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

TL;DR

This study uncovers the mechanisms behind charge transfer and electron-phonon coupling in single-unit-cell FeSe films on SrTiO3, revealing their roles in high-temperature superconductivity and guiding future superconductor design.

Contribution

It identifies the origin of interface charge transfer and enhanced electron-phonon coupling in FeSe/SrTiO3 using spectroscopic techniques, providing new insights into superconductivity mechanisms.

Findings

Strengthened Ti-O bonds contribute to electron-phonon coupling.

Band bending causes charge transfer from SrTiO3 to FeSe.

Band renormalization correlates with superconductivity onset.

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 (UPS) and element-sensitive X-ray photoemission spectroscopy (XPS), 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 towards designing novel superconductors in heterostructures with band-bending induced charge transfer and interfacial enhanced electron-phonon coupling.