  The significant role of interfacial coupling on the superconductivity
enhancement in FeSe films on SrTiO3 has been widely recognized. But the
explicit origination of this coupling is yet to be identified. Here by surface
phonon measurements using high resolution electron energy loss spectroscopy, we
found electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3
can penetrate into FeSe films and strongly interact with electrons therein. The
mode-specific electron-phonon coupling (EPC) constant for the ~92 meV F-K
phonon is ~0.25 in the 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 in the interfacial superconductivity enhancement.
