Role of SrTiO3 phonon penetrating into thin FeSe films in the enhancement of superconductivity

The significant role of interfacial coupling in the enhancement of superconductivity in FeSe films on SrTiO3 has been widely recognized, but the explicit origin of this coupling is yet to be identified. Here, by surface phonon measurements using high-resolution electron energy loss spectroscopy, we found the electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3 can penetrate into FeSe films and strongly interact with the electrons therein. The mode-specific electron-phonon coupling constant for the similar to 92 meV F-K phonon is similar to 0.25 in single-layer FeSe on SrTiO3. With increasing FeSe thickness, the penetrating field intensity decays exponentially, which matches well the observed exponential decay of the superconducting gap. It is unambiguously shown that the SrTiO3 F-K phonon penetrating into FeSe is essential for interfacial superconductivity enhancement.