comparison to the as-grown state. It reflects the enhancement of the substrate-mediated electron–phonon coupling, which further promotes the superconductivity
Summary
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. The angle-resolved photoemission spectroscopy (ARPES) observation of shake-off bands suggests the coupling between FeSe electrons and optical phonons at the energy of ∼100 meV in STO12 This phonon mode could be responsible for the further enhanced superconductivity in 1–3uc FeSe films on STO14, compared with heavily electron-doped FeSe18, 19, 24. The dielectric layer enhances the charge density of the SC layer by charge transfer, and provides the electron–phonon coupling with its intrinsic high-energy phonon mode
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